Like traditional light sources, the common optical parameters of LEDs include light measurement, radiance measurement, and color measurement. Among them, the most important and basic is the average luminous intensity and total luminous flux in the luminosity measurement. The following will introduce how to perform LED optical measurement.
①Measurement of average luminous intensity
In many LED product manuals, the average luminous intensity (or average luminous intensity) is often used to measure how much light the LED emits in a certain direction. However, in many occasions, this concept has not been correctly applied, and the measured physical quantity is not the average luminous intensity under the standard definition.
The average luminous intensity is measured by placing a detector at a certain distance from the LED to measure the luminous flux projected on the detector; then the solid angle is calculated based on the area of the detector and the distance between the detector and the LED, and finally the two Calculated by dividing by. Because these measurements are usually made at relatively close distances, in most cases, the light-emitting surface of the LED is not small enough relative to the distance between the detector and the LED, so the LED should be regarded as a Extend the light source instead of the point light source. This is the so-called “near field situation”. At this time, the measurement does not obey the “inverse square law”. Moreover, when the LED is very close to the detector, the light intensity may vary as viewed from different positions on the detector. Not only that, the results of the measurement and the use of these measurement data are completely dependent on the environment at the time of measurement. Therefore, it is very important to measure the average luminous intensity of LEDs, which is to define an accurate measurement environment, which is suitable for most LEDs so that the data of different products can be compared with each other.
In order to solve this problem, the International Commission on Illumination (CIE) defined a new physical quantity in its LED measurement standard CIE127-1997 to describe the physical measurement in the “near field”. This physical quantity is the average luminous intensity of the LED. At the same time, the standard also defines two standard measurement environments. These two standard measurement environments are based on the current experimental process in industrial production and are described from the perspective of LED manufacturers and users. These two measurement environments are called CIE standard environments A and B for LED measurement. In these two environments, ILEDA and ILEDB are used to represent the average light intensity of the LED.
When measuring in these two environments, a circular incident hole detector with an area of 100m㎡ (about 11.3mm in diameter) is used. The LED must face the detector, and the mechanical symmetry axis of the LED must pass through the center of the detector’s entrance hole (Figure 1). The distance between the detector and the LED is related to the specific measurement environment. In CIE standard environment A, the distance between the detector and the LED is 316mm; in CIE standard environment B, the distance between the detector and the LED is 100mm. In both cases, the measured distance is the distance between the front section of the LED and the plane where the entrance hole of the detector is located.
The average light intensity of the LED can be calculated by the following formula
In the formula, E is the average illuminance measured by the detector, lm/m²; d is the distance, mo for environment A, d=0.316m; for environment B, d=0.100m.
The measured solid angle corresponding to environment A is 0.001Sr, and the measured solid angle corresponding to environment B is 0.01Sr. In order to ensure the consistency of the results, usually the angle is more important. The equivalent plane angle corresponding to environment A is 2°; the corresponding value of environment B is 6.5°.
②Measurement of total luminous flux
(1) Goniophotometer method
The Goniophotometer can directly measure the total luminous flux without relying on the coordinate system of the light source. The light source is equivalent to being placed in the center of a virtual sphere, and the detector of the goniophotometer scans the entire surface of the sphere by rotating. On a certain spherical surface element dA, the luminous flux micro-element dφ from the light source can be expressed as
Through integration, the total luminous flux φ can be obtained
(2) Integrating sphere method
Another simpler way to measure the total luminous flux of an LED is to use an integrating sphere to compare the LED under test with a standard LED with similar spatial and spectral distribution. If the LED to be tested does not match the standard LED, the spatial chromatic aberration correction coefficient can be calculated. Spatial chromatic aberration correction coefficients are usually more difficult to quantify, so different integrating spheres need to be estimated.
Because the diameter of the integrating sphere used for LED measurement in many laboratories does not exceed 10cm, an auxiliary LED of the same model can be placed in the integrating sphere to correct the self-absorption of the LED under test. Experiments have shown that the optical ball with two entrance holes and one exit hole for the detector is the most suitable. Figure 2 lists three different integrating sphere structures. The structure shown in Figure 2(a) is not suitable for integrating spheres with a diameter of less than 30cm. Figure 2(b) and (c) show possible placement methods of the baffle in the integrating sphere with a diameter of 10cm and the auxiliary LED that permanently corrects the self-absorption of the LED under test. When a smaller integrating sphere is required, and the spatial distribution between the standard LED and the LED to be tested has a small difference, Figure 2(c) shows the most suitable for measuring the total luminous flux. If a larger diameter integrating sphere coated with a highly reflective coating is used, the size of the center baffle can be greatly reduced, making it closer to the ideal environment. Under this condition, the total luminous flux error obtained by comparing the standard LED and the LED to be tested does not exceed 5%.