Lighting allows us to live and work comfortably, day and night. The illuminance levels in a lighting space are evaluated with an illuminance meter to create a comfortable space. Simple illuminance meters are built into smartphones and automobiles, and are used to control the luminance of displays and to turn lights on and off. Illuminance is a physical quantity that expresses the incident light illuminating an object surface, and is recommended by law as the "minimum illuminance" according to the type of work and the purpose of space use, and by Japan Industry Standard (JIS) as the "recommended illuminance." Illuminance meters support our daily lives as important and familiar measuring instruments, with some being designated as Specified Measuring Instruments. In addition, the market for illuminance meters and illuminance sensors is expected to grow in size as a result of advances in automation technology with light, as evidenced by the fact that automobiles sold in recent years are now mandated to automatically turn on their headlights.

In order to use an illuminance meter correctly, it is necessary to calibrate it with a reference standard source for illuminance. The "luminous intensity standard lamp" has long been used as a standard source because it provides a defined standard spectrum and has characteristics such as a clear reference point for the measurement distance. The calibration system for illuminance meters using luminous standard lamps has worked effectively in industry for a long time. However, the global phase-out of incandescent lamps has made it difficult to maintain this calibration scheme. AIST, the national metrology institute in Japan, has received many requests from illuminance meter manufacturers and testing laboratories for the development of a new standard source that can provide the spectrum of CIE standard illuminant A and be used in the calibration of illuminance meters.

The National Institute of Advanced Industrial Science and Technology (AIST) and Nichia Corporation have developed a light source with light-emitting diodes (LEDs) that replaces existing luminous intensity standard lamps by reproducing the standard spectrum of incandescent lamps as defined by the International Commission on Illumination (CIE).

The comfortable lighting level in living and working spaces is evaluated by an illuminance meter. Illuminance meters (illuminance sensors) are familiar measuring instruments that are incorporated into smartphones and used for dimming displays, etc. It is mandatory for new automobiles sold after April 2020 to have automatic headlights in order to avoid no lights at dusk hours for a safety reason. The system is equipped with a function that automatically turns on the headlights when the ambient light level is less than 1 000 lx. In order to properly use illuminance meters from the viewpoint of safety management, it is important to measure and control illuminance accurately, for example, by having them calibrated traceable to the national measurement standards.

Manufacturers and testing laboratories calibrate illuminance meters using "luminous intensity standard lamps," which are incandescent lamps calibrated to be traceable to national measurement standards. However, incandescent lamps are no longer produced worldwide, and as a result of this phase-out, the discontinued luminous intensity standard lamps have become a global concern.

In response, AIST and Nichia Corporation have developed a light source (Illuminant A standard LED) that provides the standard spectrum (CIE standard illuminant A) using LEDs. The Illuminant A standard LED not only meets the specifications for spectrum and illuminance values required for calibration of illuminance meters, but also improves the aging rate to about 1/20 of a luminous intensity standard lamp by an appropriate seasoning process of the LED package. As a result, it is expected to extend the recalibration periods and improve the measurement uncertainty in manufacturers and testing laboratories.

Details of this technology were published in Measurement on August 16, 2024.

Journal: Measurement
Title of paper: LED-based standard source providing CIE standard illuminant A for replacing incandescent standard lamps
Author： Kenji Godo, Kenichi Kinoshita, Yuri Nakazawa, Kohei Ishida, Ai Fujiki, Mako Nikai, Yumiko Niimi, Hideki Teranishi, Tetsuya Nishioka
DOI: 10.1016/j.measurement.2024.115479