Figure 11.5 Photograph of 4scaleBlack with 16 sec exposure for optimal recording of the luminances in Scale D (Middle right).

The 4scaleBlack measurements show that the same scene luminance generates different (exposure-dependent) digits. This is important because this display was intended to measure the minimal glare for an 18,619:1 scene. Despite the fact that 80% of the target area does not contribute to glare, we measure that there is a problem with the ME2SL technique. The problem of glare can be seen easily in an exposure that does well in recording the light from the darkest circle. Figure 11.5 is the 16 second exposure. Scale D, (middle right) is a good record of the target. Scale C is overexposed with a large scene reflection on top of it. Glare from Scale B makes its saturated image size larger than the scenes image, while more glare makes the image of Scale A much larger. There is a larger non-inverted reflection on top of Scale A. The corners of the image are much darker than the center, even though the scene's black mask is uniformly opaque. Glare limits the range of light falling on the image plane of the camera.

Sowerby (1956), in his "Dictionary of Photography" discusses the reflection of light in lenses as the "diversion of an appreciable portion of the incident light from its intended path". The small percentage of light reflected from each air-glass surface is called a parasitic image. Parasitic images that are completely out of focus give rise to a general fog that limits the dynamic range of the image falling on the image plane. The actual 4scaleBlack image (Figure 11.5) shows a magnified inverted in-focus parasitic image, as well as, the out-of-focus fog. Multiple exposures improve the digital quantization, and thus the camera’s performance. Nevertheless, multiple exposures have no effect on the dynamic range of the image falling on the sensor. Veiling glare is a constant fraction of scene luminance that varies with scene content.

Wiley Figures