This matches physiological evidence that there are three types of color-receptor cells in the eye. In most ordinary light there is a mix of various frequencies and, at a macroscopic scale, one can indicate this mix of photons by drawing a graph of intensity versus frequency. If light is generated very carefully, it sometimes consists of photons that all have the same energy so that the light is of a single frequency. We will refer to such light as mono-spectral .
Laser light is the standard example of mono-spectral light.
Second, the green cells will respond to mono-spectral light of many different wavelengths. If we take a variable-frequency light of constant intensity and measure how much response we get from a typical green cell in response to this light, we can plot this response against frequency and call it the frequency response of the green cells. If we've done this and we then send the eye a mix of light rather than a pure mono-spectral light, we can compute the total response by summing up the product of the incoming light intensity and the frequency response at each frequency. The area under this product curve is show on the bar indicator at the right. Naturally, this leads to three different total responses for any given input frequency distribution, as shown in figure 2.