1 Values Transfer
The luminous intensity is the main optical parameters of LED device, because of the point light source and anisotropy of some LED, in the near field conditions, LED spectrophotometry test process can produce a lot of errors. So CIE recommended using the concept of the LED average intensity to as LED intensity measurements foundation. Currently, the best LED test technology is to conduct the contrastive test between the measuring LED and the close reference standard sample tube in the spectra and space distribution and its power. The calibration of luminous intensity test instrument and reference standard sample tube must use the same methods and standards.
The luminous intensity unit is kandahar(candela, symbol is CD), it is one of the seven basic units of the international system of units. Before the year of 1979, there were only nine countries which had established the light intensity benchmark, our country has long established the light intensity benchmark and 2856 K color temperature. Therefore, 2856 K light intensity light is determined by the state or province (city) level measurement department as the standard of the luminous intensity transfer tool.
a）Use the light intensity standard lamp of 2856 K color temperature to calibrate and set LED testing system.
b）Use the photoelectric detectors of the CIE standard photometric observer response spectrum to respectively conduct the contrastive test for the selected LED devices and light intensity standard lamp with the color temperature of 2856 K. For the LED device of different spectral power distribution, get their spectrum correction coefficient (SCF, Spectral correction factor), so as to transfer the light intensity value to the LED device and take this as LED light intensity reference standard. But as a reference standard tube, the LED device must have strict requirements. They mainly are:
— Good stability, must be strict screening and aging, must be provided the least aging time
— Use the LED device with the superposition of the optical axis and mechanical axle as the reference sample tube. Or use the light intensity (radiation) distribution graphics approximated by the round device.
— When the reference standard sample tube is working, it must be in the constant temperature and drive current (20 mA) to ensure that it has a constant optical output.
c）Establish the special laboratory to conduct the arbitration test and the real calibration of all kinds of reference standard sample tube.