What Are Lumens in Projectors?

In projectors, lumens are used to measure "luminous flux," or the perceived output of light. Luminous flux is not a measurement of total light output, but a measurement of what light can be seen by the human eye. In most projectors the light output is expressed in ANSI lumens, which are lumens measured in a particular method described by the American National Standards Institute.

History

In 1992, the American National Standards Institute (ANSI) defined a method for measuring the lumen output of projectors (both film and video). The method involves adjustments of brightness and contrast, measurements of a white field at nine designated spots on the screen, and the averaging of the measurements multiplied by the screen area.

Function

The ANSI method for measuring lumens is very accurate and is therefore used in the marketing of projectors. Manufacturers almost always list the ANSI lumen output of their projectors as a point of comparison.

Math

The lumen is defined as 1 lm = 1 cd·sr = 1 lx·m2 ("cd" is candela, "sr" is steradian, "lx" is lux and "m" is meter).

Misconceptions

In the marketing of home theater projectors, ANSI lumens can be somewhat misleading. The ANSI lumen measurement represents the optimum potential of the projector and does not reflect variables such as screen size, screen material, ambient light and viewer eye fatigue. Some projectors feature video optimization technology that can skew the ANSI lumen rating.

Considerations

In setting up a home theater, the lumen output of a projector can sometimes be irrelevant. It is better to measure the luminance of the light reflecting off one's particular screen. This measurement is expressed in footlamberts (fL).