Progress in the detection of LED optical properties at home and abroad

1 Overview

Since the first red LED was introduced in 1976, after 30 years of development, LED has formed a variety of spectrum products, and the power of a single LED has grown from the initial zero to several watts to several tens of watts. In 2001, white LED was successfully developed. It is expected that LEDs will eventually enter the field of lighting and even enter home lighting. The latest research results of white LED are even more exciting. The luminous efficiency of low power LEDs has reached 100 lm/W. In particular, RGB-LED research results show that LEDs, like conventional three-primary fluorescent lamps, can achieve a variety of different color temperatures and uniform illumination environments.

The progress of LED light sources and people's expectations for its application in the field of lighting have also placed new demands on the corresponding optical inspection technology. Since the optical characteristics of LEDs are quite different from those of conventional light sources, research and development of measurement methods suitable for this new type of light source are needed.

2 International Commission on Illumination (CIE) Technical Committee (TC) Related Technical Characteristics of LEDs Two Divisions of the International Commission on Illumination (CIE): D1 (Visual and Color Division), D2 (Light and Radiation Measurement Division), Study the color rendering of white LEDs and related metering issues, and have forwarded D1: TC 1-65, TC 1-62, a visual draft of the two color charts and a draft of the color rendering of LEDs.

The TC 1-62 document "Colour Rendering of White LED Light Sources" may partially replace the CIE 13.3-1995 publication. These two documents have entered the voting phase.

The TC 1-62 document "Colour Rendering of White LED Light Sources" introduces the visual experimental results of the white LED color rendering index CRI. The CRI calculation method is specified in the CIE 13.3-1995 publication. If the result of the calculation of the CRI by the white LED is inconsistent with the visual result, the document determines that this contradiction exists. The technical report concludes that CIE CRI does not apply when applying color rendering calculations including white LEDs. The Technical Committee recommended that D1 establish a new set of color rendering indices that do not immediately replace the current CIE color rendering index calculation method. As a supplement to CIE CRI, the new CRI can only be used to determine the alternative CRI calculation method after successfully applying a new color rendering index. D2 set up a special technical committee TC 2-45 to study the measurement method of LED: TC 2-45 document "Mea-surement of LEDS" is voting, it will replace CIE 127 publication.

3 LED luminous efficiency limit value

For a long time, semiconductor research experts have explored various new technologies to improve the internal and external quantum efficiency of LEDs. In 2006, there have been reports of low-power white LEDs with luminous efficiencies of 100 lm/W. In order to determine the reasonable expectation of LED luminous efficiency, it is necessary to calculate the LED luminous efficiency limit value based on photometry and colorimetry.

In October 1979, the 10th International Metrology Conference (CGPM) defined New Candela (cd). Candela (cd) is the luminous intensity of a light source emitting a single-color radiation frequency of 540.0154 × 1012 Hz (wavelength 555 nm) in a given direction, and the radiation intensity in this direction is: 1 cd = (1/683) W / sr ( Wavelength 555 nm); 1 cd = 1 lm / sr; 1 W = 683 lm (wavelength 555 nm).

If the power loss, internal quantum efficiency, and external quantum efficiency values ​​are ignored, the luminous efficiency limit values ​​of various light sources and LEDs can be calculated. Figure 1 shows the spectral power efficiency of the human eye and the spectral power distribution of the ideal isochromatic white light. Due to the spectral response characteristics of the human eye, the ideal iso-energy white light can be weight-calculated to obtain an ideal iso-energy white light limit luminous efficiency of 182.45 lm/W in the visible spectrum.

In the field of illumination, the birth of a new type of light source, its life and light efficiency are important quality indicators, but its color rendering properties for various colors is another important quality indicator of the lighting environment. The theoretical luminous efficiency of the two yellow spectral lines of the low-pressure sodium lamp can reach 450 lm/W (as shown in Figure 2), and the actual luminous efficacy exceeds 200 lm/W. However, due to its poor color rendering properties, it was eventually replaced by high pressure sodium lamps and metal halide lamps.

Investigating the new light source of LED, compared with the ideal white light at the expense of some color rendering index Ra, the limit luminous efficiency of white LED will be higher, about 200lm. For a white LED actually used in the field of illumination, it is reasonable to set the target value of the luminous efficiency at 150 to 160 lm/W.

In addition to white LEDs for lighting applications, the luminous efficiency of LEDs of various spectra can also be estimated from the data shown in Figure 2. Figure 3 is a graph showing the ultimate luminous efficiency values ​​for red, green, and blue (643 nm, 535 nm, 460 nm) LEDs. 4 LED and traditional light source comparison

(1) The LED is small in size and has various external dimensions, which are suitable for different applications (as shown in Figure 4).

(2) LED has a variety of colors, ultraviolet, purple, green, yellow, red to infrared, white LED spectrum shown in Figure 5.

(3) LED optical parameters are related to temperature (as shown in Figure 6 and Figure 7);

(4) LED optical parameters are related to the viewing angle;

(5) LEDs have various light distribution curves, and there is no determined optical axis (as shown in Figure 8).

The above characteristics of the LED bring many problems to the measurement of the optical characteristics of the LED.

5 Measurement of LED optical characteristics The optical characteristics of LEDs should be considered from the following characteristics:

(1) luminous intensity;

(2) total luminous flux;

(3) spectral characteristics, chromaticity coordinates, dominant wavelength;

(4) Spatial distribution of luminous intensity and total luminous flux.

5.1 Luminous intensity

Due to the structural characteristics of the LED, in order to improve its luminous efficiency, a reflector is installed at the bottom, which is actually a luminaire itself. The light emitted by each area has different focus points, it is not a point source. Therefore, in evaluating the luminous intensity of LEDs, the inverse square law of distance in photometry is not applicable. Two current internationally accepted measurement conditions are specified in the CIE127 publication as shown in Figures 9 and 10.

The measurement results using the above two measurement conditions can be compared internationally. The measurement conditions of A and B are not strictly in accordance with the definition of luminous intensity, and are therefore referred to as "average luminous intensity" (ALI).

Correction of the measuring probe: The measurement error of the "average luminous intensity" (ALI) due to the matching error of the measuring probe V (? attitude) (as shown in Fig. 11), the matching error of V (? posture) The measurement results of red and blue LEDs are more serious, and the spectral correction method can improve the measurement accuracy. Correction of the spectral matching error of the detector and calculation of the color correction coefficient (CCF): Es=k■Ps(?姿)V(?姿)d? posture(1)Ps(?姿) is the relative spectral power distribution of the standard source Ec=k■Pc(?姿)V(?姿)d? posture(2)Pc(?姿) is the relative spectral power distribution of the light source to be tested; ■=■=k■(3)S(?姿) For the relative spectral sensitivity of the detector, for measuring the signal value of the standard light source and the light source to be tested, the exact illuminance value is: Ec=k■Es(4) Requirements for the LED luminous intensity measuring instrument:

(1) The measured solid angle should be correct dΩ=0.001sr (A condition) dΩ=0.01sr (B condition)

(2) Measuring the mechanical axis is correct;

(3) Effective anti-stray light design;

(4) Precision V(λ) photodetectors;

(5) Providing spectral data of the V(λ) photodetector to facilitate correction of the measured value;

(6) Power supply with high stability.

5.2 Measurement of LED luminous flux

The distributed photometer can be used to accurately measure the total luminous flux of the LED (the detector spectral response curve has been corrected). This is an absolute measure of the total luminous flux of the LED, but the test instrument is expensive, and the integrating sphere is commonly used in the industry for measurement.

(1) The size of the integrating sphere is as large as possible, which can reduce the absorption of the screen and the foreign matter error;

2) The greater the reflectance of the surface of the coating, the less the difference in the response rate of the inner surface of the sphere. Currently in the LED test, the surface reflectance of the coating is even greater than 98%.

(3) Pay attention to the installation position of the LED to be tested, and align the emitted light with the area where the inner surface of the integrating sphere responds uniformly;

(4) Apply auxiliary light source to reduce screen absorption and foreign object error.

5.3 Measurement of spectral characteristics, chromaticity coordinates, and dominant wavelength

According to the technical exchanges of the three international LED expert conferences of the International Commission on Illumination (CIE) and relevant international comparison results, the following recommendations are as follows:

(1) The national metrology department should adopt a dual monochromator measurement system;

(2) Monochromator measurement system can meet industrial sector applications;

(3) The chromaticity test results of the 1 nm and 5 nm spectral measurement bandwidths are relatively close, and the 5 nm bandwidth measurement can be used;

(4) The contrast measurement of the dominant wavelength is small;

(5) The relative error of the CCD measuring instrument is large. Figure 12 is a comparison of some international comparisons: CCD instruments for white LEDs.

6 standard LED

6.1 Theoretical and technical basis for measurement of LED optical properties

(1) According to the above analysis of the optical characteristics of LEDs, national metrology departments and industries can apply conventional luminosity, chromaticity and radiance instruments to the total luminous flux, spectral characteristics, chromaticity coordinates, dominant wavelength, color temperature and other parameters of the LED. measuring.

(2) For the measurement of LED luminous intensity, since the LED illumination characteristics do not follow the photometric distance inverse square law, the CIE127 document recommends using the A and B conditions to measure the LED average luminous intensity (ALI).

(3) In order to improve the measurement uncertainty and improve the measurement efficiency during the measurement of average luminous intensity, luminous flux, etc., CIE established Tc2-45, Tc2-46, Tc2-50 and other technical committees to carry out related research and evaluation. Work, as well as research on standard LEDs.

(4) The basic theory of photometry, colorimetry, and radiosity is the basis of LED measurement. The standard A source is an important benchmark for determining the spectral power distribution characteristics of standard LEDs.

(5) Accurate standard LED luminous flux values ​​can be determined by distributed photometric measurements. As a supplementary test method, the United States (NIST), Hungary, the United Kingdom (NPL), Germany (PTB) and other countries and China are actively carrying out standard LED research work.

6.2 Requirements for standard LED characteristics

(1) The operating temperature of standard LEDs is generally greater than the ambient temperature, and there are technical solutions for cooling;

(2) Samples of standard LEDs need to be aged for several hundred hours, and the stabilizers are selected for post-calibration work;

(3) The standard LED must have the same spectral power distribution as the test sample, and a variety of standard LEDs of different colors need to be established. Especially for white LEDs, since it can be composed of different spectra, it is almost impossible to develop a universal white LED standard;

(4) The standard LED must have the same luminous intensity distribution curve (light distribution curve) as the test sample. If the color (spectrum) of the LED to be tested differs from the standard LED (spectrum), the photometric detector needs to be spectrally corrected.

6.3 Advantages of applying standard LED measurements

(1) There is no need to perform spectral correction on the photometric probe;

(2) There is no need to strictly position the photometer reference plane. Figures 13 and 14 are the application of standard LEDs.