Lamp calibration lab speeds up LED characterisation
Well-calibrated lights ensure that the 60-watt-equivalent LED bulb in your desk lamp truly is equivalent to 60 watts, for example, or that there is proper runway lighting for the pilot in a fighter plane. NIST scientists recently renovated the lab they use for these tests, automating most of the measurements so they can be done in half the time it took prior to the lab’s renovation.
LED manufacturers need to ensure that the lamps they make are really as bright as they designed them to be. To do that, they calibrate those lamps with a photometer, a tool that measures brightness at all wavelengths while taking into account the human eye’s natural sensitivities to different colours. Until recently, the NIST lab has measured lamp brightness with reasonably low uncertainties, ranging from 0.5% to 1.0%, on par with mainstream calibration services.
Now, thanks to the lab revamp, the NIST team has reduced those uncertainties by a factor of three, to 0.2% or less. The scientists have also cut the calibration time significantly. With the old system, it took almost a full day to do a single calibration for a customer.
“Most of that was devoted to setting up each measurement, swapping out a light source or a detector, manually checking distances between the two, and then reconfiguring the equipment for the next measurement”, explained NIST researcher Cameron Miller.
But now, the lab consists of two automated equipment tables, one for the light sources and the other for the detectors. The tables travel on a rail system that positions the detectors anywhere between 0 to 5 meters away from the lamps. The distances can be controlled to within 50 millionths of a meter (micrometers), which is about half the width of a human hair. Zong and Miller can program the tables to move in relation to each other without requiring continuous human intervention. What used to take the better part of a day can now be done in hours.
With additional capabilities in their lab, NIST researchers expect their customer base to expand, for example to calibrate hyperspectral cameras, which measure many more wavelengths of light than typical video cameras, which usually capture only three or four colors. The new lab also allows researchers to more easily and efficiently calibrate smartphone displays and television and computer monitors.
new automated equipment tables. This table holds all the
sources of light that are either being tested or being used
to test a detector. Credit: Jennifer Lauren Lee/NIST.
To calibrate a customer’s photometer, NIST scientists used to illuminate the detector with a broadband light source. Unlike a laser light, this white light is incoherent, which means that all the different wavelengths of light are out of step with each other. Ideally, to make the most accurate measurements, researchers would use light created by a tunable laser, whose wavelength can be controlled so that only a single wavelength of light at a time shines onto the detector. Using a tunable laser increases the signal-to-noise ratio of their measurements.
However, in the past, a tunable laser could not be used for calibrating photometers, because the single-wavelength laser light interferes with itself in a way that adds different amounts of noise to the signal depending on which wavelengths were used.
As part of their lab improvements, NIST’s Zong created a custom photometer design that has minimized this noise to the point where it is negligible. This has made it possible, for the first time, to use a tunable laser for photometer calibration with small uncertainties.
Another novelty, light source distance measurement uses microscope-based cameras. One microscope sits on the light source table and focuses on a position marker on the detector table. A second microscope sits on the detector table and focuses on a position marker on the light source table. Distances are determined by adjusting the detector’s apertures and the position of the light sources to the focal points of their respective microscopes, offering micrometer-level accuracy. The new distance measurements have also allowed researchers to gauge the LEDs’ “true intensity,” a single number that indicates how much light the LED is putting out irrespective of distance.
The lab also boasts a goniophotometer, which allows to measure how much light is emitted by an LED lamp at different angles.
National Institute of Standards and Technology (NIST) – www.nist.gov
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