How is a photometric test arranged?
A fixed layout of fixture, photometer and darkness, so a light’s intensity can be measured the same way every time.
A photometric test measures how much light a fixture sends in each direction. To make the result repeatable, the arrangement is fixed: the fixture sits on a goniometer that turns it through measured angles, a photometer reads the light at a set measuring distance, and the room is darkened and baffled so that only the beam is measured.
The fixture runs at its specified test current and is left to stabilise before readings begin. Its reference axis is aligned with the goniometer and the photometer, then intensity in candela is recorded at each angle to build the isocandela data.
Frequently Asked Questions
Q1What is a photometric testing arrangement?
The physical layout used to measure the light output of a fixture: the fixture on a goniometer, a photometer at a fixed distance, a controlled power supply and a darkened room, with a logger recording the readings.
Q2What does the goniometer do?
It turns the fixture, or the photometer, through precise horizontal and vertical angles so that intensity can be measured at each point of the beam pattern.
Q3Why must the test room be dark?
Stray light from walls, floors or other sources adds to the photometer reading and falsifies the result. Dark surroundings and baffles make sure that only the light from the fixture is measured.
Q4Why is the measuring distance important?
Intensity in candela is defined for a point source, so the photometer must be far enough from the fixture for it to behave like one. The distance is set by the test standard and kept the same throughout the test.
Q5Why is the fixture warmed up before testing?
Light output and colour change as the lamp or LEDs reach working temperature. Readings are taken at the specified test current once the output has stabilised, so the result can be repeated.
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