02-05-2012, 03:53 PM
How Bright is that LED Bulb
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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.)