Lamp calibration lab speeds up LED characterisation: Page 2 of 2

September 06, 2019 //By Julien Happich
calibration lab
Last June, the National Institute of Standards and Technology (NIST) began offering a faster, more accurate and less labour-intensive calibration service for assessing the brightness of LED lamps and other solid-state lighting products.

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.

A light source called an FEL lamp shines from one of the
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) -

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