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 Technical Characteristics of LEDs Related to the International Commission on Illumination (CIE) Technical Committee (TC)
Two divisions of the International Commission on Illumination (CIE): D1 (Visual and Color Division), D2 (Light and Radiation Measurement Division), are studying the color rendering of white LEDs and related metering issues, and have forwarded D1: TC1-65, TC1-62 These two study color charts for the visual measurement and draft of the color rendering of LEDs.
The TC1-62 document "ColourRendering of WhiteLEDLightSources" may partially replace the CIE 13.3-1995 publication. These two documents have entered the voting phase.
The TC1-62 document "ColourRenderingofWhiteLEDLightSources" 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 CIECRI 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 CIECRI, the new color rendering index can be used to determine the alternative CRI calculation method after successfully applying a new color rendering index. D2 set up a special technical committee TC2-45 to study the measurement method of LED: TC2-45 file "Mea-surementofLEDS" is voting, it will replace CIE127 publication.
3LED 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:
1cd=(1/683)W/sr (wavelength 555nm);
1cd=1lm/sr;
1W = 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.
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