The Chromaticity Of White Light

When shopping for lamps, you’re often given choices for their color temperature, which can be described as warm white, cool white, or daylight. Alternatively, you might come across color temperature values like 2700K or 5000K.

But here’s the thing: our eyes have a nifty feature called “color constancy” that makes us overlook the color of light. For instance, a glass of wine will look red, whether outside during a sunny barbecue or enjoying a candlelit dinner. So, why should you bother with the color of light?

One big reason is that it can affect your health and ability to keep your internal body clock in sync. This mechanism developed in our evolution when we were exposed to bright, bluish daylight in the morning and warm, yellowish light from a campfire in the evening before bedtime.

Usually, we get enough blue light when we’re outdoors, especially in the morning. But if you’re stuck indoors without access to natural light, you’ll need to get your light from lamps. Lamps with high color temperatures, like 4000K or higher, usually have a lot of blue light. And here’s the kicker: avoiding exposure to blue light in the evening is crucial if you want a good night’s sleep. Conversely, low-color temperature lamps, like dimmed incandescent bulbs, have very little blue light and can provide enough light in the evening while steering clear of the sleep-disrupting blue light starting in the late afternoon.

Another reason to think about light color is for style and design in lighting and architecture. For visual appeal, all your lamps should have the same color. Sometimes, lamps get swapped out without considering their color, which can give a not-so-great impression of a space. So, paying attention to color is essential to ensure everything matches seamlessly.

When discussing the color of white light, we use a set of “chromaticity coordinates.” One of these coordinates is correlated color temperature, which historically refers to incandescent lamps’ filament temperature.

The other necessary chromaticity coordinate is called “Tint,” whose value wasn’t much of a concern when we used incandescent lamps because they naturally had no significant tint. But with fluorescent and LED lights, which don’t have an inherent color temperature, we must be precise about specifying and controlling it. Manufacturers define chromaticity tolerances for their products to make things easier, mainly following standards set by organizations like ANSI/NEMA.

This book dives deep into white light’s chromaticity, how we specify it, the acceptable variations, and the industry standards that guide us. It also introduces an improved method for calculating correlated color temperature and tint, building upon the original method described by Robertson.