Lighting's Color Science

Whether you are manufacturing luminaires for general lighting, building an LED-driven sign, or installing architectural lighting in a new home, understanding color science can make it easier to decide which LEDs are best for a particular application or otherwise help in project planning.

In the context of lighting, color can be thought of as physical stimuli. Light sources like LEDs, incandescent bulbs, and florescent bulbs emit light radiation at various wavelengths, and humans perceive that radiation. Alter the wavelength emitted, and the human eye sees a different color.

The Human Eye is the Model for Color Science

The human visual spectrum includes light radiation from about 400 nanometers (nm) to about 700nm. Within this spectrum, short wavelengths (say 450nm) represent the color blue; middle wavelengths are generally green, and long wavelengths are described as red. In the physical sense, the human eye is nothing short of amazing when it comes to perceiving light at various intensities within this color spectrum.
A person’s camera-like eye is an incredibly complex physiological and biochemical structure that makes the most advanced semiconductor or electronic technologies available seem ridiculously simple and clumsy by comparison.

The eye’s physical structures, such as the cornea, iris, ciliary muscles, lens, vitreous humor, retina, fovea, and even cells like the tiny rods and cones – which are named for their relative shapes – work together in a beautifully orchestrated way. For example, a person’s eye provides roughly 15 and a half times the resolution of the best 1080p HD LCD televisions available. And, thanks to the iris, which acts as an aperture, and the receptive rods and cones, the eye’s dynamic range – or its ability to see both very bright and very dark things simultaneously – is superior to any single sensor camera in the world.

On a biochemical level, the eye is even more amazing, as it continually executes precise, multi-phase chemical reactions that must take place in just picoseconds – one picosecond is approximately the time it takes light to travel the width of a human hair – in order to pass impulses to the brain’s visual cortex.

In color science, as it might be applied to choosing LEDs or designing luminaires for example, we are most interested in the behavior of the cells called cones, which are located in the retina at the back of the human eye. Cones can be divided into short, medium, and long wavelength photoreceptors that perceive blue, green, and red colors, respectively. Other shades can be thought of as being more or less intense variations of red, green, or blue – often cited examples are pink, which is a very bright red, and brown, which is a rather dark red – or as a result of color blending.

Once one knows how the human eye captures and processes color, a color model can be created to describe the colors people see and understand how those colors can be matched. It should also be noted that there is a mental aspect to human color perception as well as a physical one.

For example, the human mind will "balance" colors. So that a cream or very light yellow light may look (be perceived as) white in some circumstances, but when blended with pure white will better retain its actual color.

A Standard Way of Representing Color

Colorimetry can be described as the science of representing the colors that humans perceive with three sets of numbers that correspond in a limited way to the cones’ short, middle, and long wavelength responsitivity along with monochromatic data from the aforementioned rods and a measure of brightness.

In 1931, an organization called the Commission Internationale de l'Eclairage (CIE) or International Commission on Illumination established what is known as the CIE XYZ color space, which, among other things, provides a two-dimensional xy representation of the luminance (comparative brightness) and chrominance (color quality) of visible light.

The CIE XYZ color space, which looks something like a shark’s fin when graphed, makes it easier for light manufacturers to describe the range of light a given source, for example an LED, emits. It also makes it easier to reproduce colors captured (measured) with an instrument like a spectroradiometer or tristimulus colorimeter. In a practical sense, a lighting engineer might use these tools, along with the color space, to design architectural lighting for a fountain.

Apply the Science

In applied lighting, colorimetry may help with the selection of light source colors, potentially matching a client’s requirements. A lighting contractor familiar with CIE XYZ, as an example, could explain to clients, in a very precise way, how colored lighting might blend in the finished project.

Engineers regularly depend on the CIE XYZ color space or derivatives when selecting semiconductor materials, since diode composition determines an LED’s color.

In fact, color science, whether recognized or not, is everywhere to be seen.