Determining Light Brightness and
By Robert Cong and the Jameco Technical Team
Intensity of LEDs
Lighting Specifications ExplainedMany people often wonder what all those LED specifications mean. Watts, lumens, flux, wavelength, candle power, candelas, millicandelas are just a few of the many terms and units of measurement used to describe light intensity. Although photometry is much more complicated than I can even begin to explain, here are just a few things you should look for to determine the brightness of a light source.
When looking at brightness specifications of LEDs, the most common specs available are luminous intensity (usually measured in units of candelas or millicandelas) and viewing angle (measured in degrees). The brightness of 1 candela is roughly around the same brightness as a common candle. A millicandela, or mcd, is 1000 times less bright than a candela, hence the prefix "milli-".
Since light is not always evenly dispersed, the viewing angle of a light source is very important. The output of light is determined by the location of the beholder, so if you want a single light source that will light up an entire room, make sure the viewing angle is wide enough to provide such light.
This is also a matter of the lens; diffused lenses provide a wider viewing angle than a clear lens, but the tradeoff is that diffused lenses may make the LED appear dimmer than it normally would
One candle is the basis of light intensity
LED with Diffused Lens
LED with Clear Lens
The wattage of an incandescent lamp or LED lamp tells you how much power consumption that particular lamp will draw, not necessarily the power output of the lamp. This is how a smaller wattage rating on an LED lamp can give a higher lumen output than incandescent lamps; LEDs save more energy and are also brighter.
Another spec to look at is luminous flux, or luminous power, which can be found if the luminous intensity and viewing angle are known. Luminous flux is the power of light as perceived by the human eye with respect to the wavelength of light being emitted, and is usually measured in lumens.
Note: Without getting too mathematical, the viewing angle in degrees is converted to steradians, then multiplied by candelas to get lumens.
Here is a helpful site that allows you to easily convert from candelas to lumens.
As you can see, the viewing angle makes a great difference in the luminous flux. An LED with 5000mcd and 60° viewing angle is about 4 times as powerful as one with just a 30° viewing angle.
Wavelengths do not necessarily provide much information on the brightness of a light source, but rather on the color of the light source, as well as the hue of that specific color. Given that certain colors are brighter than others makes the wavelength another specification to take into consideration.
Visible Light Spectrum Chart
For those who want to get more involved, there are light measuring tools available such as a lux meter that can measure up to 50,000 lux or a lux meter that can measure up to 200,000 lux!
Extech LT300 Professional Light Meter
Robert is a graduate from Cal Poly, San Luis Obispo in Electrical Engineering. His interests include sports, movies, music, and playing with cool, new gadgets.