Color, more than any other topic in visual communication, is frequently misunderstood. This is essentially why we need chromaticity.
Color is an essential tool for signaling. Traffic lights convey a great deal more information in color than they could in black and white.
Color is one of the ways we detect motion. We see moving objects as moving because our eyes respond to changes in color against the same background.
Color is difficult to translate from camera to colored inks and the printed page or from camera to computer screen. And when we see colors, our brains overlay our experiences and feelings to interpret colors in ways that no other individual on earth can duplicate.
So, how in the world could anyone ever standardize colors to enable everyone to know the colors we want, the colors we see, and the colors that convey standard information?
What Is Chromaticity?
Chromaticity is an objective, numerical specification of the quality of a color that stays constant without regard to the light that illuminates it. Color science uses chromaticity to explain how we judge the material from which the objects we see are made.
Chromaticity is a combination of two measurements. One is hue, which is formally described as the degree to which light can be detected by the human eye, or a stimulus, similar to or different from stimuli that are described as the colors red, yellow, orange, green, blue, or violet. These colors can be described in terms of the wavelengths of light that create them. Wavelength can be objectively measured.
The other measurement that goes into chromaticity is colorfulness. The International Commission on Illumination (CIE) defines colorfulness as “attribute of a visual perception according to which the perceived color of an area appears to be more or less chromatic.” This just means that color depends on more than the wavelengths of light that are being reflected to the human eye. It also depends on the intensity of the light falling on the surface reflecting them.
Why Does Color Science Need a Concept of Chromaticity?
Color is a universal language. All around the world, drivers stop when traffic lights flash red and go when traffic lights flash green. Any time of night or day, sighted people become more active and alert – even if they would rather be sleeping – when their eyes detect even a faint trace of the blue light that heralds sunrise.
Chromaticity is an important concept for explaining concepts as varied as how smartphone users respond to colors on their screens at different times of day to why some children become nearsighted and others do not. Color scientists use the concept of chromaticity to explain why a white surface might appear to have color, based on the colors of surrounding surfaces. Chromaticity is even used to describe the naturalness of tanned skin treated with sunless tanning products compared with skin tanned naturally in the sun. But the most critical application of chromaticity in color science is explaining how operators of trains and planes respond to safety signals in different colors, making sure lights emit the colors of light that pilots and conductors are trained to see.
Chromaticity confirms that light emits the color you need to see. Light fixtures that have the same chromaticity will emit the same color of light. Chromaticity makes such a big difference in safe operation of transportation that there are international standards for chromaticity set by the Federal Aviation Administration (FAA), the Defense Logistics Agency (Mil), the International Civil Aviation Organization (ICAO), the Association of American Railroads (AAR), and the Institute of Transportation Engineers (ITE), among many others.
Chromaticity Results From Three Factors
Three factors determine the chromaticity of any light fixture: the source of the light illuminating it, the transmissibility of the lens or material covering it, and the human eye that sees it. It is not possible to change the way the eye perceives or fails to perceive color, but it is possible to adjust the light source and the material used to make the lens to achieve a specific chromaticity.
Every source of light has its own unique spectral power distribution. Every light source emits a unique collection of wavelengths of light that humans perceive as color.
Light sources can be broadband, emitting light across a range of wavelengths. Or they can be discrete, emitting only a narrow band of wavelengths.
One strategy for controlling chromaticity is to use a white light source, such as a white LED or an incandescent bulb. White light is a broadband light. It contains all the other colors. White light sources can be covered by a colored lens that transmits only wavelengths of a desired color.
It is also possible to produce specific wavelengths of light with a color LED, fitted with either a clear lens or a colored lens to transmit light in a narrow range.
Every wavelength of light produced by a light source contributes to color. A white LED light that produces a phosphor-generated peak of green to red wavelengths and a peak of blue wavelengths from the semiconductor chip will not contribute to chromaticity the same way as an incandescent bulb that produces light across the visible spectrum.
Why is this important to know?
LED lighting isn’t always a substitute for incandescent bulbs. This is especially true when a very specific chromaticity output is needed.
When changing a light source, the chromaticity of both the old and the new light source must be considered. Changing the lens over the light source may take care of this problem.
Lens Color and Color Absorption
It isn’t just the light source that influences chromaticity. It is also the color of the lens, or cover over the light source.
Only the wavelengths of light that are transmitted by the lens are visible, but each wavelength that passes through the lens contributes to color.
A blue lens, for instance, transmits blue light while absorbing orange, yellow, and red. But the light transmitted by a blue lens could be blue or blue-green, depending on the light source.
The composition of the glass in the lens determines its unique transmission spectrum. Glass can be manufactured with a thermal striking process and nano-crystals so it absorbs light of shorter wavelengths. This kind of glass has sharp-cut absorption edges that emit red, yellow, and orange light.
Glass can also be colored with transition metals or rare earths. Referred to as bandpass filters, these lenses have broad absorption bands that produce green, amethyst, and blue colors.
There is an intrinsic challenge in dealing with lenses. Different materials that appear to be the same color to the naked eye, such as plastic and glass, will not usually produce the same colors of light. No matter how similar two lenses look on visual inspection, they are likely to produce different colors when they are illuminated. To make sure the light output of a fixture with its lens meets chromaticity requirements, it is necessary to consider the transmission spectrum of each material and how it pairs with its light source.
Coated lenses reflect or absorb light at the surface of the glass. Glasses are intrinsically colored. As a result, the thickness of the glass used in a lens affects its chromaticity. Thicker lenses absorb more light. They emit darker colors of higher saturation.
Also unlike plastic coatings, glass has a permanent color. Its transmission spectrum and its chromaticity stay the same even after prolonged use.
The Human Eye
Eye sensitivity also contributes to chromaticity. Not everyone has the same ability to perceive color, so eye sensitivity is not a factor that lighting engineers can control.
Color perception begins when light strikes the retina, the collection of rods and cones at the back of the eye.
Rods are sensitive in low-light conditions. They provide most of our night vision. The rods respond best to light with a wavelength of about 500 nanometers, which is blue light.
Cones are sensitive in bright-light conditions. They provide most of our day vision. There are three types of cones:
- Long cones have peak sensitivities around 560 nanometers, at red wavelengths.
- Medium cones have peak sensitivities around 530 nanometers, at green wavelengths.
- Short cones have peak sensitivities around 420 nanometers, at blue wavelengths.
Rods and cones send electrical signals to the brain to be interpreted as light. Any time the rods and cones are struck by the same wavelength of light (the same color), they are stimulated in the exact same way.
But because colors evoke emotions, color interpretation differs from person to person. Chromaticity provides the measurement that enables consistent production of color that creates the most consistent experience of color. We can make use of this measurement because of a property of color known as metamerism.
Most of the light everyday objects reflect into our eyes isn’t in the visual spectrum. Our eyes are flooded with infrared and ultraviolet light that our rods and cones cannot detect.
The visible light reflected into the eye isn’t all the same wavelength, either. The rods and cones are stimulated more by some wavelengths and less by others, but they send a single signal to the brain. They reduce a complex spectrum of light to three signals that can be represented by three numerical values. The brain interprets these three signals together to interpret them as a single color.
It is not necessary to reproduce the original light to reproduce the original color. This property, known as metamerism, leads to some surprising possibilities.
It is possible, for example, to create the visual experience of yellow without yellow light. A light source emitting red and green light can stimulate the rods and cones in the same way as yellow light, making it possible to create a yellow image without a yellow light.
A model called CIE 1931, better known as RGB (red-green-blue) color matching, uses light at just three wavelengths, 700 nanometers (red), 546.1 nanometers (green), and 435.8 nanometers (blue), to recreate the experience of any color the human eye can see. The math gets a little complicated, since it has to take into account hue, brightness, and luminosity, but these three colors became the basis of the x,y,z color coordinates invented by Microsoft and Adobe.
Mastering chromaticity measurements is the foundation for using LED lighting for pleasing visual effects. Know your lighting, and know your colors, and you can use any modern lighting system successfully.