Photopic

C is the concentration of limiting reactant in mol/L, c is the chemiluminescence quantum yield in ein/mol, and P is a photopic factor that is determined by the sensitivity of the human eye to the spectral distribution of the light. Because the human eye is most responsive to yellow light, where the photopic factor for a yellow fluorescer such as fluorescein can be as high as 0.85, blue or red formulations have inherently lower light capacities. 

The theoretical limit of light capacity has been estimated for an ideal reaction that provides yellow light with a photopic factor of 0.85 in a quantum yield of one at 5 Af concentration as 173,000 (Im-h)/L, equivalent to the light output of a 40-W bulb burning continuously for two weeks (237). The most efficient formulation available, based on oxaUc ester chemiluminescence, produces about 0.5% of that limit, with a light capacity of 880 (Im-h)/L (237). 

The GIE Standard Observer. The CIE standard observer is a set of curves giving the tristimulus responses of an imaginary observer representing an average population for three primary colors arbitrarily chosen for convenience. The 1931 CIE standard observer was deterrnined for 2° foveal vision, while the later 1964 CIE supplementary standard observer appHes to a 10° vision a subscript 10 is usually used for the latter. The curves for both are given in Eigure 7 and the differences between the two observers can be seen in Table 2. The standard observers were defined in such a way that of the three primary responses x(X),jy(X), and X), the value ofjy(X) corresponds to the spectral photopic luminous efficiency, ie, to the perceived overall lightness of an object. 

Fig. 10-5. Comparison of for 0.1 ppm NO2 and Rayleigh scattering by air. The photopic eye response represents the range of wavelengths over which the eye detects light. Source Husar, R., White, W. H., Paterson, D. E., and Trijonis, J., "Visibility Impairment in the Atmosphere," Draft report prepared for the U.S. Environmental Protection Agency under Contract No. 68022515, Task Order No. 28.

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. 

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. 

In Eqs. (7)—(10), 5(A) is the spectral power distribution of the illuminant, and R A) is the spectral reflectance factor of the object. Jc(A), y(A), and 5(A) are the color-matching functions of the observer. In the usual practice, k is defined so that the tristimulus value, Y, for a perfect reflecting diffusor (the reference for R A)) equals 100. Using the functions proposed by the CIE in 1931, y(A) was made identical to the spectral photopic luminous efficiency function, and consequently its tristimulus value, Y, is a measure of the brightness of objects. The X and Z values describe aspects of color that permit identification with various spectral regions. 

In human eye units, watt becomes lumen. At 555 nm, the peak sensitivity of the photopic response, there are 683 lm/W. A watt is a joule per second, and a photon has energy in units of joules. A watt tells us how many photons per second are coming out of the OLED. A lumen... 

Radiant intensity can be described as the amount of power (watt) heading in your direction, i.e., per steradian, from a light source. The total amount of power emitted by the source is the radiant flux (watt). If you integrate the radiant intensity over all solid angles, you get the total radiant flux. If it is weighted by the photopic response, then it is the luminous intensity and the luminous flux. 

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. 

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... 

Takahashi Not really. It is too narrow. There are some really old action spectra from the Karolinska Institute that were done in rats. Under photopic conditions the action spectrum is much narrower than any opsin nomogram. This is common in rodents it is because of the contribution of multiple pigments at high light intensities. This is what the hamster action spectrum looks like it peaks near 500 nm, but it is narrower than an opsin nomogram. 

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... 

International Commission on Illumination 1990 CIE 1988 2° Spectral luminous Efficiency Function for Photopic Vision. Technical Report 86, International Commission on Illumination. 

L-cone 625 nm -0.8 (50 microns) This work, photopic level... 

Researchers have been trying to explain how retinal (or retinol) are chromophores in the visual system for more than 40 years with remarkably little success. Dartnall et. al. even claimed that the same retinal-based chromophore accounts for both the broad photopic spectral sensitivity of the eye as well as all three of the color-sensitive photodetectors found in the eye. There may be a problem of semantics related to this claim. In an attempt to synthesize an analog of rhodopsin, they formed a protonated azomethine using retinal and a simple amine, n-butylamine the resulting peak absorption was at 440 nm. It was hoped the material would have a peak near 500 nm (see further discussion below). Its absorption coefficient was not specified in Zollinger. Although this chromophore exhibits apeak in its absorption near that of the S-channel, it does not explain how the chromophores of the other spectral channels are formed. It is an irrelevant compound. 

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). 

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. 

---