While color is extremely important, it is not the only information that our visual system and brain handle. They are also responsible for determining the luminance of light. In general terms, luminance can be defined as how bright a light appears to a viewer. However, luminance is not just a function of the intensity of light. It is also dependent on the biology of the human visual system.
It turns out that the human visual system has two systems for determining luminance. One system operates in bright light; the other system operates in low light. This is due to the eye having two types of photoreceptors: cones and rods. As covered previously, the cones are less sensitive to light. Thus, they are used primarily in bright light conditions (e.g., during the day). The rods are far more sensitive to light. Thus, they are used in dim light conditions (e.g., at night). Consequently, humans perceive luminance differently at night than they do during the day.
The rods are too sensitive for the bright light of day. Therefore, during the day, luminance is determined by the cones. To determine day luminance, the brain simply adds the signals from all three types of cones (i.e., red cones, green cones, and blue cones). However, luminance is not just a simple matter of how much light strikes the cones. This is due to two factors: cone response and the number of each of the three types of cones.
Figure 1 shows the cone response of the three cones to light. This figure reminds us that the cones are not equally sensitive to all wavelengths. The cones are more sensitive to the green wavelengths and less sensitive to the shorter blue and longer red wavelengths.
The luminance response of the eyes is further complicated by the fact that the eye does not have an equal number of each of the three types of cones. About half of all of the cones are red cones. Most of the rest are green cones. Only a few of the cones are blue cones.
The result of the cone's response and the disparate numbers of the red, green, and blue cones is that day luminance is dependent on both the intensity and the wavelength of the light that strikes the eye. In short, our eyes see certain colors as being brighter than others. This is demonstrated in Figure 2 which shows the cones' luminosity response to light. The curve in Figure 2 clearly shows that the cones respond much more strongly to the greens and yellows than to the blues and magentas. Thus, a green light will be perceived as being brighter than a blue light of equal intensity.
The good news is that since the cones are active in bright light we see both color and luminosity during the daylight hours.
During low light situations, such as during the evening hours, the cones are not sensitive enough for the dim light. Therefore, night luminance is determined by the rods. Now, just as with the cones, the rods are not equally sensitive to all the wavelengths of light. This is show in Figure 3. As can be seen in the figure, the rods are more sensitive to the blues and greens than to the yellows and reds.
Consequently, as with day luminance, night luminance is dependent on both the intensity and the wavelength of the light that strikes the eye. Thus, at night, our eyes will also see certain wavelengths as being brighter than others.
While there are some similarities between day and night luminance, there are two major differences. First, since the luminance curves of day and night vision are different, we see objects as having different luminances at night than we do during the day. Second, since there is only one type of rod, there is no color vision at night. Instead, everything is seen in shades of gray.