Photopic Scotopic vision

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. 

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. 

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

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... 

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... 

Purkinje Effect Effect of the behaviour of the human eye, the ratio of photopic vision to scotopic vision, named after the Czech physiologist Jan Evangelista Purkinje. 

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. 

Rods are long and thin, and are more concentrated towards the periphery of the retina. As seen in Fig. 5.2, the maximum density is at 25° eccentricity, and decreases towards the outer edges of the retina. Rods only have one type of photopigment and can therefore only detect differences in luminance (i.e. these see only in monochrome). During night-time, when illumination is low and the iris wide open, the rods are used for vision (called scotopic vision). Peripheral vision in a dark environment is much better than foveal vision due to the low distribution-density of rods in the central part of the human eye. However, rods are about two-and-a-half times more sensitive than cones (Fig. 5.3, after data published by ) szechy and Stiles, 1982, p. 256). They do not contribute to photopic vision because of over-exposure when illumination is high (a process called bleaching). 

Mesopic vision n. Vision at luminosities intermediate between luminosities required for completely photopic or completely scotopic vision sometimes called twilight vision. 

At low levels of illumination, objects may differ from one another in their lightness appearances but give rise to no sensation of hue or saturation. AH objects appear to be of different shades of gray. Vision at low levels of illumination is called scotopic vision, as distinct from photopic vision, which takes place at higher levels of illumination. Table 1.32 compares the luminosity values for photopic and scotopic vision. 

TABLE 1.32 Relative Luminosity Values for Photopic and Scotopic Vision... 

Only the rods of the retina are involved in scotopic vision the cones play no part. As the fovea centralis is free of rods, scotopic vision takes place outside the fovea. Visual acuity of scotopic vision is low compared with photopic vision. 

It is not clear why scotopic vision is so much more readily affected by vitamin A deficiency than photopic vision since retinal is a constituent of the three cone pigments as well as of rhodopsin. Normal vision requires the presence of ll-cir-retinal and the sjmthesis of foiu different opsins. 

The quantity Fg can be any of the above radiometric quantities photometric quantity is obtained by replacing the term radiant with the term luminous (e.g. luminous flux luminous intensity /, luminance L, illuminance E etc.). The crucial criterion for the assessment of a CMS is the final impact on the (human) driver. Therefore, the physical quantities to be considered are the photometric ones. However, there are significant differences between the luminosity functions for photopic vision (high intensity, daylight) and scotopic vision (low intensity, night scenes)—see Fig. 1. 

Fig. 1 Luminosity functions for photopic vision (E(2), black curve) and scotopic vision (E (A), green curve). Image from [3]...

The very light sensitive rods are excluded from color interpolation Their neuronal signals are overlaid as monochromatic (not as color opponency) into the information feed to the optical nerve (see Sect. 3.1). From this it can be deducted why human vision fades from photopic (day light) over mesopic (twilight) to scotopic vision (low light) with a gracefully degradation from color to monochromatic vision and an increase in sensitivity [2, p. 216]. 

Scotopic Scotopic vision is the vision of the dark-adapted eye, in contrast to photopic vision when the eye is light-adapted. 

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

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).

Another aspect of visual function that requires further study in children in relation to lead exposure is light and dark adaptation. Several animal studies suggest that rods (which mediate scotopic, dark-adapted vision) are more susceptible than cones (which mediate photopic, light-adapted vision) to lead... 

---