How Bright is that LED Bulb?
How bright is it? That’s a simple question that deserves a simple answer.
How do we compare the brightness of Compact Fluorescent Light (“CFL”) bulbs to incandescent lights? We say that a 13 watt CFL with the brightness of a 60 watt incandescent bulb is a “60 watt-equivalent, 13 watt CFL.” The CFL bulb uses only 13 watts of power (what you pay for,) but it generates the light of a 60 watt incandescent bulb. Therefore, the CFL uses only 13/60 = 22% of the power of the incandescent to generate the same brightness.
For white LED’s we use the same system, and rate our bulbs by “watt-equivalent,” with incandescent light as the reference just like before. So, a 7-watt LED that generates the brightness of a 70-watt incandescent bulb is a “70 watt-equivalent, 7 watt LED.” The LED uses only 10% of the power of the incandescent to generate the same brightness. That 90% energy savings is typical for LED’s, versus incandescents. (Incandescents are inefficient light sources: they generate over 90% heat and less than 10% light.)
That may be all you wanted to know about “how bright it is.” Just compare watt-equivalents to get an idea how that LED will compare with an incandescent in brightness.
You may notice we've marked a few of our bulbs down in "watt-equivalents." The reason is that, while our center beam measurements support the former ratings, our customers are using the bulbs for some general illumination applications, and not just task, accent and spot lighting. Therefore, to give a better indication of substitutability, we've taken the numbers down. Call if you have questions.
Directional versus “General” lighting.
LED’s are inherently directional. Directional lighting means illumination on the work-plane or an object mainly from a single direction.
Incandescent bulbs are inherently non-directional—they send the light out in all directions from the glowing filament. Incandescents (and Compact Fluorescents) are better for “general” lighting, such as filling a space with light.
Measuring Light: Lumens versus Footcandles
Directional lights should be measured using Footcandles (the light that arrives where you directed it.) “General” lighting should be measured using lumens (the light sent out in all directions.)
Since LED’s are inherently directional, they make excellent spot and flood lights and less-effective “general” lights. Therefore, it doesn’t make much sense to measure LED floods and spots in lumens. We measure their brightness in illuminance, convert to luminance, and then to watt-equivalents.
To simplify things, we assume that the light delivered per watt for an incandescent is constant.
What’s the matter with using Lumens to measure directional light?
Lumens are a spherical measurement, not a directional one. Incandescents generate a lot of lumens, but much of that output is useless or even annoying. You may be surprised by how much useless light comes out of the back of the incandescent reflector bulb. In a ceiling can, that light bounces around your ceiling can and heats up your ceiling. You might also notice how much light an incandescent sends sideways from the bulb. That light creates annoying unwanted glare. We can do much better than lumen output for measuring directional light. That’s why we measure luminance.
What does “Brightness” really mean?
Let’s refine what we mean by “brightness.” The photometric quantity most closely associated with brightness perception is luminance, measured in units of luminous intensity (candelas) per unit area (square feet or square meters). Brightness at the center of the beam is called Center Beam Candle Power (CBCP) (in candelas, formerly candles.) It’s easy to measure illuminance with a light meter, but you have to multiply it by the distance from the light squared to get the luminance at beam center, the CBCP.
A light meter measures illuminance, in foot-candles (or, in the metric system, lux.) Illuminance is the light arriving at a surface, expressed in lumens per unit area; 1 lumen per square foot equals 1 footcandle.
At EnvironmentalLights.com, we measure illuminance 6 feet from the light. We multiply the footcandles by the distance (6 feet) squared to get the Center Beam Candle Power, or brightness. A typical 65 watt incandescent R30 flood light has illuminance of 11 footcandles 6 feet from the bulb, so the luminance at beam center, (Center Beam Candle Power) is 396 candelas [11 footcandles x (6 feet) x (6 feet)]. That’s how “bright” it is.
Beam Spread
The beam angle is the angle across the cone of light from one beam edge to the opposite beam edge. The beam edge is the cone-shaped surface where the beam intensity is 50% of what it is at the center of the beam. (50% is somewhat arbitrary, but seems to be the most commonly used figure.)
A flood light has a beam spread of around 22 degrees or more. A spot has a beam spread of less than 21 degrees. (21° is also arbitrary, but commonly used as the dividing point between floods and spots.)
If you compare a flood and spot with the same light elements (and lumen output,) the flood has a lower “brightness,” as defined by Center Beam Candle Power (luminance) because the bulb is spreading the light over a larger cone. That may not seem entirely fair to the flood light, but research has linked human perception of “brightness” with luminance, so using CBCP is valid. And converting CBCP to Watt-equivalents using a constant ratio is also valid for our purposes.
It is interesting to note that when we sell a flood and spot with the same light elements and prices, the floods out-sell the spots, but the spots “look” brighter. This suggests that people care about the beam spread at least as much as the overall perception of brightness.
Summary
The question, “How bright is that light?” can be answered for a flood or spot light by multiplying the Center Beam Candle Power, in candelas, by (0.17 watts/candelas), which is the typical ratio of watts to candelas at beam center for incandescent reflector bulbs.
Example:
An LED with a measured illuminance of 15 footcandles 6 feet from the face of the bulb has a CBCP = 15 x 6 feet x 6 feet = 540 candelas. That LED is a 92 watt-equivalent bulb (540 cd x 0.17 watts/cd = 92 watt-equivalents.)
Scientists may argue over the approximations and simplifications we have made; however, this is a useful, easy to understand way of understanding how bright a directional light is without getting too technical.
Compare “watt-equivalents” to answer the question “How bright is it?” Now that’s simple.
EnvironmentalLights.com aims to bring you the
- best energy efficient lights,
- best information, and
- best service.
We test your LED bulbs so you don’t have to. And we tell you what they can do, by the numbers. But the ultimate arbiter of the quality of light is YOU. So try some of our LED lights and see which you like best.
Glossary of Terms Relating to “Brightness”
accent lighting: directional lighting to emphasize a particular object or to draw attention to a part of the field of view. See directional lighting.
beam angle: the angle between the two directions for which the intensity (candlepower) is 50% of the maximum intensity as measured in a plane through the nominal beam centerline (center beam candlepower).
beam spread: (in any plane) the angle between the two directions in the plane in which the candlepower is equal to a stated percent (usually 10%) of the maximum candlepower in the beam, the Center Beam Candle Power.
brightness: see luminance.
center beam candlepower (CBCP): Center beam candlepower is the luminous intensity at the center of a beam, expressed in candelas (cd).
direct lighting: lighting by luminaires distributing 90 to 100 percent of the emitted light in the general direction of the surface to be illuminated. The term usually refers to light emitted in a downward direction. (See accent lighting.)
directional lighting: illumination on the work-plane or on an object predominantly from a single direction.
footcandle, fc: a unit of illuminance equal to 1 lumen per square foot. One footcandle equals 10.76 lux.
general lighting: lighting designed to provide a substantially uniform illuminance throughout an area, exclusive of any provision for special local requirements.
glare: excessive brightness that may be caused by either direct or indirect viewing of a light source; any brightness or brightness relationship that annoys, distracts or reduces visibility.
illuminance: light arriving at a surface, expressed in lumens per unit area; 1 lumen per square foot equals 1 footcandle, while 1 lumen per square meter equals 1 lux.
luminance, L: light reflected in a particular direction; the photometric quantity most closely associated with brightness perception, measured in units of luminous intensity (candelas) per unit area (square feet or square meters).
lumen, lm: a unit of luminous flux; the overall light output of a luminous source is measured in lumens. A unit measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time.
lux, lx: a unit of illuminance equal to 1 lumen per square meter. One lux equals 0.093 footcandle.
watt, W: a unit of electrical power (energy) equal to 1 joule per second. Volts times amps.
work plane: the plane at which work usually is done, and on which the illuminance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 meters (30 inches) above the floor.