# Determining Light Brightness and Intensity of LEDs

## Lighting Specifications Explained

Many 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-".

1000 millicandelas = 1 candela

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

Common Units used to Measure Light Brightness

 Unit Translation Candela Brightness of a common candle Millicandela 1/1000th of a candela Lumen 1 candela•steradian Lux 1 lumen/square meter Footcandle 1 lumen/square foot