Lighting-speak involves a great deal of bewildering and confusing terminology: color temperature, lumens, color rendering index, spectrum, and so on.
On these pages we’ve tried to keep the jargon to a minimum. Nevertheless, there is some terminology we’ve been unable to avoid, and other terminology that we know you’ll encounter as you read about lighting, or discuss lighting with others.
Here we’ve tried to collect the most common, if unfamiliar terms that you may encounter and provide straight-forward explanations of each.
If we’ve missed something, or if something remains confusing, drop us a line at contact@NightSkySantaFe.Org and we’ll work to get you an explanation and, if it is of general interest, add it to this page.
Common Lighting Terminology
Possibly the most commonly used and commonly misused terminology in all of the lighting terminology kingdom. It is so commonly used and misunderstood that we’ve devoted an entire page to it.
In a few words, Color Temperature is a way of comparing the color of the light from some source to the color of light emitted by a body heated to a given temperature. The color temperature of a light is the temperature of the incandescent light “closest” in color.
Keep in mind that “closest” does not mean “close”: the distinctive yellow of the high-pressure sodium lights now in use on Santa Fe streets has a CCT close to candlelight, but no one would ever mistake high-pressure sodium light for candlelight.
Color temperature is about color, not brightness. A candle held far away has the same color as one held nearby; however, the distant candle will appear less bright than the closer candle. Likewise, two candles held together will have the same color as a single candle, but will appear twice as bright as a single candle alone.
When discussing street lighting there are only a few numbers to keep in mind:
1850 – 2000 K: The golden-amber of candlelight
2200 – 2500 K: Bright yellow
3000 – 3500 K: The white to slightly blue-white color of an early morning of evening clear sky
4000 – 4500 K: The bluish-white color of a mid-morning or afternoon to late afternoon clear sky
Lumen is a measure of the total amount of light a lamp emits. Just as the amount of water pouring from a faucet is measured in gallons per hour, so the amount of light put-out by a lamp is measured in lumens.
Watts are the way we measure electrical power. They are important when we want to know how efficient a light is: that is, how much electrical power it takes to produce so many lumen.
Back in the day, when the incandescent bulb was king, it was common to use watts to describe both the amount of electricity used and the brightness of a bulb. We could do that because there it was pretty much the case that each watt of energy in produced 14 lumens of light out.
Now that we have different types of lighting that simple equivalence no longer holds. For instance, a 9 watt indoor LED light might produce as much light (lumens) as a 60 watt incandescent bulb.
Lux is how we describe how much light there is on a roughly one-yard square surface. You can safely think of it as how brightly illuminated something is.
When we talk about how brightly lit a street should be, we talk about the “number of Lux” on the street level, or “at grade”.
Color Rendering Index
Our eyes evolved to recognize color under incandescent light. Under different kinds of lighting – e.g., fluorescent lights, or high-pressure sodium lights – our eyes become less capable of distinguishing different colors. The Color Rendering Index, or CRI, is a measure of how well the eye can distinguish object colors when illuminated under different kinds of light. A CRI of 100 is as good as it gets. A CRI greater than 90 is considered excellent. A CRI of 80 to 90 is considered good.
CRI is independent of color temperature: CRI tells us about how well we can identify the color of something that is illuminated by the light; CCT tells us about the color of the light itself.
CRI has typically not been a consideration for outdoor street lighting because the CRI for the most popular previous technology – high-pressure sodium – lights was generally a very poor 25.
Modern commercial LED street lighting can produce light with CRIs in the high 60s to 85 or more, which greatly improves their safety across all color temperatures.