The Role Of The Integrating Sphere In LED Test

To judge whether an LED light source is what we need, we usually use the integrating sphere for test, and then analyze it according to the test data. So what role does the integrating sphere play in the test.

The basic working principle of the integrating sphere is shown in the figure below: after the light is incident from the input hole, the light is uniformly reflected and diffused inside the sphere, forming a uniform light intensity distribution on the sphere. Therefore, the light obtained from the output hole is a very uniform diffuse beam. Moreover, the incident angle, spatial distribution and polarity of the incident light will not affect the output beam intensity and uniformity. At the same time, because the light is emitted after being evenly distributed inside the integrating sphere, the integrating sphere can also be used as a light intensity attenuator, and the ratio of output intensity to input intensity is about: the area of light output hole / the surface area inside the integrating sphere.

Generally speaking, when the optical diffuser is used carefully, it can reduce the small error caused by the uneven distribution of incident light sources or beam offset on the detector, so it can improve the accuracy of measurement. However, in more precise measurement, the integrating sphere must be used as the optical diffuser to reduce the above error.

When the integrating sphere is used to measure the luminous flux (lumen), the measurement result can be more reliable. The integrating sphere can reduce and remove the measurement error caused by the shape of light, divergence angle and the difference of responsiveness at different positions on the detector.

The integrating sphere can generally give the following seven important parameters: luminous flux, luminous efficiency, color temperature, color rendering index (RA or re), TLCI, RF and RG. (in fact, there are many other parameters, such as voltage, color coordinates, peak wavelength, main wavelength, dark current, CRI, etc.) today we mainly discuss the significance of these seven parameters to the light source and their influence on each other.

Luminous flux: luminous flux refers to the radiant power that can be felt by human eyes, that is, the total radiant power emitted by the LED, unit: lumen (LM). Luminous flux is not only a direct measurement, but also the most intuitive physical quantity to judge the brightness of LED.

Luminous efficiency: luminous efficiency, that is, the ratio of all luminous flux emitted by the light source to the total input power, is the calculated quantity, unit: LM / W. For LED, the input electric energy is mainly used for light emission and heating. If the light efficiency is high, it indicates that there are few parts used for heating, which is also a manifestation of good heat dissipation.

Color temperature: measure the physical quantity of light color. When the radiation of the absolute blackbody is exactly the same as that of the light source in the visible area, the temperature of the blackbody is called the color temperature of the light source. Color temperature is a measured quantity, but it can be calculated by color coordinates at the same time.

Color rendering index RA (average value of R1 ~ R8): used to describe the ability of the light source to restore the color of the object. It is determined by comparing the appearance color of the object under the standard light source. Our color rendering index is actually the average value calculated by the integrating sphere for the eight light color measurements of light grayish red, dark grayish yellow, saturated yellow green, medium yellow green, light blue green, light blue, light purple blue and light red purple. It can be found that it does not include saturated red, commonly known as R9. Since some lighting requires more red light (such as meat lighting), R9 is often evaluated as an important parameter. Re parameter is rarely used (it is the evaluation of 15 colors R1 ~ R15. Generally, the integrating sphere does not qualitative re. If the effect of higher light quality is pursued, it clearly means that this parameter also needs to consider the evaluation of 99 groups of colors).

The color temperature can be calculated by color coordinates, but careful observation of the color map will find that the same color temperature can correspond to many color coordinates, while a pair of color coordinates only correspond to one color temperature. Therefore, it is more accurate to use color coordinates to describe the color of light source. The display index itself has nothing to do with the color coordinates and color temperature, but when the color temperature is higher and the light color is colder, the red component in the light source is less, and the display index is difficult to be very high. For the warm light source with low color temperature, the red component is more, the spectrum coverage is wide, and it is closer to the spectrum of natural light, so the color display index can be naturally high. This is also the reason why LEDs above 95ra on the market have low color temperature.

TLCI: TLCI (television lighting consistency index), the latest measurement standard for evaluating television and film light sources, can reflect the degree of light and color reception and color restoration of the camera.

Tm-30-15: a new evaluation method (RF and RG) for the color rendering ability of light sources of the North American Lighting Society (ies).

(1) RF: color fidelity index, range: 0 ~ 100, similar to the explicit concept.

(2) RG: color saturation index. 100 indicates the same saturation. It is considered bad if it is too high or too low.

RA is the evaluation (average) of the indicators of 8 colors R1 ~ R8; Re means extend, which is to evaluate the color rendering index (root mean square value) of 15 colors R1 ~ R15. Re index puts forward higher requirements for light quality.