LED Lighting University - LED Light Bulb Brightness and Illumination by EagleLight

Various Definitions of LED Brightness: lux, lumen and watt

In this lesson we are going to define some of the many terms used in light brightness such as watt, lumen, and lux.

Here we will discuss:

• measuring the amount of light produced from an LED light source
• measuring the amount of light we see from an LED light source
• measuring the amount of usable light produced from an LED light source

This discussion is not a rigorous scientific analysis, it is intended to offer an understanding of a few of the various terms used related to LED light bulb brightness.

Watt as a measure of LED brightness

A measure of brightness used in the past was the wattage of a light. With more efficient lighting products like CFL and LEDs, use of the watt is no longer an accurate means of determining brightness across multiple lamp technologies. For example: a 75 Watt incandescent bulb, 18 watt CFL and a 9 watt LED may all produce the same amount of usable light. Obviously watt is not longer a viable means to measure the brightness of a light source.

LED Lumen as a measure of brightness

The LED lumen measures the total amount of visible light from a light source. Unfortunately some light may be used to light areas not even visible to the viewer. Such as the lighting of the inside of a fixture. This is why the measure of the light found on the projected surface is often a much better measure of applicable light. LED lumens often times are less than the lumen measure of an other light technology for the same amount of brightness on the working surface. This is due to the directed nature of LED packaging. This direction of the LED lumen results in greater use of the available light.

LED Lux as a measure of brightness

The measure of the amount of visible light on a surface, like the lux, is a good way to determine the amount of "usable" light created by a light source.

Light Measurement	Definition	Form of Measure
Radiant Flux	the total power of light: visible and non-visible light	watt (W)
Luminous Flux or Luminous Power	the perceived power of light adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light	lumen (lm)
Luminous Intensity	the amount of lumen on a square meter	lux (lx)
Luminous Efficiency	the amount of visible light produced per each watt consumed	lumen/watt (lm/W)

The radiant flux is the total amount of light generated, this includes light that is not visible to the naked eye like infrared and ultraviolet.

The luminous flux is an adjustment to the measurement of radiant flux that measures only that light perceived by the human eye. The LED lumen (Lm) is a measure of the amount of LED light produced that we can potentially see.

The luminous intensity is an adjustment to the luminous flux that measures the amount of light on a surface. Think of this as the usable light.

When purchasing LED light bulbs you may see that the lumen listed on the LED does not match that of an incandescent, but the lamps offer the same about of light on your desk or table. This is because the light from LEDs are often "directed light" where an incandescent light shines light in all directions: only a part of which is used to light up what you are intending.

When purchasing light bulbs the lumen is one measure to determine relative brightness, however use of the lux gives you a measure of the light hitting the surfaces you are trying to light.

When choosing a lightbulb, determine the appropriate lux or lumen required and then select that bulb with the fewest watts. Lower wattage means lower electricity bills. Checking LED lumens directly against traditional bulb lumens is only appropriate under certain lighting conditions and certain light bulb shapes and fixture designs.