why Daylight Color Temperature is best for work!

Here's Why:

1. Daylight directly stimulates human performance.

2. The Eye was made for Daylight.

If you understand how the rods and cones of the eye work, you'll see that high color temperature light, (greater than 5600K), is nearly always best for shops. Since we are creatures made to live and work in daylight it makes sense that the eye would be made to best process this kind of light. So as a public service, here is a semi- scholarly explanation on how "color temperature" affects vision You can use this information to get the best lighting possible when ever you need lights from whom ever you buy.

Daylight (6000K) LEDs don't cost any more and they are worth finding. (Later I will discuss the unfortunate reason why they are so hard to find.)

Below I discuss color temperature, how to compare the lumens between lights with different color temperatures and how many foot candles of daylight illumination is required to achieve "natural" lighting.

Color Temperature

When buying lights, you may notice a specification called "Color Temperature". It describes the relative blue spectrum versus red spectrum content of light. Low color temperate light sources ( eg. 2700K, 3500 K) are a reddish yellow and often simulate incandescent light spectrums or the color of sunlight just as the sun rises or sets. The color temperature of the sun is 5780K. Here on earth, light from the Sun is mixed with blue light from the sky and light from clouds. So at noon, the color temperature of daylight ranges from 5600K to 6000K. And during an overcast afternoon the color temperature of outdoor light is about 6500K. Light with a color temperature of 6000K or 6500K is actually very, very blue because the sky is blue. But as we shall see, our remarkable brains compensate and interpret daylight as being white under the right conditions.

Not All Lumens of Light are the Same

Light sources are usually rated in terms of "lumens". But our eyes can use both rods and cones depending upon how bright the illumination is. A nd the rods and cones of the eye see light differently. The upshot is that high color temperature light will be perceived as brighter and clearer at the light levels that should be installed in shops.

For instance, if you have a shop illuminated with 6500K bulbs and then put in a single 5000K bulb for comparison, the 5000K bulb will look yellow and dim as if it's "dirty".

To help you understand why, look below at the images of a flower as perceived by the eye under different light intensities:

In full sunlight (a), you can see that reds are brilliant and that the leaves have a more yellowish hue. In full sunlight, there is so much light available that the brain can afford to waste it. The brain will only use the cones of the eye, which are responsible for color vision. But as light levels decline , see photo(b), you'll see that the red flower loses its brilliance and that the leaves turn from yellow-green to blue-green. This is known as the "Perkinje Effect".

At less than full sunlight, the brain uses both the cones and the rods. The rods are used for black & white vision and are completely blind to red light and not so sensitive to yellow light. But they are very sensitive to light closer to the blue end of the spectrum. In other words, lower color temperature light becomes less visible indoors and higher color temperature illumination is perceived as being brighter.

A general rule is that you can use 25% fewer daylight fixtures

Kruithof's Principle

But there are limits on the smallest level of high color temperature light you can use before it starts looking stark.

We've seen that the actual color of outdoor light changes throughout the day -- from reddish-yellow to deep blue, yet our amazing brains will interpret all of this light as being "white" to facilitate visual acuity. However, when light levels are low, the brain expects to see reddish-yellow light. If you have too much reddish-light, we interpret the illumination as being "harsh".

At higher light levels, the brain expects to process bluer light. But If you illuminate a space with just a little high color temperature light, the resulting illumination will appear "stark" and unnatural. The light from the fixtures will actually look blue.

For every light intensity, there is a sweet range of color temperatures for which light appears white. This is summarized in the following Kruithof curves plotting light level vs color temperature:

Elsewhere on this website I show you how bright a work light needs to be. For difficult tasks, you want 100 foot candles (1076 lux). At that level of illumination, the light from standard 6000K LEDs is perceived as being white.

Look for Daylight (5600K+)

Unfortunately, most manufacturers don't make 5600K+ work lights. That's because they make weak lights that only put out a few hundred lumens. With such dismal output, they have to use 4000K LEDs due to Kruithof's Principle. So you get low levels of light that you can't see very well.

The story is the same for shop lights. In the old days, daylight light sources were not very energy efficient. So most lights were 4000K or 4100K. Once LED technology appeared, manufacturers have continued making 4000K lights with just a handful adopting the inferior 5000K spectrum.

But now quality daylight LEDs are available. We must move the market by demanding them.

Caveats

My recommendation for daylight color temperatures presumes that you have ordinary white walls. If you do something really crazy, like paint your shop with red and yellow stripes, you might be better off with light spectrums heavy in red and yellow light that will reflect off the walls.

Daylight directly stimulates human performance.