Condign volumes 1 to 3

Read from the scan by GLM-OCR; expect the odd misread word.

# UNCLASSIFIED

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CONDIGN

important, for example, in the context of lights which are 'chopped' by helicopter blades (~50Hz for a main rotor four-bladed Helo).

6. Flash Duration Dependency An important finding is that a given amount of light produces the same after-image (except for the first 15 sec), irrespective of whether it is delivered within 15.7 millisec or spread over 1.68 sec. This, is consistent with the hypothesis that the after-images of a brief stimulus from the 15th second until its disappearance at t=100 to 300 seconds later, depends upon photochemical effects. It does not depend upon adaptation or neural mechanisms in the retina or brain (as a result of the activity it is presumed the brain undertakes immediately after an eye stimulus).

7. It is probable, from the evidence available, that, after the first 15 seconds, neural effects contribute to the perception of the image seen. Up to five seconds after a stimulus (flash) it is nevertheless believed that neural as well as retinal (chemical) image perceptions are formed particularly if the event is from an unexpected source. At t=10 seconds the after-images are nearly alike (when using the alternatives used for experimentation). By 15 seconds they are seen as alike.

8. Observer Variability Observer contrast, for 50% probability of detection (against a plain background), is a function of object luminance, size of illuminated field, object size, edge sharpness, shape, time seen position in field of view, colour, motion and experience. Detection thresholds are largely independent of shape up to an expected ratio of 7:1.

## SUMMARY OF RESULTS

9. Based on the limited data available; to a first order level:

- After-images in the eye are based on the total amount of light available independent of time.

- The persistence depends on the chemical consequences in the eye's receptors, not in (5-10 sec case) the adaptation of the image by the nerves/brain response.

- For very short flashes (e.g. less than 4 millisec) high intensities have more effect than lower intensity stimuli when viewed against either bright or dim backgrounds.

- Colours observed following a flash of a particular colour are dependent on the viewing background thereafter.

## RELEVANCE TO UAP SIGHTINGS

10. A large number of UAP reports are of very intense lights of diverse colours, some of which are only reported as being of very short duration. Some of these could be the after-images of even briefer flashes. Longer duration reports apparently (see para. 6) of up to five minutes in duration, could be attributed to flash stimuli, followed by an after-image. It would appear that sightings are unlikely to be caused by after images for longer than five minutes. After 10-15 seconds the human involved will be interpretging and adapting what is seen well beyond what is actually registered chemically, due to the flash, within the retina.

11. There is, of course, the possibility of the after-image being 'replenished' by successive intense flashes. However, many flashes are separate (i.e. single) events, e.g. lighting, unexpected (i.e. non-continuous) electrical flash-over from (national grid) power lines or overhead train power lines.

12. It seems unlikely that, on the occasion of multiple witnesses of UAPs, that they will all receive the same phenomena (after-image); since apart from human observer variability in eyesight, it is unlikely that they would all receive the same stimulus at the same time unless they were located very close together.

[1] "Discrimination of After-Images" G.S. Brindley, J. Physiology 147 (1959).