The lumen is the unit scientists use in the study of luminous flux from a given light source. Luminous flux is the amount of radiative energy passing a given point every second that the human eye can see. The National Institute of Standards and Technology measure lumen output of an unknown source by using an integration sphere. The unknown source is placed inside the sphere while a light source with a known amount of lumen is set up to shine into the sphere. Two photometers which count light particles inside the sphere coming from each light source are also needed.

Measure the photometer signal from the known light source outside the sphere. As an example, say the known light source photometry reading is 500,000 counts.

Measure the light particle count for the unknown light source that is inside the sphere. For the example calculation, use 700,000 counts for the unknown source.

Divide the unknown photometer reading by the known photometer reading. This leads to 700,000 divided by 500,000 or simply, 1.4.

Multiply the known lumen output of the light source exterior by the sphere to obtain the lumen output for the unknown light source. Assume the lumen output for the known source is 10,000 lumens. The lumen output for the unknown source is then 10,000 lumens times 1.4 or 14,000 lumens.

#### Tip

This guide assumes the integration sphere is operating in ideal conditions.