On The Possibility That The McMinnville Photos
Show A Distant Unidentified Object (UO)
by Bruce S. Maccabee. (c) B. Maccabee, 2000

Original link at

This paper was originally published in the proceedings of the 1976 UFO conference of The Center For UFO Studies (CUFOS). This version has been slightly modified in April 2000 for this publication.

This is the first of two technical and historical papers on the Trent photo case that were presented to and published by CUFOS located in Chicago, Illinois.


During the Air Force funded investigation of UFO reports at the University of Colorado in 1967-1968 (the "Condon Report"), photo analyst William Hartmann studied in detail photographic and verbal evidence presented by two former residents of McMinnville, Oregon, Paul and Evelyn Trent. He concluded, mainly on the basis of a simplified photo metric analysis, that "all factors investigated, geometrical, psychological and physical, appear to be consistent with the assertion that an extraordinary flying object, silvery, metallic, disk shaped, tens of meters in diameter and evidently artificial, flew within sight of two witnesses." An important part of his analysis included calculations of the expected brightness of the image of the bottom of the Unidentified Object (UO) that appears in the first photo. He pointed out that the elliptical image of the bottom was brighter than expected if the object were close and therefore a small model. The excessive image brightness led him to conclude that the object was at a great distance (over a kilometer), His conclusion was criticized by Philip J. Klass and Robert Sheaffer who argued that veiling glare (caused by surface dirt and imperfections in the lens which scatter light from bright areas of the image into all other areas of the image) could have increased the brightness of the image of the UO, making it appear distant. This investigation revisited and improved upon Hartmann's method with the following modifications: (a) the bottom of the UO in the first photo has been assumed to be as intrinsically bright as possible without being a source of light (i.e., assumed to be white); (b) laboratory measurements have been used to estimate the magnitudes of veiling glare added to the various images of interest; (c) a film exposure curve has been used to determine relative image illuminances; and (d) a surface brightness ratio, determined by field measurements, has been included. The results of the new photo metric analysis suggest that the bottom of the UO is too bright for it to have been a non-self-luminous white (paper) surface of a nearby object. Hence it could have been distant.


In June 1950, four weeks after they were taken, the photos illustrated in Figures 1 and 2 appeared in the local newspaper in McMinnville, Oregon.

SEE THE FOLLOWING IMAGES: Trent1.jpg is Trent Photo 1 showing the elliptical bottom image Trent2.jpg is Trent Photo 2 showing the side view Trnt1BLWUP.jpg is a blowup of the UO in photo 1; blowup by Wm. Hartmann Trnt 1 BLWUP.jpg is a blowup of the UO in photo 2. The lines were drawn by Hartmann FIGURE 1 in the text IS Trnt Sketch1.gif, a tracing showing sighting directions FIGURE 2 in the text IS Trnt Sketch2.gif, a tracing showing sighting directions

Subsequently, they appeared in Life Magazine and in many publications, devoted to UFO reports. Although they clearly depict an unusual object, they were never treated as scientifically valuable because it was always considered probable that they were photos of a hoax object (e.g., "a garbage can lid"). Nevertheless they did gain a large measure of scientific "stature" when, in 1968, Hartmann (1) concluded that the object may have been distant and, therefore, large (i.e. not a hoax). Since the publication of Hartmann's conclusion in the Condon Report (1) these photos and the verbal evidence associated with them have been the subject of a continuing controversy. A brief history of the analysis of the photos is given in Figure 3 (below).


A Brief History of the McMinnville Photos

Publication: 8 June 1950. The Editor stated that "expert photographers declared there has been no tampering with the negatives. (The) original photos were developed by a local firm. After careful consideration, there appears to be no possibility of hoax or hallucination connected with the pictures. Therefore, the Telephone-Register believes them authentic." - The Telephone Register, McMinnville, Oregon

Subsequent Immediate Publications

The Portland Oregonian, Portland, Oregon, 10 June 1950 (contains further verbal information); The Los Angeles Examiner, Los Angeles, California, 11 June 1950 (contains further verbal information); Life, June 1950 (contains further verbal information)

The photos were "lost" in the files of the International News Photo Service and subsequently in the files of UPI until they were "found" by the Condon UFO study project in 1968.

Condon UFO Report -Conclusion by Wm. Hartmann, case investigator: Certain physical evidence, specifically relative photographic densities of images in the photographs, suggests that the object was distant; if the object was truly distant, a hoax could be ruled out as beyond the capabilities of the photographer. (NOTE: Hartmann's report contains a good summary of the verbal evidence available up to 1967.)

Sheaffer-Klass Conclusion (1974):(1) There are some possible inconsistencies in the verbal evidence and several important discrepancies between the verbal report and the photographic evidence. (2) Hartmann's photo metric analysis was incomplete. Specifically, (a) shadows on the garage wall (facing east) suggest that the pictures were taken in the morning, not in the evening as claimed; (b) the apparent shrinkage of the shadow nearest the edge of the garage suggests that there were many minutes between photo 1 and photo 2; and (c) veiling glare could have made the image of the bottom of the UO excessively bright thus leading Hartmann to erroneously conclude that the object was distant.

Present Investigation (1976): New testimony (published in a companion paper to this) has been obtained and the original negatives have been studied photogrammetrically as well as photo metrically. The present investigation has confirmed that there are shadows on the garage wall (agree with (a) above), but has found that, to within the resolution of the measurements (using a traveling microscope), the shadows other than the one at the edge of the garage did not
move with respect to the garage wall between photos (the shadow at the edge of the garage does appear narrower in photo 2) (disagree with (b) above). The present investigation has reviewed and confirmed the general validity of Hartmann's analysis. When the effects of veiling glare and the ratio of brightness' of vertical and horizontal surfaces have been accounted for the Hartmann type analysis again indicates a large distance (disagree with (c) above).

(Resume Text) The initial analysis was carried out by a photographer (Bill Powell) who worked for the McMinnville Telephone-Register (now the News Register). Hartmann confirmed the original analysis and went on to conclude that the object was asymmetric and that it was probably not rotating about a (nearly) vertical axis (i.e., was not thrown into the air). Hartmann pointed out that the possibility for a simple hoax existed since the photos show the UO as appearing to be "underneath" two nearby power wires. However, he carried out a simplified photo metric analysis which led him to conclude that the object was distant and that "the simplest, most direct interpretation of the photographs confirms precisely what the witnesses said they saw." A modified version of Hartmann's analysis will be presented in the next section to illustrate the use of photometry. In 1974 Philip Klass (2) published an analysis of the verbal evidence by himself and of the photographic evidence by Robert Sheaffer (2,3). They found a puzzling inconsistency between the photos and the verbal description: the photos show clear shadows on the east wall of the nearby garage, which implies that the pictures were taken in the morning, while the witnesses claimed that the pictures were taken in the evening. Sheaffer argued, on the basis of measurements of the width of the shadow of the eaves rafter at the corner of the garage, that there was a considerable time lag between photos rather than "less than 30 seconds" as claimed (see Figure 3). However, Sheaffer's most important "discovery" was that dirt on the camera lens, or a poor quality lens, could have caused light from the bright sky surrounding the image of the UO to "spill over" onto the image of the UO, thus making the UO image excessively bright. In Hartmann's analysis the excessive brightness was attributed to the effect of the atmosphere on the apparent brightness of an object if it were distant. By attributing the excess brightness to a camera defect, Sheaffer was able to argue (qualitatively) that the distance calculation was in error and that "in reality" the object was close to the camera. He was, thus, able to remove the main inconsistency with the simple hoax hypothesis: the object, a model UFO, was hanging from wires that were less than twenty feet from the camera. In late 1973, unaware of the work of Sheaffer and Klass, I decided to undertake an investigation of the McMinnville case because (a) the pictures are so clear the object is either a hoax device or an unusual object (no misinterpretation seems possible; e.g., it's not a plane at an odd angle), and (b) Hartmann had devoted considerable effort and analytical research to the photos and had concluded on the basis of this physical evidence that the object was distant (not a hoax). Considering the general tone of the Condon Report (skeptical), I felt that Hartmann must have been quite confident to publish the conclusion he drew from his analysis. He could have decided to do no photo metric study and then he would have been "safe" in saying that the case provided "no probative evidence" and that, furthermore, it was probably a hoax. Or, he could have reported the photo metric study with such disclaimers as "the photos are so poor (scratched, worn, etc.) that the photo metric study is probably in error by a considerable amount." (NOTE: Dr. Condon wrote in the executive summary chapter that photo analyst Everitt Merritt, who was not a part of the Colorado University UFO research project, had already "thrown out" the photos as being too fuzzy for worthwhile photogrammetric analysis. But photogrammetric analysis, which makes use of angular separations of images, is different from photo metric analysis, which makes use of relative image brightness'. I am certain that Condon knew the difference between photo metric and photogrammetric analysis. It appears that he tried to "cover up" the success of one [photo metric] with the "failure" of the other [photogrammetric] by not mentioning Hartmann's analysis in the executive summary of the research.) Dr. Hartmann did point out that his analysis might only be correct to within a factor of four, but, even with an error bar this large, several hundred meters was the closest distance compatible with his analysis. Since Hartmann had essentially endorsed the photos as probably genuine, I decided to try to either confirm or refute his result in a study of my own. Since I was somewhat skeptical myself, I fully expected to be able to show that either the atmospheric theory he used or the photo metric measurements were wrong (or incorrectly applied). After a several year study, I have concluded that the general form of Hartmann's analysis is valid. However, I have found that he ignored or was unaware of several "details" of the necessary photographic analysis which will be outlined in the following section. I was not able to confirm the specific numbers which he gave as relative brightness' of various images on the photos. At least part (perhaps a major part) of this discrepancy is due to a difference in measurement technique: Hartmann measured transmission values of small portions of the images of interest and then divided by the transmission "somewhere" along the horizon; he thus did not have good estimates of average brightness' of the images. I used a scanning densitometer with a very small aperture and averaged over many scans across an image of interest. However, despite the (not large) difference in the relative brightness' obtained in the two independent investigations, the conclusions have turned out to be essentially the same, as will be seen.

II. Photo metric Analysis of the McMinnville Photos In the spring of 1975 I was able to locate, with the incidental help of Mr. Klass, the original negatives. (They were in the possession of Philip Bladine, the editor of the newspaper.) Consequently, all density values given in this paper are from those negatives. They were measured on a Joyce-Loeble densitometer that was repeatedly calibrated with a Kodak standard diffuse neutral density "wedge." Although many areas of both photos have been scanned to establish consistency between the exposures, etc., only the density values pertinent to the range calculation will be listed here. These values along with other pertinent photographic data are listed in Table I. The analysis is based on Hartmann's method with the following modifications: (1) I have used an exposure curve relation for the negatives based on a published D-LogE curve for Verichrome film whereas, Hartmann implicitly assumed gamma = 1 (Film gamma relates exposure level or image density to illumination of the film or image brightness. See illustrations labeled "TrntGamma6Curve.gif" and TrntGAMACurves.gif.) Other possible film types are Plus-X and Plus-XX, both Kodak films, but the exposure curves of these are similar to that of Verichrome; measures of the fog density suggest that only Plus-XX and Verichrome are compatible with densities found in unexposed regions; Verichrome was the least expensive, hence most likely to have been used; Verichrome has low sensitivity to red light.); (2) Since the negatives are pale (1,4), that is, the density range starting from the fog level is not as large as expected for a sunlit day, I have assumed that the negatives were slightly underdeveloped and have, therefore, used an exposure curve for gamma = 0.6, even though it was standard procedure to develop to a gamma of about 1 (4); (3) I have used a photographic formula to relate image illuminances to object brightness; (4) I have incorporated laboratory derived estimates of veiling glare; and (5) I have incorporated the brightness ratio of a shaded vertical surface to a horizontal surface seen from below. The ratio was obtained from field measurements. This brightness ratio was ignored by both Sheaffer and Hartmann. The first step in the analysis is to determine the relative illuminances on the film plane which produced the image densities. Simple photographic theory corrected for the effects of veiling glare predicts that

E' = image illuminances = K(B + G) cos^4(A) (1)

where K is a constant for a particular picture (and is assumed to be the same for both photos here; this involves f-number and shutter time), B is the brightness in the absence of glare of the object being photographed, G is the amount of veiling glare added to the image, cos^4 is the cosine raised to the fourth power and A is the angle between the lens axis and the direction to the object. Defining Ei = E'/[Kcos^4(A)], and substituting the empirical exposure curve relation between measured image densities and their causative illuminances, yields the total image "brightness" given in Eq. 2 (see Table I). The brightness in the absence of glare is then found by subtracting the glare on the image, as in Eq. 3 (see Table I).


IMAGE OF: DENSITY, D ANGLE,A,FROM LENS AXIS Shadow on wall of 0.025 +/- 0.03 A = 17.6 deg. distant white house, ph#1 weighting factor = 1) Same as above, ph#2 0.024 +/- 0.01 12.5 deg Sky near and above U0 0.061 +/- 0.01 0 deg Horizon in each photo 0.43 to 0.46 10 deg (use 0.45 as avg) Bottom of UO in photo 1 0.315 +/- 0.001 The atmospheric Extinction Coefficient (12 mile visibility from weather report), b = O.2/km. The distance to white house across the Salmon River Parkway is about 360 meters The focal length of the lens = 103 (+/-) 5 mm The f# was probably about f/ll The shutter time was probably 1/125 Relative exposures or "total image brightness'" have been calculated from

Ei = Eo {exp[2.303(Di/gamma - k/Di^3)]}/{Kcos^4(A)} (2)

where Ei is the image exposure, Di, is the measured density for Di>0.1, Eo and k are constants that depend upon the film development "constant," gamma. Table IV contains a listing of values of E, and k for various values of gamma. The relation between image brightness, B, image exposure, Ei, and veiling glare on the image, Gi is

B = Ei - Gi (3)

The amount of veiling glare added to an image is proportional to the brightness, Bs, surrounding the image: Gi = gi x Bs, where values of gi for particular sizes and shapes of images in particular surrounding brightness distributions have been measured in the laboratory. With a brightness distribution similar to that of the photos (bright above the horizon, dark below the horizon), a laboratory simulation has shown that, when a lens is sufficiently dirty to produce guo ~ 0.12, i.e., glare in the UO image is about 12% of the surrounding brightness, then g(distant house)~ 0035 and g(horizon) ~ 005.

Let the ratio of the brightness of a vertical, white, shaded surface (the wall of a white house) to the brightness of a horizontal white surface viewed from below (hypothetical UFO model with a white paper bottom) be called Rb.

Field measurements show that 2.4 < Rb < 4.7. In the calculations done here I have used Rg = 2.4 to be conservative. Use of a larger Rb would result in calculated distances greater than reported here.

Atmospheric brightening formulas for range r (the formulas first used by Hartmann) are:

(a) B(r=0) = intrinsic brightness = Bh + (B(r)-Bh) e^(br) (4)

(b) r = range = (1/b)Ln{[B(r=0)- Bh]/[B(r) - Bh]} (5)

where B(r) is the measured brightness at range r, Bh is the horizon brightness and b is the atmospheric extinction coefficient.

(Resume Text) To illustrate the photo metric method I shall first summarize Hartmann's analysis, and then I shall present a range calculation based upon the simplified analysis. Hartmann pointed out that the upper bright side of the object appears brighter than the side of the nearby tank and that the elliptical shaded bottom is the brightest shadow in either photo. He attributed the excessive brightness of the bottom of the UO to atmospheric brightening. (NOTE: the contrast between the brightness of an object and that of the sky, assumed to be brighter than the object, approaches zero as the distance to the object increases, i.e., the apparent brightness of the object increases until it matches that of the sky at a great distance.) By definition the intrinsic brightness of an object is the brightness measured from a very short distance. By assuming the intrinsic brightness of the bottom of the UO was the same as that of the shaded bottom of the tank, and using the formula which attributes increased brightness to atmospheric effects over a long distance (Equation 5 in Table 1), he estimated that the range to the object was about 1.3 km, based on his estimate of b (0.289/km). (NOTE: all his brightness' were normalized to the horizon brightness so Bh = 1 in his version of Eq. 5). He then pointed out that if the UO were nearby under the wires, the bottom must have been very white, even brighter than the shaded white surface of the distant house which appears near the bottom of the photos. I have modified Hartmann's analysis by assuming at the outset that the bottom is as bright a surface as would have been available to the photographers (white paper) without being itself a source of light. (Note: the witnesses described the bottom as being copper colored or darker than white. Use of a darker bottom in the following analysis would result in a greater calculated distance.) This assumption has led me to compare the relative brightness of the bottom of the UO with the relative brightness of a hypothetical nearby horizontal shaded white surface as seen from below. The brightness that a horizontal white surface seen from below would have had under the circumstances of the photo has been estimated from the relative brightness of the vertical shaded white surface of the distant house (and also from the shaded white surface of the wall nearby Trent house) and from the brightness ratio Rb in table 1. If, in a naive way, the intrinsic brightness of a vertical white shaded surface (house wall) is equated to the intrinsic brightness of a horizontal white surface as seen from below (whereas the horizontal surface actually may be somewhat less than half as bright), that is, if Rb is set equal to 1 , and if the effects of veiling glare are ignored (G in Eq. 3 is set equal to zero), then the range of the UO can be calculated from Eq. 5 using as B(r=O) the brightness of a nearby vertical shaded white surface (the Hartmann method). The shaded wall of the distant house was used by Hartmann to estimate the relative brightness of a hypothetical nearby vertical surface (see the illustration labeled "TrntWhteHouse.gif) by correcting the relative brightness of the wall for atmospheric brightening using Eq. 4 (Table I). If the object were hanging under the wires then, by this (naive) reasoning, the brightness of the hypothetical nearby vertical surface should equal the brightness of the bottom of the UO, and Eq. 5 would yield r = 0. Such a result would be consistent with the hoax hypothesis. However, Hartmann found that the brightness of the image of the bottom of the UO was actually greater than the brightness of his hypothetical nearby vertical surface. Hartmann's calculation is duplicated in Table II except that I have used b = 0.2/km rather than 0.289/km. The table lists the pertinent relative "brightness'," Ei (uncorrected for glare), the correction of the distant house wall "brightness" for atmospheric brightening, and the range calculated from Eq. 5. The calculated range, 1.4 km., agrees with Hartmann's result and is clearly inconsistent with the nearby UO hypothesis.

Table II

Modified Hartmann method Assume the bottom is white and use gamma = 0.6 Ehorizon = 0039 (+/-) 0.002; Edistant house shadow = 0.018 (+/-) 0.001; Euo = 0.022 (+/-) 0.001 ; Esky = 0.070 (+/-) 0.001. Atmospheric Extinction Coefficient (based on visibility range): b = 0.2/km Distance to White House: 0.36 km Now use the measured brightness of the distant shaded vertical white wall to obtain the brightness of a hypothetical nearby white shaded surface by removing the atmospheric brightening (Eq. 4 of Table I): 0.039 + (0.018 - 0.039)e^(0.2x0.36) = 0.0164 (+ /-)0.001. Now assume 0.0164 to be the intrinsic "brightness" of the bottom of the UO and calculate its range: r = (1/(0.2/km.) x Ln[(0.0164 - 0.039)/(0.022 - 0.039)] = 1.42 (+/-) 0.6 kilometer.

(Resume text) Accurate calculations of object brightness' require corrections for veiling glare, as pointed out by Sheaffer. Since, in the first approximation, the phenomenon (scattering) which produces veiling glare simply adds light (from the brighter areas) to the darker areas, it is only necessary to subtract the amount of glare from an image to find the object brightness (Eq. 3). The problem is to find the amount of glare on an image. After some considerable thought and experimentation I found a way to estimate the glare on the Trent photos using laboratory simulations. In order to estimate amounts of glare on the images of interest in these photos, I have conducted laboratory experiments with several camera lenses, one of which was comparable (but not identical) to the lens on the camera that took the photos. I simulated the brightness distribution of the sky with a large screen which was illuminated from behind. Below the simulated "horizon" (the bottom of the bright area) there were no sources of light. I then measured brightness distributions in the bright and dark areas when there were varying amounts of grease on the lens. (Measurements were made with a linear photo detector and a small aperture that could be moved about in the focal plane of the lens.) The light that "turned up" in the dark areas was the glare light, G, which would have appeared on any images that might have been present in the dark areas (although no such images were present in the laboratory simulation). Values of G were proportional to the "sky" brightness, Bs, so that at each point on the image plane a glare index, gi, could be defined as gi = Gi/Bs. For the present work it was important to have values of gi for images 2 degrees below the horizon (the angle of the image of the distant house) and for images at (or just below) the horizon, when the glare index for an image of the angular size and shape of the elliptical bottom of the UO was a particular value. I carried out the experiments as follows. First I placed an ellipse of dark paper with the angular size of the UO in photo 1 on the bright screen. I then put some dirt or grease on the lens in order to increase the glare and measured the amount of glare light at the center of the image of the dark ellipse. This was defined as the "glare index" for a particular amount of grease/dirt. I also measured brightness variations in simulations of other images in the photos with the same grease/dirt. Of particular interest was the image of the large telephone pole in Photo 1. Measurements of the brightness variation of the image of the pole showed that below the horizon the image was of a nearly constant brightness, and that above the horizon the image increased in brightness as the angular altitude increased. I attributed this increase in brightness to an increase in the glare light added to the pole image (thus implicitly assuming that the brightness of the pole was intrinsically constant from its bottom to its top; however, I have observed that, probably because of weathering, the brightness of many wooden telephone poles increases with height along the pole; the result, in these calculations, of my assumption of constant intrinsic brightness is an overestimate of the actual glare and therefore an underestimate of the calculated distance to the UO). I simulated the pole image in the laboratory setup by placing strip of black paper of the same angular width and height as the pole on the bright screen above the simulated horizon. I measured the brightness of the image of the simulated pole on the focal plane of the simulated camera. Since the paper was black and the only illumination was from behind the paper the measurement would have given zero brightness if there had been no glare.

However, by changing the amount of grease on the lens, I was able to adjust the brightness of the simulated pole image; the more grease the brighter then simulated image. Thus, a distribution of values of gp along the pole image (glare on the pole image vs height) was measured for each amount of grease. Then the laboratory determined values of gp vs. altitude alog the pole were multiplied by a value of Bs determined from the sky brightness of Photo 1 to obtain the amounts of glare, Gp, that would have been added to the actual pole image in Photo 1. By adjusting the amount of grease on the lens, I was able to obtain a set of values of gp, that is, a graph of gp vs height, which, when multiplied by the sky brightness of Photo 1, yielded a theoretical brightness increase that was close to the increase in brightness of the actual pole image, that is, the graph of pole image brightness vs height. (See Figure A16 in the Appendix.) In other words, I was able to approximately fit the laboratory data to the measured increase in pole brightness. I then measured the glare index (the glare in the simulated UO image as described above) for the same amount of grease on the lens. I also measured the glare below the horizon at the angular distance of the distant house below the horizon. (Briefly, I used the pole glare distribution in the photo to determine the amount of grease in a simulation and then measured the UO glare in the simulation and calculated from that the glare in the photo image of the UO.) The amount of grease which yielded the most correct set of values of gp for the pole image also yielded guo = 0.12 for the image of the UO, and the other values of gi given in Table I. These values have been used in the following analysis, even though other measurements have strongly suggested that guo = 0.12 is probably too high. (Typical values of veiling glare in an image the angular size of the UO in Photo 1 would be less than 0.09.) Moreover, measurements of the brightness variations in certain other images in the photos suggest that guo = 0.12 is be too high (0.07 might be better). More details of the result of the glare experiments are presented in the Appendix to this paper. The effect of the inclusion of veiling glare is readily apparent when it is applied to the image illuminances, Ei, shown in Table II. For example, the horizon brightness is found to be Eh - Gh = Eh - ghBs (where, from Table I, gh= 0.05) = 0.039 - (005)(007) = 00355. Similar calculations yield the relative brightness' given in Table III. Note that in this first order theory the small loss of brightness from the bright areas is ignored, so Esky = Bsky.


Relative Object Brightness' with Esky = Bsky = 0.07: Bhorizon = Eh - Gh = Eh - ghBs = 0.039 - (0.05)(0.07) = 0.0355; Bdistant house shadow = 0.018 - (0.035)(0.07) = 0.0155. After atmospheric distance correction, Bnearby vertical shadow = 0.014; Buo = 0.0136 CALCULATED DISTANCE = "ZERO"

From Table III one can observe that a major effect of the inclusion of veiling glare is to make the brightness of the bottom of the UO equal to (or slightly less than) the brightness of a vertical shaded white surface. Naive use of Eq. 5 with B(r=0) = 0.014 and B(r) = Buo = 0.0136 would yield a range of zero (negative numbers are not allowed), so Sheaffer's conjecture that the apparent distance of the UO could be explained by veiling glare has merit. (NOTE: If guo were 0.07 and the other values of gi were proportionately lower, the range would not be zero but about 400 meters.) If there were no other correction factors this would be the end of the analysis. However, field measurements with a spot photometer have shown that it is incorrect to equate the brightness of a shaded vertical white wall with the brightness of a horizontal surface as seen from below. A shaded vertical wall which is on the order of ten feet above the ground and which is not closely surrounded by trees is illuminated by direct sky light as well as by light reflected from the ground. On the other hand, the horizontal bottom surface of a body which is less than ten feet above the ground is illuminated only by light reflected from the ground. Since the ground reflectivity is not particularly high (15-30% for grassy ground), one would expect the illumination on the horizontal (or nearly horizontal) bottom of an object to be less than that on the vertical surface. Thus, from a priori reasoning one should not equate the relative intrinsic brightness of a white shaded vertical surface to the relative intrinsic brightness of a white shaded horizontal surface seen from below. To provide a quantitative estimate of the ratio of brightness of a vertical surface to a horizontal surface, Rg, (see Table I) I made field measurements with a calibrated panchromatic 3.5 degree field of view spot photometer. I measured the brightness of the white wall of a house when the wall was shaded by the eaves and when the sun angle and sky conditions were similar to those at the time of the UO photos. Under the same environmental conditions, I measured the brightness of an opaque white paper surface held about seven feet above the ground. Many measurements of the surfaces were made with the result that the house wall was found to be 1.5 to 2 "stops" (photographic terminology) brighter than the bottom of the white surface, depending upon the exact nature of the ground (grassy, dirt, etc.) and upon the sky brightness distribution. Allowing a 1/4 stop possible error in the readings, the brightness ratio lay within the range 2^1.25 = 2.4 to 2^2.5 = 4.7 (see Table I). To be "conservative" I have used Rb = 2.4 in these calculations. (NOTE: This ratio was measured with panchromatic meter. If a filter had been used to simulate the orthochromatic Verichrome spectral response, the measured ratio might have been as much as 30% greater.) The measured brightness of the bottom of the horizontal surface did not change noticeably when the surface was tilted by as much as 20 degrees. From Table III the relative brightness of a nearby vertical white shaded surface was 0.014. From the field measurements this value should be divided by a number at least as great as 2.4 to obtain the relative brightness of a nearby horizontal white shaded surface, which is assumed to be the brightness of the bottom of the nearby UO. With Bh = 0.0355, Buo = B(r=0) = 0.0136 (see Table III), with B(nearby horizontal surface viewed from below) =0.014/24 = 0.0058, and with b = 0.2 (Table I) the range calculation yields about 1.5 km. Variations in the calculated range with variations in the parameters of the range equation are as follows: (a) the calculated range increases as the glare decreases; for example, if there were no glare the range would be calculated from Euo = Buo = B(r) = 0.022, Bh = 0.039, (from Table II) B(r=0) = 0.0164 /2.4 = 0.0068 and Eq. 5 would yield about 3.2 km.; (b) the calculated range increases with increases in the ratio Rb; for example, if Rb = 3, using the brightness' in Table III and B(r=0) = 0.014/3 = 0.00467, Eq. 5 yields arrange of about 1.7 km.; (c) the calculated range increases with gamma as indicated in Table IV.


gamma Eo k Buo/B(r=0) Range Diameter*
Thickness** 1.0 0.00436 0.0025 2.75 2.4 km 68 m 9.6 m
0.7 0.0063 0.0017 2.60 1.5 44 6.2
0.6 0.0068 0.00055 2.34 1.52 43 6.1
0.5# 0.0076 0.0002 2.18 1.06 30 4.2
0.3# 0.0093 0.00009 <1 0.0 0 0 IF
Nearby under the overhead wires: 5m 14 cm 2 cm

*angular diameter in photo 1 is 0.0283 radians (in photo 2, 0.0248 radians) based on the assumed focal length of 103 mm which is the approximate value for the cameras of the type used (assumed to be a Kodak Monitor or Vigilant) **angular thickness excluding "UO pole" in photo 2 is 0.004 rad. based on the 103 mm focal length #curves for these values of gamma were synthesized by extrapolation from published curves which show gamma in the range 0.6 to 1.0. These results are included for completeness. However there is no evidence at all that the gamma would have been lower than 0.6. In fact, it is more likely that gamma was greater than 0.6. See note 11.

Table IV also contains a list of ratios of the brightness' of the bottom of the UO to the expected brightness' if the object were close and had a white bottom (the brightness' of a nearby horizontal shaded white surface). Since the expected relative brightness' were calculated using a white surface (the distant house wall or the nearby house wall - see Appendix) as a reference, the ratios imply that the bottom of the UO was "brighter than white" whenever reasonable values of gamma, i.e., gamma > 0.6, were used in the calculation. White surfaces reflect most of the incident light (both white paint and white paper have reflectivity's in the range(6) of 60-80%). If we assume, for example, that the white paint on the distant (or nearby) house reflected only 60% of the incident light, then a brightness ratio greater than 1/0.6 = 1.67 would imply that, if the UO were a small nearby model. then its bottom was a source of light (it could not reflect more light than 100% of what was incident on it; 1.67 X 60% = 100%). As shown in Table IV, for reasonable values of gamma the calculated ratio Buo/B(r=0) exceeds 1.67 by a considerable margin. Actually 1.67 is an upper bound on the ratio if the distant house reflected 60% of the light because any white surface which the witnesses would have available to place on the bottom of their hypothetical nearby UO would have a reflectivity lower than 100%. If the bottom were white paper, the reflectivity would be, at maximum, about 80%, in which case the maximum expected ratio of the brightness of the bottom to the expected brightness would be 0.8/0.6 = 1.33. (NOTE: If the white painted surface were known or assumed to be dirty, the reflectivity would be decreased and the brightness ratio increased. For example, to obtain the brightness ratio 2.34 which is obtained when gamma = 0.6 (see Table IV) with 80% reflective paper on the bottom of the object, the distant wall reflectivity would have to be as low as 0.8/2.34 = 0.34. On the other hand, measurements of the image density of the shaded wall of the nearby Trent house, after correction for veiling glare, yielded an upper bound on the relative brightness of a shaded white vertical surface of 0.0171, which is only 0.0031 units higher than the value 0.014 in Table III. This house was reportedly painted in the year previous to the sighting date, so the paint must have approached its maximum reflectivity. Use of this value, after dividing by 2.4, with the other brightness' in Table III yields a distance of about 1.3 km, and a brightness ratio of 1.9, which is still larger than 1.67 and 1.33.) The implication of the brightness ratios for reasonable values of gamma is that the bottom of the UO was itself a source of light if it were nearby (e.g., within 20 feet under the wires). To be a source of light it would have to have (a) contained a source of light, or (b) been made of translucent materials so that light could filter from the sky above through the bottom surface. Requirement (a) is considered beyond the capabilities of the photographer because a very small illumination apparatus would have been required and because the illumination mechanism, a small light bulb, would have produced a very uneven distribution of light over the bottom surface in contradiction to the fact that there are no "hot spots" of brightness in the image of the bottom (see TrntDensUO1.gif and TrntDensUO2.gif). Requirement (b) above is considered a possibility if the upper body of the UO were a translucent material. (7) Any holes through the upper body would allow direct sunlight through, and these would cause brightness "hot spots" on the bottom surface. On the other hand, a translucent or transparent material such as glass would probably not "look" the same in a side view as the object appears in photo 2 (apparently shiny like the nearby tank, but not a mirror - like specular surface). Any hypothetical translucent UO must appear, in a side view, as bright and "shiny" as does the object in photo 2 (also, it must be shown that an appropriately translucent or transparent material in the proper shape was available to the photographers). Independent tests of the density distributions of the images of the object and its surround and of the density distributions of nearby objects in the photos have been made (8). Color contouring (using a computer to assign specific colors to specific density ranges) has shown that (a) the "back" end (left hand end in photo 1) of the object appears slightly non circular (actually it comes to a slight or shallow point), and (b) the edges of the image are rough or jagged (the color contour boundaries are not smooth curves), whereas the edges of the images of nearby objects, and particularly of the wires "above" the UO, are relatively smooth. Observation (b) may be related to an atmospheric effect on images: the distortion of an image increases quite rapidly as the object distance increases up to about a kilometer, and then the distortion increases very slowly or not at all with further increases in range. The atmospheric conditions assumed for a hoax (morning, no wind) may have been conducive to the production of image distortion. (9) Thus, the jaggedness of the edge of the UO image may be an indication that it was more than several hundred meters away, thus contradicting the hoax hypothesis. (NOTE ADDED IN THE YEAR 2000: this was considered a theoretical possibility 25 years ago. Now I consider it unlikely that any edge fuzziness could be directly related to distance.)

In conclusion, to echo Hartmann, the simplest interpretation of these photos is that they, indeed, show a distant object. However, simplicity does not necessarily imply truth. Further research will be necessary to resolve this case once and for all.

NOTE: APPENDIX A provides further data and analysis regarding the brightness of a white vertical surface and also provides data to support the veiling glare analysis presented in the text.

The following images also provide further information: TrntDensUO1.gif TrntDensUO2.gif TrntGamma6Curve TrntWhteHouse.gif TrntGammaCurves.gif

Bibliography and Footnotes

1. Scientific Study of Unidentified Flying Objects, E.U. Condon, Ed. (Bantam, 1969, pg. 396)

2. P.J. Klass, UFO's Explained, Random House, New York (1974)

3. R. Sheaffer, private communication

4. C. Grover, private communication (Grover was a Navy professional photographer)

5. Note that the range increases with assumed darkness of the bottom of the UO. If the bottom were black, B(r,O) = 0, the range would be about 2.4 km with gamma = 0.6

6. Handbook of Chemistry and Physics, Forty first Edition, (Chemical Rubber Publishing Company, Cleveland, Ohio 1960)

7. Measurements have been made of the brightness' of the bottoms of several model UO's made of uniformly translucent materials. The models were oriented with respect to the sun in the same way as it would have been if the UO in photo 1 were a model lit by the morning sun. The brightness of the bottom of each model was measured as a function of position, with the "front" part being that part closest to the sun (in photo 1 the front part of the elliptical image is at the right hand side). The front part of the bottom was found to be from 20% to 40% brighter than the back part for each model. However, the brightness variation of the image of the bottom of the UO in photo 1 is only (+/-)5% with the back somewhat brighter than the front. These experiments, and the comparison with the image of the UO, suggest that if the UO were a nearby model it was not made of a uniformly translucent material.

8. W. Spaulding, GSW Inc., Phoenix, Arizona, private communication. An electron microscope test of the negatives has shown that the grain structure is consistent with that of known Verichrome film, but not with Plus X.

9.However, experiments (e.g. R. S. Laurence and J. W. Strohbehn, A Survey of Clear Air Propagation Effects Relevant to Optical Communications, Proc. IEEE 58, 1523 (1970)) have shown that there is a period of time just after sunrise when the turbulence is quite low. The pictures may have been taken during this period. If this were so, even a very small amount of atmospheric edge distortion would correspond to a rather large distance to the object.

10. I thank Charles Grover, William Hartmann, and Robert Sheaffer for instructive comments on earlier versions of this paper. I also thank NICAP for free access to their files and for assistance in obtaining the negatives.

11. Note added in proof: the fog density of the negatives is consistent with the range of values expected when gamma = 0.5 to 0.6, but is larger than expected when gamma = 0.3. The brightness of the illuminated part of the distant white wall and the brightness of the shaded part of the same wall have been calculated for gamma = 0.3, 0.4, and O.6. The calculated brightness ratios, (illuminated/shaded), are, respectively, 10(+/-)2, 3(+/-)0.5, and 2(+/-)0.2. A field measurement of the same ratio under conditions similar to those when the pictures were taken yielded 1.5 to 2. Thus both the fog density measurement and this brightness ratio measurement indicate that gamma is greater than 0.3 and perhaps even greater than 0.6.

Post publication Notes: a) Experiments with a Kodak Vigilant lens of 153 mm focal length yielded the same or lower values of veiling glare than assumed in this paper. b) Shadows on a surface that faces the east when the sun was in the west have been observed when a cumulous cloud was in the sky to the east of the surface.

NOTE 1 ADDED IN APRIL, 2000: A larger paper in which I discussed the rest of the story, including cloud shadows and verbal testimony, was presented at the second conference of the Center for UFO Studies which occurred in 1981. That paper was eventually published by the Center in the Spectrum of UFO Research in 1988. See "The McMinnville Photos," the companion paper to this one.

NOTE 2 ADDED IN APRIL, 2000: A very recent re-investigation of the Trent sighting (ca. 1999) has demonstrated that the camera used was probably not a Kodak type but rather a "Roamer 1" built by Universal Camera Corp. of New York for several year starting in 1948. It was a very inexpensive camera with a minimum f stop of f/11 and a fixed shutter time of 1/50 sec. The focal length was rated at 100 mm. The camera was designed to be held in the "landscape" orientation (long dimension horizontal) and the direction finder was to be viewed from above, that is, the the operator held the camera at stomach or chest level and looked downward into the viewfinder to point the camera at the scene before taking the photo. The fact that the focal length of the camera was 100 mm rather than the 103 mm assumed here has no effect on the photo metric calculations in this paper. Use of this shorter focal length does make the calculated size of the UO 3% larger, e.g., in Table IV all the diameters and thickness' should be multiplied by 1.03. I thank Brad Sparks, Joel Carpenter and David Silver (President of the International Photographic Historical Association) for successfully identifying the camera that was actually used.


To the paper "On the Possibility that the McMinnville Photos Show a Distant Unidentified Object (UO)"

by Bruce Maccabee (c) B. Maccabee, 2000

(This was written in 1976-1977 - exact date not recorded - but was not included in the previous publication. It is published here for the first time. There have been some clarifying comments added in April, 2000.)

This appendix is provided to supply certain supplemental information that will prove useful in evaluating the analysis presented in the main text, in particular the analysis related to the determination of the amount and effects of veiling glare. The information is provided in a series of figures, each of which is described below . Further information is available from the author.

In the main text the relative brightness of a vertical , white shaded surface was estimated from the image brightness of the shadow on the distant house wall. There has been some question an to whether or not the wall was "truly" white. Therefore I have made another estimate based upon the image brightness of the nearby (Trent) house that appears at the right hand side of photo 2. This house was (according to Mrs. Trent in 1975) painted white only about a year before the pictures were taken. An image of the corner of the house just below the eaves appears in the second UO photo at the right hand side. (The corresponding image in the first UO photo was cut off the original negative sometime after publication in the Telephone Register newspaper, which shows the corner of the house in both photos.) Figure A 1 illustrates the calculation of the brightness of a vertical white surface from the brightness of the image of the western corner of the south wall of the nearby house. Although the veiling glare correction is larger (because it is immediately adjacent to the sky), there is no atmospheric brightening correction. The brightness of a horizontal, shaded white surface based on this nearby house image differs only slightly from the value obtained using the image of the distant house. As pointed out in the main text, certain evidence suggests that 12% may be an upper bound on the glare index (defined as the brightness of a perfectly intrinsically black UO image divided by the adjacent sky brightness; see the text and see below). The evidence for this is presented in Figures A3 -A5. These figures contain data on the relative brightness' of images (garage roof, wall) which, if there were no veiling glare, would have (approximately) constant intrinsic brightness (because of constant reflectivity) over angular distances of at least several degrees away from the object/sky boundary. Figure A2 illustrates the variation of the brightness of the garage roof in each picture. The angular distances are measured along the scan directions indicated by the arrows. Figures A3 and A4 illustrate the brightness variation of the garage wall at the level of the rafter ends, and Fig. A5 illustrates the brightness variation of the shadow that is just under the edge of the roof and Just above the rafter ends. The variation in brightness is mostly caused by veiling glare...light from the adjacent sky is scattered by camera optics into the image of the darker roof or wall.


The southwest corner of the wall and eaves of the nearby (Trent) house appears at the right hand side of photo 2. This house was reportedly painted white within the year before the pictures were taken. Since the sun wa slightly north of west (or if in the morning, slightly north of due east at an angular elevation of about 25 degrees), and since the roof had a small eave, the south wall was shaded from the direct sun. It was, however, illuminated by skylight and ground reflected light. Thus the intrinsic brightness of the wall should be the same (or perhaps slightly greater, since there was no eave shading it) as that of the shaded part of the wall of the distant white house. From the density measurements and transfer curve:

Ewall image = 0.021 (1/2) degree from the edge

Esky image = 0.065 adjacent to the wall

When the glare index is 125, a dark area next to a uniformly bright area has a glare of about 6% at a distance of 0.5 degrees into the dark area image. The sky brightness is not uniformly bright. Nevertheless, a good approximation to the intrinsic relative brightness of the wall is:

Bvertical,white,shaded surface = Ewall image - Gwall image = Ewall image - g Bsky image = 0.21 - (0.06) (0.06565) = 0.0171

This value is somewhat larger, but in good agreement with the value, 0.0014, calculated from measurement of the brightness of the distant house shadow after correction for glare and atmospheric effects. Using this value divided by 2.4 in the range calculation yields 1.3km.

The image brightness' illustrated in all these figures would be roughly constant if there were no veiling glare. However, since these images are adjacent to the image of the bright sky, and since there was VG, the brightness' increase with decreasing angular distance to the image of 'the sky. Included in these figures are image brightness variations predicted from laboratory simulation data on the glare light distribution for various values of glare index, guo, which is the glare in an ellipse that simulates the UFO image in photo 1. Figures A3 and A4 show that the brightness variation of the garage wall is more consistent with guo= 7% than with guo = 12 % or 20 %. Figures A6-A13 contain glare curves obtained in laboratory simulations of the luminance distribution in photos 1 and 2. Figures A6-A8 illustrate the glare brightness variation along the image of a synthesized "telephone pole" for various values of glare index. These pole simulation curves were used to predict the brightness variation of the image of the pole in photo 1. The predicted variations, illustrated in Fig. A16, were obtained by fitting the laboratory glare curves to the measured Image brightness at the horizon using the formula Eimage= Bintrinsic + Gimage = Bi + g(x,y) Bs , where Bi, the intrinsic brightness of the object is an adjustable constant (it is constant for a particular graph of brightness versus position on the pole), Bs is the brightness of the sky about 10 degrees above the horizon and g(x,y) is the glare distribution for a given 'sky' luminance distribution and for a given image shape and size as a function of x-y coordinates in the film plane. If y represents angular displacement in the vertical direction, then, along the vertical pole image, Epole image = Bpole + g(y)Bs. The function g(y) for the three glare index values illustrated was, obtained from Figures A6-A8. (NOTE: This formulation of the quantitative estimation of veiling glare has a theoretical basis in the observed fact that most of the glare in a image comes from the light sources immediately adjacent to the image, such as from the sky within a few degrees of the UO, for example. In other words, the scattering which produces the glare tends to be a small angle or "forward scattering" phenomenon. The more grease there is on a lens the larger this scattering angle becomes. For typical lenses experiments suggest the angle is a few degrees.) For example, for guo = 20 %, curve A shown in Fig. A16 is given by Eimage = 0.00151+ g(y)(0.06), where g(y) is the variation of glare with height (y) in Figure A6 (i.e. the graph in Figure A6) and 0.06 is the sky brightness above the pole. To obtain curve 3 in Fig. A16, I have calculated the expected image brightness from Emage = 0.00259 + g(y )(O.06), where g(y) is the variation along the simulated pole in Figure A7. Also in Figure A16 is the expected brightness variation along the pole image when the glare index is 7%. Clearly the best fit to the data (dots) is for 12%. As can be seen in Fig. A16, all the "theoretical" curves fit the data at the horizon. However, none of them fit the data below the horizon; the photo data indicate a very constant image brightness below 1 degree below the horizon. Thus the data below the horizon are consistent with a glare curve for a glare index even lower than 7%. If the glare index were that low, the increased brightness of the pole image above the horizon in the photo would have to be explained as a combination of glare and intrinsic brightness increase with height along the pole. I have noticed that creosoted poles often become lighter colored near the top as a result of weathering away of the creosote, so it is possible that some of the increased brightness of the pole image with altitude was due to an actual increase in brightens of the pole, in which case the glare index should be lower than 12%. Unfortunately there is now no way of measuring the actual brightness variations, if any, of the telephone pole. Figures A8-A10 illustrate the laboratory measurements of VG in a large dark image adjacent to a large bright area, which is an approximate simulation of the garage roof. These curves were used to Calculate the glare curves in Fig. A2. Figures A11, A12, and A13 illustrate the glare variations obtained in laboratory synthesis of the image of the garage when scanned along the rafter ends. These curves labeled "B" in the figures were used to calculate the glare curves illustrated in Figures A3-A5. Figures A14 and A15 illustrate the variation of VG with the angular size of an image for various types of simple geometric images silhouetted against a large, constant brightness field. The VG increases as the image size shrinks, although for sizes much smaller than 0.1 degree the VG is expected to remain nearly constant. As an image increases in size the glare shrinks, but it does not go to zero since some light is always scattered. The glare curve can be roughly divided into "short range" and "long range" regions. The short range glare decreases rapidly with increasing image size. When a lens is clean the short range glare is evident for images smaller than a degree in angular extension (depending upon the shape ), as illustrated in Fig. A14. However, when a lens is very dirty the "short range" glare may extend for many degrees, as illustrated in Fig. A15. Also illustrated in Fig. A15 is the observation that an increase in dirt or grease on the lens does not substantially change the functional form of the glare for angular sizes less than 1 degree. Note that the effects of the "short range" glare are also evident in the separation between curves A and B in Figures A11 - A 13. Figures A 14 and A 15 also illustrate the previously mentioned fact that the glare in an ellipse comparable to the image of the bottom of the UO (22 degree aspect ellipse with a major axis length of 1.6 degree) is about the same as in a 1 degree disc.

CUFOS Paper Part 2

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