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Gonio-Photometer

The gonio-photometer (or goniometer for short) is able to measure the illuminance from each portion of a lamp or fixture. There are three main components to the goniometer: (1) the rotating table that the fixture or lamp is placed on, (2) the long arm with a mirror on the end that rotates around the fixture and (3) a light sensor that measures the light reflected by the mirror.

The light source (whether it is in a fixture or not) is placed in the middle of the goniometer, sitting on the rotating table. The rotating table can be adjusted up and down to make sure that the light source is in the very center of the goniometer. When the lamp is positioned this way, the mirror at the end of the rotating arm is positioned so that it reflects the light from the light source directly to a light sensor at the opposite end of the room.

The arm with the mirror rotates from bottom to top around the lamp, stopping every few degrees so the light meter can take a measurement. The time-lapse picture above was taken from almost the same position as the light sensor. The picture shows how the sensor can see the lamp from every angle that the mirror reaches. One rotation from bottom to top of the mirror is called a "sweep."

After the first sweep, the mirror is lowered back to the bottom and then the rotating table rotates a pre-set number of degrees (depending on the symmetry of the light source and how detailed we want our measuremens to be). After the table rotates, the arm with the mirror does another sweep of measurements.

This process is repeated until we obtain a map of the entire lamp. A complete test takes anywhere from ten minutes to two hours, depending on the detail we need (greater detail takes more time).

The graphic above demonstrates the sort of output we obtain from our measurment. Each of the three graphs shows only one sweep of the mirror arm. Even though the mirror arm only rotates 180 degrees around the lamp, if the fixture is bi-symmetrical we are able to reflect the results across the 0-180 degree axis and obtain a 360 degree graph of the fixture illuminance.

The longer the radius from the center to the plot line at a certain angle, the greater the intensity at that angle. So to find out the illuminance at 90 degrees for the graph at the left, follow the line from the center of the graph to the 90 degree mark. The plot line intersects the scale at about 80 candela (a unit of measurement for light). This can be done at any angle around the graph.