Understanding Kelvin Temperature, Lumens, And Beam Spread
KELVIN TEMPERATURE, LUMENS, AND BEAM SPREAD
When making decisions about LED lighting, there are three important concepts to understand: Kelvin temperature, lumen output, and beam spread (lighting spread). Each light fixture or lamp will list figures for all of these, so let’s take a closer look at each term and what the numbers tell you.
When we talk about the color temperature of light, as expressed in degrees Kelvin (or “K”), we’re referring to the color of the light. The measurement is a scientific one based on the color of light radiated by something called an ideal black body at different temperatures—but don’t worry about the technicalities here. The thing to remember is that orange or yellow light is produced at lower temperatures (under 3000K) and blue light is produced at higher temperatures (over 4500K).
This would be easy to remember except that when we talk about light in everyday conversation, we describe it in exactly the opposite way. We call the yellowish light of incandescent light bulbs “warm” (in part because the bulbs themselves are warm when they’re turned on) and the bluish light of fluorescent bulbs “cool” (in part because the bulbs are cool when they’re turned on). Our use of “cool” and “warm” here has nothing to do with the Kelvin temperature of light.
Keep this in mind when you’re comparing the Kelvin temperatures of different fixtures: the lower the temperature, the “warmer” the light. Candlelight is about 1850K, noon sunlight is about 5000K, overcast light is about 7000K and a blue sky is about 10,000 K.
One of the drawbacks about the early LED lighting is that the LEDs often had a bluish cast to them—they had a high Kelvin temperature. It’s still true of less expensive LEDs. If you don’t want that blue light, make sure you select lamps or fixtures with a relatively low Kelvin temperature. The range of temperatures in landscape lighting is from about 2700K to 4200K and up, with 2700K being yellower, “warmer” light, 3000K being a clearer “pure white” light and 4200K being bluer, “colder” light. You’ll want to pick the lighting style you like most—though it’s worth noting that the “warmer” LEDs tend to be more expensive.
Before the advent of LEDs, most light bulbs were incandescent, and you could tell how bright they were by how much power they consumed. So if you needed to replace a lightbulb, you’d look at the wattage on the top—35 watts, let’s say—and go to the hardware store to pick up a new 35 watt bulb. So people got used to talking about lamp brightness in terms of watts, which worked fine at the time even though watts aren’t really a measure of brightness.
When LEDs came on the scene it generated a lot of confusion because they could generate a lot more brightness using a lot less power—so we needed a different measurement for brightness. That’s why we started using a unit that measures brightness called “lumens.” Using lumens, we can now compare all kinds of different lamps based on their brightness. Let’s look at a chart comparing the lumen outputs of the most common landscape lighting bulbs:
The last concept you’ll need to understand is beam spread. If you think of the beam emitted by a landscape lighting lamp or fixture as a cone, or, when seen from the side, a triangle, the beam spread is the measure of the angle at the top of the cone or triangle as it leaves the fixture. It’s measured in degrees—not degrees Kelvin, but degrees of a circle, from zero to 360.
In landscape lighting, different beam spreads are used for different purposes. If you want to illuminate a wide area down low, like a rock wall, you’ll want a wide beam spread (usually called a “flood” light), most commonly 60 degrees. If you’re lighting a small tree, you will probably want a medium beam (35 degrees) that will reach the top of the tree with more light. If you’re lighting a tall tree or something narrow like a column, you’re looking for a narrow beam spread—a spot light—15 degrees.
With these concepts in mind, you’ll be ready to confidently specify your lamps or fixtures, and be sure that you’re getting the illumination you’re looking for.