The Nature of Light
Page 7: The Human Eye
The retina of the human eye absorbs photons to form images and perform other functions. In the eye, sensors that absorb photons to form images are called rods and cones, and sensors that absorb photons for nonimaging purposes are called ganglion.
The cones are used for daytime vision, which is called photopic vision. The rods are used for night vision, which is called scotopic vision.
The response (sensitivity) curve for daytime (photopic) vision was shown in Figures 2.8 and 2.9 of this report. In that mode, the human eye is mostly sensitive to greens and yellows.
The response curve for night (scotopic) vision is shifted toward the blue somewhat, making the eye more sensitive to blues and greens in very low light (instead of mostly sensitive to greens and yellows).
Scotopic vision only works in darkness. Under bright light conditions, such as in the illumination of LEDs, only photopic vision functions. Thus, the slight blue-shift of scotopic vision cannot be used to see if extra blue light is emitting from LEDs.
The cones are responsible for detecting color. There are three types of cones, one type for detecting red, another for detecting green, and the other type for detecting blue.
Each of the three detectors can detect colors that are close to the primary colors just mentioned. For example the cones that detect red can also detect colors that are close to red (but with less sensitivity than detecting red).
The brain mixes the results of detecting the primary colors to interpret other colors. Since there are more than three colors in nature, some natural colors will appear the same to humans. That inability to discern some natural colors is called metamerism.
There are fewer cones to detect blue light than to detect the other two primary colors. This causes photopic vision to under-sample blue light.
Figure 7.2 shows that there are many more cones to detect green light than to detect blue light.
Figure 7.3: Retinal ganglion cells.
Ganglion receptors govern the bodys circadian system, telling the body when to have a deep sleep and when to be awake. These receptors rely on blue light, historically from the sun.
Now that increasing blue light is being generated at night by artificial lighting, peoples circadian systems are being thrown off, preventing deep sleep, causing health and productivity declines.
Figure 7.4: Ganglion/circadian response curve.
Figure 7.4 shows the retinal ganglion/circadian response curve relative to the photopic (bright illumination) vision response curve and the spectral emission curve of a white LED. Blue light leaking from the LED triggers the circadian system without photopic vision noticing.
Affordable sensors need to be developed to identify situations in which circadian systems would be triggered at the wrong time, since photopic vision is not able to identify those situations.
Blue light is also a risk factor in causing macular degeneration by causing apoptotis. This has been known for many decades.
At one time, cataracts surgeons tried to implant blue-blocking intraocular lenses (IOLs) to help prevent apoptotis, but found that patients then became ill from not getting enough blue light. Now patients are encouraged to wear blue-blocking eye glasses.
Affordable sensors need to be developed to help people determine when to wear blue-blocking glasses.