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. In June, NIST has started providing faster, more accurate, and labor-saving calibration services for evaluating the brightness of LED lights and other solid-state lighting products. Customers of this service include LED light manufacturers and other calibration laboratories. For example, a calibrated lamp can ensure that the 60 watt equivalent LED bulb in the desk lamp is truly equivalent to 60 watts, or ensure that the pilot in the fighter jet has appropriate runway lighting.

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. This service involves measuring the brightness of the customer’s LED and other solid-state lights, as well as calibrating the customer’s own photometer. 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. Scientists have also significantly shortened the calibration time. In old systems, performing a calibration for customers would take almost a whole day. NIST researcher Cameron Miller stated that most of the work is used to set up each measurement, replace light sources or detectors, manually check the distance between the two, and then reconfigure the equipment for the next measurement.
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. The distance can be controlled within 50 parts per million of one meter (micrometer), which is approximately half the width of human hair. Zong and Miller can program tables to move relative to each other without the need for continuous human intervention. It used to take a day, but now it can be completed within a few hours. No longer needs to replace any equipment, everything is here and can be used at any time, giving researchers a lot of freedom to do many things at the same time because it is completely automated.
You can return to the office to do other work while it is running. NIST researchers predict that the customer base will expand as the laboratory has added several additional features. For example, the new device can calibrate hyperspectral cameras, which measure much more light wavelength than typical cameras that typically only capture three to four colors. From medical imaging to analyzing satellite images of the Earth, hyperspectral cameras are becoming increasingly popular. The information provided by space-based hyperspectral cameras about Earth’s weather and vegetation enables scientists to predict famines and floods, and can assist communities in planning emergency and disaster relief. The new laboratory can also make it easier and more efficient for researchers to calibrate smartphone displays, as well as TV and computer displays.

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 (colors), and its brightness is very clear because measurements are made using NIST standard photometers. Unlike lasers, this type of white light is incoherent, which means that all light of different wavelengths is not synchronized with each other. 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. The use of tunable lasers increases the signal-to-noise ratio of the measurement.
However, in the past, tunable lasers could not be used to calibrate photometers because single wavelength lasers interfered with themselves in a way that added different amounts of noise to the signal based on the wavelength used. As part of laboratory improvement, Zong has created a customized photometer design that reduces this noise to a negligible level. This makes it possible to use tunable lasers for the first time to calibrate photometers with small uncertainties. The additional benefit of the new design is that it makes the lighting equipment easier to clean, as the exquisite aperture is now protected behind the sealed glass window. Intensity measurement requires accurate knowledge of how far the detector is from the light source.
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. A common solution is for researchers to first measure the illuminance of the light source and illuminate a surface with a certain area. 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.
This method uses a microscope based camera, with a microscope sitting on the light source stage and focusing on the position markers on the detector stage. The second microscope is located on the detector workbench and focuses on the position markers on the light source workbench. Determine the distance by adjusting the aperture of the detector and the position of the light source to the focus of their respective microscopes. Microscopes are very sensitive to defocusing, and can recognize even a few micrometers away. The new distance measurement also enables researchers to measure the “true intensity” of LEDs, which is a separate number indicating that the amount of light emitted by LEDs is independent of 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. In the coming months, Miller and Zong hope to use a spectrophotometer for a new service: measuring the ultraviolet (UV) output of LEDs. 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.