October 28, 2010
Lighting designers — ever wonder where those photometric reports, that tell you the intensity and light distribution of a lamp or luminaire, actually come from? It’s all done on this machine: a goniophotometer.
It’s one of a select few located in the United States, and our lighting studio class took a field trip to Allentown, PA, to the offices of Luminaire Testing Laboratories (LTL), to find it. In simple terms, this machine measures the light output of a lamp or luminaire in one direction by bouncing light off of this enormous mirror (that rotates 360 degrees around the light source), into a photosensor at the opposite end of the room.
Why so complicated, you ask? Well, the rule for photometering (measuring) the output of a light source is the measurement must be taken from a distance of at least five times the length of the source. That way, to the photosensor, all of the light appears to come from one point on the mirror. Without this machine the testing lab would have to be five times as large to accomodate that distance for some of the bigger luminaires.
At any rate, it’s a very expensive, heavy duty machine that’s fun to watch as it spins around. Here’s a video:
Apparently this lab only conducts five to seven tests on it per day. As this is one of the primary lighting testing labs in the country, I can only imagine what their calendar looks like!
The other large piece of equipment in the lab used to photometer lamps and luminaires is the integrating sphere.
It measures all of the light emanating from a lamp or luminaire in every direction at once — this defines the lumen output of the source. A lumen, for those who don’t know, is a measure of the power of light perceived by the human eye.
The sphere is perfectly round and coated with a highly reflective white paint on the inside. Each lamp or luminaire, then, is rigged in the center of the sphere, the two halves of the sphere are rolled and locked together, and since all of the light bounces evenly around inside of the sphere, a measurement can be taken from a single photosensor mounted in the side of it. The number of tests done on this machine in a day varies, because each lamp has to heat up to a stabilized temperature before it can be measured. Depending on the type of lamp this can take anywhere from minutes to hours.
Sadly one of the lab’s spheres was damaged due to one luminaire that leaked grease on the inside of the sphere after heating up. The sphere is so big that they had to cut a hole in the drywall to get it out of the room to ship it out to be re-coated. That’s one costly mistake to repair, as a single integrating sphere of this size can cost as much as $25,000!
The trip was certainly a learning experience in the more technical and engineering sides of lighting. A special thanks to the staff of Luminaire Testing Labs for letting us visit!