Photopic luminosity

Figure 5.5.10-2 compares the typical putative spectrums based on such a linear analysis, (3, y, p compared to the actual chromophores, Rhodonines 5, 7 9 [with Rhodonine(l 1) shown for completeness. It is not significant in human vision except for aphakic patients.] Hunt describes the (3,y p spectrums as probable sensitivity curves of the three types of cones. He did not discuss any rod spectrum in his figure. The probable sensitivity curves appear to have been normalized individually. The peak in the p spectrum appears to be at a longer wavelength than frequently suggested. However, it is still at too short a wavelength to support the known spectral response of the human eye as illustrated by the Photopic Luminosity Function. 

Because the major application of OLEDs/PLEDs is for displays, the response of the human eye, described by the photopic luminosity function [15], must be taken into account. The photopic luminosity function is shown in Fig. 4.4. By using the photopic luminosity function, the radiance (watts/(sr m2)) is converted into the luminance (candela/m2, cd/m2 or lumen/(sr m2)). Therefore, photometry is used to measure the forward viewing luminance at the surface of an OLED/PLED. 

FIGURE 1.114 The photopic luminosity fiinrtion. (Source Adapted from Fink, D.G. 1952. Television Engineering, 2nd ed. McGraw-Hill, New York.)... 

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. 

We finally arrive at the result we want, since we can now set up "Tristimulus Filters" to use in defining colors. We can now define "y as our standard luminosity curve for the human eye (photopic vision). Note that x, the red tristimulus value, has a certain amount of blue in it in order to duplicate the response of the red preceptor in the retina. 

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). 

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. 

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... 

The luminosity function that has been accepted as standard for photopic vision is given in Fig. 1.114. Tabulated values at 10-nm intervals are given in Table 1.32. This function was agreed on by the International Commission on Illumination (CIE) in 1924. It is based on considerable experimental work that was conducted over a number of years. Chief reliance in arriving at this function was based on the step-by-step equality-of-brightness method. Flicker photometry provided additional data. 

In the scotopic range of intensities, the luminosity function is somewhat different from that of the photopic range. The two curves are compared in Fig. 1.115. Values are listed in Table 1.32. The two curves are similar in shape, but there is a shift for the scotopic curve of about 40 nm to the shorter wavelengths. 

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]...

---