Scotopic

Fig. 1. Representative spectral sensitivities of the human retinal receptors, (-) scotopic (rod) vision, and P, y, and p cone sensitivities. The wavelengths of...

Photopic (right) and scotopic (left) luminous efficiency functions. 

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. 

By measuring a number of individual observers, we can obtain what we call a "Standard Luminosity Curve". Photopic vision peaks at 5500 A whereas scotopic vision peaks at 5200 A. 

Fox DA, Katz LM. 1992. Developmental lead exposure selectively alters the scotopic ERG component of dark and light adaptation and increases rod calcium content. Vision Res 32 249-255. 

Sole, P., D. Rigal et al. (1984). Effects of cyaninoside chloride and heleniene on mesopic and scotopic vision in myopia and night blindness (French). J. Fr. Ophthalmol. 7(1) 35-39. 

Vision results from signals transmitted to the brain by about 125 million sensors located in the retina. These photoreceptors are of two types, called cones and rods. Cones work under intense light - that is, during daylight hours - and this mode of vision is called photopic vision. Rods work under dim lighting conditions, and this is called scotopic vision. 

Scotopic vision does not permit the resolution of colors. Rods are much more sensitive to dim light (see Figure 1.3) than cones, but they do not have different types of pigments that are sensitive to different wavelengths so color cannot be distinguished in dim lighting conditions. 

Figure 1.3 The sensitivity of the eye for photopic (cones) and scotopic (rods) vision. The arrows indicate the wavelengths of maximum sensitivity.

We can now estimate the freqnency that corresponds to the radiation detected nnder photopic or scotopic vision. From expression (1.1), we obtain that... 

Ahmad M, Grancher N, Heil M et al 2002 Action spectrum for cryptochrome-dependent hypocotyl growth inhibition in Arabidopsis. Plant Physiol 129 774-785 Bellingham J, Whitmore D, Philp AR, Wells DJ, Foster RG 2002 Zebrafish melanopsin isolation, tissue localisation and phylogenetic position. Brain Res Mol Brain Res 107 128-136 Crawford BH 1949 The scotopic visibility function. Proc Phys Soc Lond B62 321—334 Miyashita Y, Moriya T, Yamada K et al 2001 The photoreceptor molecules in Xenopus tadpole tail fin, in which melanophores exist. Zool Sci 18 671-674 Wald G 1945 The spectral sensitivity of the human eye a spectral adaptometer. J Opt Soc Am 35 187... 

Night vision improvement. In a doubleblind study, graduated THC administration at doses of 0-20 mg (as Marinol) on measures of dark adaptometry and scotopic sensitivity was evaluated. Field studies of night vision were performed among Jamaican and Moroccan fishermen, and mountain dwellers with the LKC Technologies Scotopic Sensitivity Tester-1 (SST-1). Improvements in night vision measures were noted after THC or cannabis. The effect was dose-dependent and cannabinoid-mediated at the retinal level k... 

Russo, E. B., A. Merzouki, J. M. Mesa, K. A. Frey, and P. J. Bach. Cannabis improves night vision a case study of dark adaptometry and scotopic sensitivity in kif smokers of the Rif mountains of northern Morocco. J Ethnopharmacol 2004 93(1) 99-104. D Souza, D. C., E. Perry, L. MacDougall, et al. The psychotomimetic effects of intravenous delta-P-tetrahydrocannab inol in healthy individuals implications for psychosis. Neuropsychophar-... 

Duncan, J.L., Yang, H., Doan, T., Silverstein, R.S., Murphy, G.J., Nune, G., Liu, X., Copenhagen, D., Tempel, B.L., Rieke, F., Krizaj, D., 2006, Scotopic visual signaling in the mouse retina is modulated by high-affinity plasma membrane calcium extrusion. J Neurosci 26, 7201—7211. 

The cones are used for color vision in bright light conditions (photopic vision), whereas the rods are used when very little light is available (scotopic vision). 

The International Commission on Illumination (CIE) has defined a standard observer to be used for accurate color reproduction (International Commission on Illumination 1983, 1990, 1996). In Chapter 2 we have seen that the rods mediate vision when very little light is available. This type of vision is called scotopic vision. The cones mediate high acuity vision in bright light conditions. This type of vision is called photopic vision. The sensitivities for a standard observer as defined by the CIE for scotopic and photopic vision are shown in Figure 4.3. The scotopic function is denoted by V (k). The photopic... 

Figure 4.3 Sensitivities as defined by CIE, which are used to model scotopic V (X) and photopic vision VM(k) of a standard observer (International Commission on Illumination 1983, 1990, 1996) (data from International Commission on Illumination 1988).

Using these sensitivities, we can calculate the luminous flux of any light source (International Commission on Illumination 1983 Jahne 2002). Let (/,) be the radiant flux at wavelength X of the light source. Let V (X) be the sensitivity of scotopic vision and V(X) be the sensitivity of photopic vision, then the intensity of the light source is... 

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. 

The best empirical values for this spectral parameter vary somewhat due to experimental technique, particularly due to the solvent used. Although rounded due to the width of the spectral filter used, the values of Wald Brown are still the most widely quoted118. They are a = 447 nm, b = 502 nm, c = 549 nm. It would be easily to interpret the median of this waveform at the mean of the two half-amplitude points, 498 nm. It is common to find the waveform shifted slightly to a center wavelength at 495nm. However, the 502 nm wavelength is frequently reported in an attempt to relate it to the peak of the scotopic spectrum when that spectrum is measured with a broadband spectral filter. 

Using these values and the resulting parameters from TABLE 5.5.10-1, the photopic and scotopic luminosity functions can be computed precisely (See Chapter 17). 

Baylor et. al. have provided recent in-vitro isotropic absorption spectrum for Rhodonine based on Macaca fascicularis143. The data was collected over five and one-half orders of intensityusing transverse illumination at near 37 Celsius. Baylor, et. al. also provide a review of other available measurements and perform a circuitous calculation to compare the isotropic spectrum to the human scotopic luminous efficiency function attributed to Carpenter rather than the C.I.E. Based on ten photoreceptors, the peak sensitivity was estimated to be 491 nm. This is lower than most other investigators. However, data points were only collected at 20 nm intervals using filters of 10 nm nominal half-widths. This sampling plan does not support three digit accuracy in their peak value. 

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