How to Measure a Digital Projector's Brightness

By James T Wood

Projector brightness can indicate the age of the bulb.
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According to the testing standards put forth by the American National Standards Institute, the light of a projector is reported in lumens. Since lumens are the measure of light over a certain area, the lux of specific spots on a projector screen must be measured and then averaged out over the screen's area. A common photographer's light meter is sufficient for measuring the brightness of a projector with a fair degree of accuracy.

Determine the square footage of the screen on which you are going to project. Measure the length and the width, and then multiply the length by the width to get the square footage. A screen that is 5 feet high by 7.5 feet wide would have a square footage of 37.5 square feet.

Remove any external light sources to the best of your ability. Power on the projector and project a white image, a blank word processor screen maximized on your computer, for example.

Take several lux readings with your light meter, and note each reading. Do not take readings from areas of the screen that don't have a white image on them. All readings should be from the same image. Ensure that the readings are spread out over the entire screen area for the most accurate reading.

Calculate the lumens for each reading you took. Lumens are equal to the lux multiplied by the square footage multiplied by 0.09290304. You can calculate each manually or use an automated calculator (see link in Resources). For example, if your reading was 1150 lux on a 37.5 square foot screen, that would be 4006.4436 lumens.

Average your lumen readings. Add all of the readings you took after converting them to lumens, and then divide the total by the number of readings you took. For example, if you took 5 readings of 4006, 3998, 4100, 3965 and 4053 lumens, your average would be 20122 divided by 5, or 4024.4 lumens.

Items you will need

  • Light meter

  • Tape measure


Compare your lumen reading with the rating of the projector. Projector bulbs are designed to be replaced when they reach half-brightness. So, if your projector was rated at 8000 lumens, and you read only 4000, the bulb should be replaced.

Ambient light can drastically affect the brightness reading. Remove as much as possible for the most accurate reading.