Color Perception –
Many of the sensations of color we enjoy are not seen by any other creatures on the planet. This is because our perceptions of color are based on a particular combination of stimulation of our red, green and blue (or L, M, and S) cone sensors in the human eye.
Each of these cone light sensing cells in the retina has its own range of spectral sensitivity. Though the peak responsivity of the L cells may be approximately 570nm light wavelength, they provide contributing information at lower levels from approximately 425-700nm. The M cells peak around 540nm with a range of 400-650nm, and the S cells peak around 445nm with a range of 380-540nm. The rods also contribute information peaking at around 498nm.
Your sensation of any particular color is based on the complex stimulation of all of these cells and the algorithm your brain uses to interpret their signals. They must all three be properly stimulated in order to cause the sensation of any particular color perfectly. However, since all of these cell’s range of sensitivity overlap, artificially generating this precise stimulation of all three types of cells is nearly impossible.
Using monochromatic red, green, and blue LEDs to stimulate these different cells is a very poor substitute for synthesizing color sensation. If you had a 600nm red LED it would always stimulate the M cells 50% of the signal strength of the L cells it is trying to stimulate. There is no way to change this ratio. So subtle variation of the red and green blend cannot be done. The same is true for any single color light source. You cannot change the relative amount of stimulation generated to the adjacent color cells.
Normalized responsivity of cone cells
The reason there are only seven colors of the rainbow is because our vision system is less sensitive to the colors between them. Many color sensations are caused by unique combinations these colors. For example, using only the three monochromatic primary colors, red, green, and blue, you cannot completely synthesize the sensation of the color of the metal gold. The correct stimulation of the retinal sensors cannot be achieved from such a simple light source.
Most objects in the real world reflect energy across a broad range of wavelengths. In fact most objects reflect energy at nearly every wavelength in the visible spectrum to various degrees. So the color information received by the eye from an object in our visual field is very complicated and contains information that cannot be recreated by simply using three LEDs. The result of this fact is that most colors you see around you cannot be accurately replicated by a three color system. If you add addition light sources so that you have the ability to generate information in more parts of the spectrum, then you can do a better job of replicating a particular color experience. Creating a good color gold, to continue the example, might take five or six different color LEDs with their output regulated correctly.
In artificial lighting it is helpful to understand how the light source will affect how the color of illuminated objects will be perceived. These objects that would normally reflect energy across the entire spectrum, under artificial lighting can only reflect energy at the wavelengths available. Some light sources, such as fluorescents, have a strong output in the green area of the spectrum, so everything seen with those lights has a green cast. The human eye is most sensitive in the yellow area of the spectrum, so less light is needed as long as you have a strong yellow signal. That is why soft white lamps are often preferred.
Author: Percy Shadwell, Principal Consultant
Shadwell Technical Services
Consulting in Applied Physics and Electronics
Photo source: Wikipedia – Cone cell