An Investigation of the McMinnville UFO Photographs

Robert Sheaffer

Unpublished paper written November, 1969; summarized by Philip J. Klass in
chapter 15 of UFOs Explained (Random House, 1974).

Electronic version (with revisions) compiled April, 1999. Some additions compiled Nov. 5, 1999. Links revised March 30, 2002.


On June 8, 1950 the McMinnville, Oregon Telephone Register displayed on its front pages two photographs of a strange object that were reportedly taken on May 11 of that year by farmer Paul Trent. He and his wife said they saw what was described as a metallic disk-shaped flying object. Two photographs of the object were reportedly taken before it disappeared into the distance.

The witnesses contend that the two photographs were taken after sunset, at about 7:30 PM, and that the sky was overcast at the time of the sighting. Further investigation has shown these statements to be highly questionable.

This set of photographs was investigated by the University of Colorado under contract to the United States Air Force, under the direction of Dr. Edward U. Condon. This incident is Case 46, the first of the photographic case studies., investigated by Dr. William K. Hartmann.[1] He concluded, primarily on the basis of densitometric measurements of the original negative, that the witness’ story is consistent with information obtained from the photographs. A further investigation reveals that this is not the case. It is assumed that the reader is familiar with Hartmann’s report in The Scientific Study of Unidentified Flying Objects (commonly called the "Condon Report").

Weather Conditions at the Time of the Sighting

The witnesses contend that the sky was overcast, with a ceiling at about 5000 feet. Hartmann suggests that this is confirmed by the photographs. However, records kept by the U.S. Department of Commerce Weather Bureau indicate otherwise.

The Weather Bureau station at McMinnville, Oregon has a record of surface weather conditions from 7:00 AM to 7:00 PM daily. Their last observations on May 11, at 6:00 and 7:00 PM, both show perfectly clear skies. During the entire day only a few clouds are recorded from 9:00 AM to 5:00 PM, the other times the sky being clear. Partly cloudy conditions prevail the next morning. At no time is there any ceiling recorded between 9:00 AM May 10 and 10:00 AM May 12.

The reason for the discrepancy between the witness’ story and the weather records is not known. This is not the only discrepancy found for which no motive can be suggested.

The Illumination in the Photographs

The witnesses contend that the photographs were taken of an overcast sky after sunset. Hartmann has written that the photographs confirm these conditions, but acknowledges that the shadows in them represent a possible discrepancy.[2]

Distinct, sharply-bounded shadows are seen on the garage wall, the metallic tank, a distant barn, and a post in the foreground. They are all consistent in direction, indicating a light source that is approximately due east. All the illumination in the photographs is consistent with a daylight photograph taken when the sun is in the east. Measurement of the relative brightness of the two sides of the UFO in Plate 23 [the first Trent photo] are somewhat uncertain, and do not yield reliable comparisons. On Plate 24 [the second Trent photo] they are measured more reliably, and Hartmann found that the right side is definitely brighter than the left, indicating illumination from the east. The metallic tank in the foreground, with a somewhat diffuse reflecting surface, shows brilliant highlights on its broad face, suggesting that the sun was shining directly on its dull aluminum-painted surface.

Although the eastward illumination could conceivably be explained by brilliantly-lighted clouds, if clouds exist, however it is very difficult to account for the distinct shadows and highlights on the tank unless we assume that the sun is shining in the east.

The Maximum Size of the Illuminating Body

To explain the existence of the shadows seen on the garage wall and elsewhere, there must be illumination from the east. Although we have no a priori knowledge of the angular size or surface brightness of this body, careful analysis of the shadows can yield reliable order-of-magnitude estimates.

The shadows on the garage are distinctly bounded umbral shadows. The entire illuminating body is invisible from every point within them. If this were not true, the shadows would have no distinct edges, and consequently, no measurable size. This can be most readily visualized by watching the progression of an eclipse of the moon. The eye cannot detect the early phases of the penumbra, or partial shadow. The umbra of the earth’s shadow, however, is distinctly bounded and of uniform darkness, as are the shadows on the garage.

The size of this shadow allows us to estimate the maximum angular size of the illuminating body. A point source of light at infinity casts shadows that are identical in size with the object causing them. As the angular size of the light source increases, the size of the umbral shadow decreases. Its radius is given by

u = a - 2d Tan (theta)

where u is the size of the umbral shadow

a is the size of the object casting the shadow

d is the distance from the object to the shadow

theta is the angular radius of the illuminating body

The umbral shadow disappears when u is less than or equal to zero.

The shadows on the board are not measurably smaller than the boards casting them. The measurements made on an 8 x 10 inch print of Plate 24 are as follows:

(starting from left) Eave No. 1 width 2 mm; shadow not measurably smaller

Eave No. 2. width 1.6 mm; shadow not measurably smaller

Eave No. 3 could not be used for a measurement of this sort, since its shadow overlaps with that of an adjacent supporting brace.

Although the exact sizes of the boards and distances to the wall are not known, reasonable estimates can be made. The boards do not appear to be those commonly known in the U.S. as "2 by 4’s," whose actual dimensions are approximately 1.5 by 3.6 inches, for a width-to-height ratio of 0.41. The measured width-to-height ratio of the board protruding the most in the photos is approximately 0.27, which matches quite well with the assumption that the boards are "2 by 6" (actual dimensions approx. 1.5 by 5.5). We will assume this to be correct. The distance from the boards casting the shadow to the wall is harder to estimate. Taking measurements of the "2 by 6’s" supporting the roof of the house in which I live, they protrude 20 inches from the flat surface of the wall. We will assume that these same measurements apply to the Trents’ garage; they surely cannot be greatly different from that.

It is probable that a 10 percent reduction in the width of the shadow would have been measurable on the prints. The following diagram illustrates the geometry:

The angle abc above represents the radius of the illuminating body. Assuming these measurements, it is the arctangent of .004, which is 0.229 degrees. This matches extremely well with the known average apparent radius of the sun, which is approximately .25 degrees. Even an undetected reduction of 20% in the size of the shadows in the photo, which is highly unlikely, allows the radius of the illuminating body to be no larger than 0.46 degrees (diameter 0.92).

The maximum possible diameter of the illuminating body is thus shown to be less than one degree, and is probably closer to one-half degree. The area of a one-degree circle is less than 0.025% (1/4,000) that of the quarter sky facing the garage wall.

A bright cloud in full sunlight is only about 10 times the surface brightness of the sky surrounding it.[3] Furthermore, during and after sunset, the sunlight in the landscape has traveled a very long path through the atmosphere, and has thus been very evenly scattered and diffused. According to the astronomer M. Minnaert, by about 10 minutes after sunset, the sky and landscape in the east is dull and of uniform hue. Even a half-hour before sunset, clouds in the east assume a dull red color.[4] To attribute the illumination in the photos to a bright cloud, or to a bright hole in the cloud cover, especially after sunset, would require a surface brightness of these remarkable clouds on the order of magnitude of thousands of times that of the surrounding sky, which is inconceivable. It is particularly implausible that such illumination could exist around the time of sunset.

Conclusion: Because of the small maximum angular size of the illumination body and its intense brilliance, there can be no doubt whatsoever that the shadows in the Trent photographs are cast directly by the sun.

The Time of Day

The existence of the shadows in the photographs allows us to determine the time of day that the photos were taken, provided that we can accurately determine the solar azimuth. This can be done on plate 23. It can be seen that the shadow of the eave farthest right, on the very edge of the garage, has its shadow directly below it, on the extreme edge of the garage. This indicates that the sun is within a few degrees of being due east, as Hartmann’s map shows that the wall faces directly east.[5] A simple astronomical calculation shows that in McMinnville[6] on May 11 the sun is in this position at about 8:20 AM, Pacific Daylight Time. Its elevation is then approximately 25 degrees.

Distribution of Sky Brightness

Densitometric measurements indicate that the sky is brighter toward the west than it is toward the north, at the right of the photographs. Hartmann cites this as evidence that the photographs were taken after sunset.[7]

This distribution of sky brightness is also consistent with a daylight photo taken when the sun is due east. The darkest point in the daylight sky lies, when the sun is fairly low, roughly 90 degrees from the sun. Passing through this point is a ‘line of darkness’ dividing the sky into two regions : the bright one around the sun, and the bright region opposite it. The atmospheric scattering 90 degrees from the sun can be as little as about half as much as at the anti-solar point (180 degrees from the sun).[8]

Hartmann’s measurements of sky brightness are exactly what we should expect in the morning when the sun is due east. The northern sky to the right is considerably less bright than the anti-solar region to the left. Thus one of the strongest reasons for believing these photographs to be taken after sunset is also consistent with the assumption that they were taken in the early morning. The latter assumption is far more consistent with the observed shadows.

Densitometric Measurements of Object Surface Brightness

Because of the high surface brightness of the shaded bottom of the UFO in the photograph, compared with other shaded regions, Hartmann concluded that the object appeared to be at a great distance. Atmospheric scattering will cause dark objects to appear brighter when seen at great distances. As objects recede, they gradually approach the brightness of the horizon sky. While it is possible to determine the approximate distance of an object if the surface brightness of the object is known, this determination is based upon an unstated assumption: that the distribution of light as focused on the film plane is in uniform proportion to the lighting in the original scene.

A test of this assumption was made on a series of experimental black-and-white negatives taken under the following circumstances:

Series one: Normal, clean optical system.

Series two: Lens lightly smudged with petroleum jelly.

Series three: Lens more densely smudged with petroleum jelly.

The purpose of the exercise was to determine what effects dirt and grease on the lens, or other optical imperfections, might have on densitometric measurements of a dark object against a bright background. In none of these photos was the lens smudged heavily enough to cause resolution to suffer visibly on an 8 x 10 print. The photographs were taken in on a sunlit afternoon, and contain a concrete streetlight pole, a grassy foreground, and downtown Chicago buildings approximately eleven miles away. Except for the foreign substance on the lens, the photographs are identical.

It was desired to determine whether foreign substances on the optical surfaces caused bright regions to "spill out" into adjacent dark regions (the technical term for this is veiling glare), while isolated dark regions remain unaffected. It was discovered that when values are normalized to horizon sky brightness, this is exactly what happens. The results of densitometric measurements of the test photos are as follows:


Series 1 2 3  
Shaded concrete pole:

bottom,surrounded by grass

.52 .54 .59  
middle, horizon sky

is background

.55 .65 .72  
top, brighter sky

is background

.55 .64 .82  
John Hancock building,

11 miles distant

.90 .89 .93  
nearby grass .60 .59 .59  
Average scan of sky a few

degrees above horizon

1.10 1.06 1.10  

horizon sky brightness = 1.00

In Series 1, the pole measures about 6% brighter at the top than at the bottom. This is probably due to the fact that even a clean lens scatters light from bright areas to adjacent dark areas to some extent. In Series 3, the pole is 39% brighter at the top than at the bottom, a very clearly erroneous value. The value of the sky has "spilled over" into the adjacent shaded region, but the lower part of the pole is not so greatly affected because its background is not as bright. Notice that the brightness of extended regions (grass, sky), as well as low-contrast objects, such as the distant John Hancock building, are changed little if at all.

This provides an alternate explanation for the high surface brightness of the supposed UFO in the photograph. The presence of smudges on the camera lens, or other optical imperfections, will cause light from the bright sky to be diffused over a dark object seen against it. If we were to consider the top and bottom of the streetlight pole to be separate objects, we would probably conclude that the top of the pole in Series 3 is at a much greater distance than the is bottom because supposed "atmospheric scattering" had made its shaded surface very much brighter. Conclusion: If we admit the possibility that the lens was smudged or otherwise impaired when the McMinnville photographs were taken, then we no longer need attribute the seemingly anomalous brightness of the UFO to atmospheric scattering resulting from a large distance. The generally hazy appearance of the photographs tends to support the contention that the lens was dirty or otherwise impaired.

Other prosaic explanations for the seemingly anomalous brightness of the object’s underside are: Hartmann’s suggestion that the model UFO may have had a pale gray top but a bright white bottom (such as "an aluminum pie pan sealed on the bottom with white paper")[9]; physicist and pro-UFO researcher Bruce Maccabee suggests that if a translucent model were used it would explain the photometry results[10]; and researcher Joel Carpenter’s suggestion that the object suspended from the wires may be a side mirror from an old vehicle, and hence is reflecting the bright sunlit ground below.[11]

Relative Size and Position of the Object in the Two Photographs

Hartmann found in his investigation that the object appears to be in the same position with respect to the telephone wires above it in both photographs, even though the camera had changed positions. While it is conceivable that the motion of the object could exactly cancel that of the photographer with respect to the nearby wires, this fact suggests very strongly the possibility of fabrication.

It was also noted by Hartmann that the object is about 8% farther from the camera in Plate 24 than in Plate 23. Measuring the size of irregularities in the wires reveals that they are about 10% farther away in Plate 24, but this value is less certain than the other. Thus the change in the distance to the object from plate 23 to plate 24 just happens to very closely match the change in the distance to the overhead wires, another factor that strongly argues in favor of a fabrication.


In light of the above, it is clear that the witnesses’ story of supposedly photographing a UFO cannot be accepted at face value. It is difficult to see what advantage would be gained in altering the alleged circumstances of the photographs, but the scientific investigator must guard against becoming an apologist. No serious researcher would contend that a photograph is of any value whatsoever in establishing the existence of an extraordinary object unless it is solidly corroborated by the testimony of one or more witnesses.

There exists no factual basis for rejecting the following hypothesis: at approximately 8:20 in the morning of May 11, 1950, a small asymmetrical model was suspended from overhead telephone wires by two very thin threads. It was photographed once, then reoriented either by hand or by its assumption of a pendulum-type motion, and photographed again.

Of course, this does not "prove" that the photographs do not show an extraordinary flying object, but it has shown that there is no reason to believe that they do. The non-existence of such objects, as well as that of werewolves, witches, and unicorns, can never be "proven." No amount of negative evidence will seem conclusive so long as there exists a strong will to believe. In this writer’s opinion, the fact that this and so many other "classic" UFO cases have eventually been exposed should instill in us a healthy skepticism that should not be lost in investigating future reports of highly-improbable phenomena.


I would like to thank Mr. Philip J. Klass of Washington, DC for sending me photographs, information, and valuable suggestions, without which this report would not be possible. I also thank Dr. William K. Hartmann for his comments upon the first version of this paper, many of which have been incorporated.[12] I would also like to thank Dr. Erhard Stutz of Northwestern University for making available to me the use of a densitometer.

Related Information

A certain UFOlogist has accused me, in a privately-circulated letter, of inventing a nonexistent "tangle of threads" above the telephone lines on Trent photo #1 which he admits would be "a nice proof of a hoax." Click here to see this "tangle of threads" for yourself - the one that I supposedly made up.


1. Scientific Study of Unidentified Flying Objects, Edward U. Condon, editor. (Bantam, 1969).
2. Scientific Study, p 397; p. 399.
3. The Nature of Light and Colour in the Open Air by M. Minnaert. (New York: Dover publications, 1954.) Page 81.
4. Minnaert, p. 269-270.
5. Scientific Study, p. 404.
6. McMinnville is at latitude 45 deg 06’ North, longitude 123 deg 20’ west.
7. Scientific Study, p. 400.
8. The Nature of Light and Colour in the Open Air, pages 245-6.
9. Scientific Study, p. 407.
10. Bruce Maccabee, letter in Journal of Scientific Exploration, Vol. 9 nr 2, pp. 281-291 (1995).
11. Personal correspondence from Joel Carpenter, February 16, 1999.
12. After reading the original version of this paper in 1969, Dr. Hartmann indicated that he agreed with its principal points, which are stated above. (Personal correspondence, Dec. 19, 1969).

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