Semiconductor light emitting diodes (LEDs) have been widely used in indicator lights, signal lights, instrument displays, mobile phone backlights, vehicle light sources and other occasions. White LED technology is also developing, and LEDs are more and more widely used in the field of lighting. In the past, there were no comprehensive national and industrial standards for LED testing. In production practice, it can only be based on relative parameters. Different manufacturers, users and research institutions have great disputes about this, which has greatly affected the development of domestic LED industry.

Optical property measurement

Luminous flux

Method 1 integrating sphere spectroradiometer

Reference standard:

Cie84:1989 measurement of luminous flux;

IESNA lm79-08 electrical and photometric measurement of solid state lighting products

Method 2 distribution photometer

Reference standard:

Cie84:1989 measurement of luminous flux;

IESNA lm79-08 electrical and photometric measurement of solid state lighting products

Test items:

Beam angle luminous efficiency luminous flux

Annular luminous flux spectral power distribution chromatic aberration peak wavelength

Luminous flux maintains color rendering index color coordinate color tolerance

Light intensity distribution (light distribution curve)

Method distribution photometer

Reference standard:

IESNA lm79-08 electrical and photometric measurement of solid state lighting products

CIE 121:1998 lamp photometry and distribution photometry

CIE 043:1979 "projection luminous intensity test"

Gb/t 9468 General requirements for photometric measurement of lamp distribution

Gb/t 7002:2008 photometric testing of projection lighting fixtures

Test items:

Lamp energy efficiency beam angle lamp intensity distribution brightness limit curve

Estimated curve of indoor lamps, equal intensity distribution curve, total luminous flux, annular luminous flux

Plane isolux curve circular isolux curve ballast lumen coefficient (BLF) rectangular isolux curve

test method

LED is a unipolar PN junction diode composed of semiconductor inorganic materials. The relationship between voltage and current is called volt ampere characteristic. The electrical characteristic parameters of LED include forward current, forward voltage, reverse current and reverse voltage. The LED must be driven by appropriate current and voltage to work normally (as shown in Figure 1). Through the test of LED electrical characteristics, the maximum allowable forward voltage, forward current, reverse voltage and current of LED can be obtained. In addition, the best working electric power of LED can also be measured.

The electrical characteristics of LED are generally tested with a voltmeter under the power supply of the corresponding constant current and constant voltage source.

Optical characteristic test

Similar to other light sources, the test of LED light characteristics mainly includes luminous flux, luminous efficiency, radiation flux, radiation efficiency, light intensity, light intensity distribution characteristics and spectral parameters.

Luminous flux and luminous effect

There are two methods for measuring luminous flux, namely, integrating sphere method and variable angle photometer method. The variable angle photometer method is the most accurate method to test luminous flux, but because it takes a long time, the integrating sphere method is generally used to test luminous flux. There are two test structures in the existing integrating sphere method for measuring LED luminous flux, one is to place the measured led at the center of the ball, and the other is to place it on the wall of the ball.

When the integrating sphere method is used to test luminous flux, the self absorption of the light source will affect the test results. Therefore, it is often necessary to introduce auxiliary lights.

After measuring the luminous flux, the luminous efficiency of LED can be measured with the electric parameter tester. The measurement method of radiation flux and radiation efficiency is similar to that of luminous flux and luminous efficiency.

Light intensity and light intensity distribution characteristics

The light intensity of the point light source is evenly distributed in all directions in space, and the test results received with detectors with different receiving apertures at different distances will not change. However, due to the inconsistent light intensity distribution of LED, the test results vary with the test distance and detector aperture. Therefore, cie-127 proposes two recommended test conditions, so that each LED can be tested and evaluated under the same conditions, as shown in Figure 5. At present, cie-127 recommended test conditions have been quoted by LED manufacturers and testing institutions.

Spectral parameters

The spectral characteristic parameters of LED mainly include peak emission wavelength, spectral radiation bandwidth and spectral power distribution. The spectrum of monochromatic LED is a single peak, and its characteristics are expressed by peak wavelength and bandwidth, while the spectrum of white LED is synthesized by a variety of monochromatic spectra. The spectral characteristics of all LEDs can be expressed by the spectral power distribution, and the spectral power distribution of white LED. The chromaticity parameters can also be calculated from the spectral power distribution of LED.

The measurement of spectral power distribution needs to be carried out through light splitting, which can distinguish all colors of light from the mixed light for measurement. Generally, prism and grating can be used to realize light splitting.