How Humans See Color: What Lighting Designers Need to Know

Full Spectrum Light and Color Perception

Both daylight and incandescent light are illumination sources which provide full spectrum light, containing every frequency of the electromagnetic spectrum that humans can see, from approximately 385nm to 700nm. Daylight tends to be bluer, with a little less energy in the red part of the visible spectrum. Daylight contains fewer warmer hues, which is why we refer to this as “cooler” light. Incandescent light is “warmer” and contains more energy in the red end of the visible spectrum, with less energy in the lower visible wavelengths, the “cool” colors such as blues and purples.

How We Perceive Color

Humans perceive color when full spectrum white light illuminates a pigmented surface. Some of the frequencies, or wavelengths of light, are absorbed by the pigment, and some are reflected back to the eye. Our eyes contain rods and cones which interpret incoming light energy and signal the brain with the amount of incoming light and color.

Rods are more sensitive and do a better job of interpreting light and dark; they are what helps us to navigate in extremely low light levels. Cones need more energy and respond to more specific wavelengths of light, such as red, green, and blue. If there is not enough light energy, the rods are doing all the work, and what we see in very low light appears in shades of gray.

How Gel Filters Work

If you were to place a saturated blue gel in the beam of an incandescent light, the gel filters out most of the red and green energy, allowing primarily blue to pass through. Shine that blue beam of light on a white object, and it appears blue because those particular wavelengths are the only energy available to reflect off the surface back to the eye.

Shine that same blue light on a red object, and since the red energy has been filtered out, there is essentially no red to reflect back to the eye, thus it appears to be black or dark gray. Gels are an imperfect medium, so even the primary gel colors allow a small amount of other wavelengths to pass.

Why RGB LED Fixtures Fall Short

Red, Green, and Blue LEDs are very precise in the wavelength they create. Red LEDs produce a very sharp spike of energy around 630nm, Green around 560nm, and Blue around 470nm. Mix Red, Green, and Blue LEDs and you should get white; however, the green emitter is the weakest of the three, and can’t keep up with the more powerful Red and Blue LEDs, so you end up with a pinkish-purplish light which isn’t very good at illuminating much of anything.

This RGB combination is also lacking energy between Red and Green, between Green and Blue, as well as below Blue (which is Violet or Indigo) and above Red (which is Deep Red).

Adding White and Lime Emitters

Adding White LED emitters (blue LEDs with white phosphor coating) adds intensity but likely not much to the color gamut. Lime LED emitters, like White, are a blue pump with Lime phosphor. This adds a mountain of energy to the middle of the visible spectrum, so additional energy in green, yellow, and cyan. Mix Red and Green LEDs with a little Lime, and you get amazing oranges and yellows. Mix Green and Blue with a little Lime and you can produce better cyan and other lighter hues of blue.

5-Color and Beyond

RGBL luminaires do mix to a decent white light, as well as all the colors between blue and red on the visible spectrum; however, they still fall short on the far ends of what humans can see. 5-Color fixtures with RGBLI (Red, Green, Blue, Lime, and Indigo) solve that issue. Add actual Cyan and Amber emitters and you have even more energy in those parts of the visual spectrum.

Some luminaires now also have Deep Red LED emitters, which thoroughly rounds out the visual spectrum. The Deep Red LEDs aren’t very bright on their own; however, what they add to the color gamut of the fixture is a game-changer for theatrical lighting.