Photoreceptors

Figure 5.2: On the left is an electron micrograph image of photoreceptors. The right shows the structure and components of rods and cones. The outer segments contain photopigments that electrochemically respond when bombarded by photons. (Figure from [360].)

Figure 5.3: The sensitivity of rods and cones as a function of wavelength [29]. (Figure adapted by OpenStax College.)

The retina contains two kinds of photoreceptors for vision: 1) rods, which are triggered by very low levels of light, and 2) cones, which require more light and are designed to distinguish between colors. See Figure 5.2. To understand the scale, the width of the smallest cones is around $1000$nm. This is quite close to the wavelength of visible light, implying that photoreceptors need not be much smaller. Each human retina contains about $120$ million rods and $6$ million cones that are densely packed along the retina. Figure 5.3 shows the detection capabilities of each photoreceptor type. Rod sensitivity peaks at $498$nm, between blue and green in the spectrum. Three categories of cones exist, based on whether they are designed to sense blue, green, or red light.

Figure 5.4: Several familiar settings and the approximate number of photons per second hitting a photoreceptor. (Figure adapted from [162,206].)

Photoreceptors respond to light levels over a large dynamic range. Figure 5.4 shows several familiar examples. The luminance is measured in SI units of candelas per square meter, which corresponds directly to the amount of light power per area. The range spans seven orders of magnitude, from $1$ photon hitting a photoreceptor every $100$ seconds up to $100,000$ photons per receptor per second. At low light levels, only rods are triggered. Our inability to distinguish colors at night is caused by the inability of rods to distinguish colors. Our eyes may take up to 35 minutes to fully adapt to low light, resulting in a monochromatic mode called scotopic vision. By contrast, our cones become active in brighter light. Adaptation to this trichromatic mode, called photopic vision, may take up to ten minutes (you have undoubtedly noticed the adjustment period when someone unexpectedly turns on lights while you are lying in bed at night).

Figure 5.5: Photoreceptor density as a function of angle. The right of the plot is the nasal side (which corresponds to rays entering from the opposite, temporal side). (Figure based on [244])