The amount of light emitted by LEDs is independent of distance

  How many measurement scientists are needed to calibrate an LED light bulb?  For researchers at the National Institute of Standards and Technology (NIST) in the United States, this number is half of what it was a few weeks ago.

  LED manufacturers need to ensure that the lights they manufacture are truly as bright as they are designed. To achieve this, calibrate these lamps with a photometer, which is a tool that can measure brightness at all wavelengths while taking into account the natural sensitivity of the human eye to different colors. For decades, NIST’s photometric laboratory has been meeting industry demands by providing LED brightness and photometric calibration services. Until now, The NIST laboratory has been measuring bulb brightness with relatively low uncertainty, with an error between 0.5% and 1.0%, which is comparable to mainstream calibration services.

  Now, thanks to the renovation of the laboratory, The NIST team has tripled these uncertainties to 0.2% or lower. This achievement makes the new LED brightness and photometer calibration service one of the best in the world. In old systems, performing a calibration for customers would take almost a whole day.   But now, the laboratory consists of two automated equipment tables, one for the light source and the other for the detector. The table moves on the track system and places the detector anywhere from 0 to 5 meters away from the light.

Correct distance

  To calibrate the customer’s photometer, Scientists at NIST use broadband light sources to illuminate detectors, which are essentially white light with multiple wavelengths, and its brightness is very clear because measurements are made using NIST standard photometers. In an ideal scenario, for the most accurate measurement, researchers will use tunable lasers to generate light with controllable wavelengths, so that only one wavelength of light is irradiated on the detector at a time.

  Until now, like most other photometry laboratories, the NIST laboratory does not yet have a high-precision method to measure this distance. This is partly because the aperture of the detector, through which light is collected, is too subtle to be touched by the measuring device. Next, use this information to determine these distances using the inverse square law, which describes how the intensity of a light source decreases exponentially with increasing distance. This two-step measurement is not easy to implement and introduces additional uncertainty. With the new system, the team can now abandon the inverse square method and directly determine the distance.

  In addition to these new features, NIST scientists have also added some instruments, such as a device called a goniometer that can rotate LED lights to measure how much light is emitted at different angles. The potential uses of LED for generating ultraviolet rays include irradiating food to extend its shelf life, as well as disinfecting water and medical equipment. Traditionally, commercial irradiation uses the ultraviolet light emitted by mercury vapor lamps.

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