Scotopic response

Photopic vision peaks at 5500 A whereas scotopic vision peaks at 5200 A. The following, given as 6.7.7. on the next page, shows both the photopic and scotopic response curves for the human eye, as determined from a number of observers. In this case, the relative response of the observers are summed into a response called "THE STANDARD OBSERVER" and is normalized for easier usage. You will note that these eye-response curves are the result of an average of many human eye response curves. 

Apparently, photopic vision relates to "sunlight", to which the humcui had adapted through evolution, while scotopic vision related to "moonlight", that is, sunlight modified by reflection finm the Moon s surface. However, it was soon discovered that the color responses of individuals were not exactly the same. Each individual "sees" a color slightly differently from anyone else. We have learned to discriminate between colors but no one knows exactly what anyone else actually sees. 

A primary reason for the above disparities is the critically important structural organization of the chromophores when found in-vivo. These relationships make a major (several orders of magnitude) difference in the absorbance of the material and also lead to anisotropic absorption. These relationships have not been maintained by the chemists. A second reason relates particularly to the L-channel. The chromophore of that channel exhibits a more intimate relationship with the electronic portion of the photoreceptor neuron than do the S- and M-channel chromophores. As a result, the L-channel exhibits an effective absorption characteristic very different from that observed by the chemist. This characteristic also accounts for the loss of red response in the mesopic and scotopic regions. These relationships have not been emulated in the environment of the chemist. Failure to emulate these conditions leads to extraneous absorption spectra for the L-channel chromophore. A third reason is due to the frequent chemical reactions occurring in the chemists solutions that he may not be aware of. It has been rare in the past for the chemist to document the pH of the solutions he has measured. This is a particular problem as mentioned in a later section [Section 5.5.12], The chromophores of vision are members of the "indicator class of chemicals. Their spectral characteristics are intimately related to the pH of their environment. They are also complex organics. Their spectral characteristics are a function of the organic solvent used. They are also subject to chemical attack. This mechanism has been documented by Wald, et. al. and more recently by Ma, et. al. 

Figure 1 Photometry visible light measurement. Abbreviations. CIE, Commission Internationale de I Eclairage, CIE V(X), Photopic- Light Adapted Eye Response, CIE V (X), Scotopic - Dark Adapted Eye (no color) Response.

Let us now summarize the results we have achieved. We have measured the luminosity response of the human eye, in terms of photopic and scotopic behavior. We also defined a "black-body" and its wavelength emission, stipulating its absolute temperature. We then defined Standard Sources. We next designed a Color Comparator and then determined the transmission characteristics of three (3) filters required to duplicate the response of the three color preceptors of the human eye. These we called the tristimulus response of the Standard Observer. 

Sieving, P.A. 1991. Retinal ganglion cell loss does not abolish the scotopic threshold response (STR) of the cat and human ERG. Clin Vis. Sci 6 149-158. 

Figure 6. Response versus intensity function for b-wave amplitude and implicit time of the scotopic ERG in Cynomolgus monkeys. SF = standard flash at 2.57 cds/m ...

The photopic log intensity functions are shown in figure 8. While the first three values used for this function represent cone responses on red flashes, the last one represents the white flash cone response. In contrast to the scotopic time vs. log intensity function, the intensity-response function of the cones peak latency indicates a slight increase. This result is also known from studies on human retinal function (Jacobi et al., 1993). 

The procedure is adapted from the recommendations for standardization issued by the ISCEV (Marmor et al., 1989). ERGs obtained with blue-light flashes (scotopic ERG) showed a predominance of B2-wave and a shark-peak A wave. ERGs with red-light flashes are photo-scotopic mixed flash ERGs. On dark-adapted eyes, ERG with white flashes is a mixed response. Procedural alterations are required to obtain a photopic flash ERG. 

Bushnell, P., Bowman, R., Allen, J. and Marlar, R. (1977) Scotopic vision deficits in young monkeys exposed to lead. Science, 196, 333-335 Feeney, D., Longo, Cosden, M., Zenick, H. and Padich, R. (1979) Detection of the effects of lead exposure by visual evoked response latency. Physiol. Psychol, 7, 143-145 Fox, D. and Sillman, A. (1979) Heavy metals affect rod, but not cone, photoreceptors. Science, 206, 78-80... 

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