This causes variations in measured luminous intensity when measured at different distances, especially when the distance is short. The effective center of LED emission can shift from the physical center of the LED. Measuring luminous intensity of LEDs at short distances is problematic because many LEDs have epoxy lenses, and they do not behave as a point source and the inverse-square law does not hold well. This practice still prevails even though LEDs are much brighter. The tradition presumably came from the time when LEDs were very dim and photometers were not very sensitive. However, a common practice in the LED industry has been to measure LEDs at much shorter distances, such as 10 cm to 50 cm. The luminous intensity (unit: candela) of LEDs can be measured with a conventional photometric bench and the standard photometers under a far field condition, at a distance far enough so that the test LED can be regarded as a point source (typically 2 m or longer). This article discusses measurement of luminous Intensity, total luminous flux, total spectral radiant flux, and color quantities of LEDs, as well as NIST measurement facilities and calibration services for LEDs. The National Institute of Standards and Technology (NIST) has recently developed and expanded capabilities for calibrating LEDs for photometric, radiometric, and colorimetric quantities and provides various calibration services for LEDs. To assure high accuracy LED measurements, reference standard LEDs and calibration services have been in high demand. The temperature-dependent characteristics and great variability of optical designs of LEDs make it even more difficult to reproduce measurements.

Traditional standard lamps do not meet the calibration needs for LED measurements as LEDs differ substantially from traditional lamps in terms of physical sizes, flux levels, spectra, and spatial intensity distributions.

Various new types of light emitting diodes (LEDs) are being developed and introduced for general illumination and other applications, and there are increasing needs for accurate measurements of various optical parameters of LEDs.

Luminous intensity unit