How to measure LED flux and radiation power? It introduces the methods of measurement and suggestion

Usually, the luminous flux and optical radiation power is LED the most important parameter, but sometimes referred to the intensity spatial distribution. For smaller devices, the average intensity of LED is still very common. In reality, part of the LED flux is just a constant increase in the number, but has not yet been widely measured. For solid state lighting source, photometric and colorimetric characteristics of radiation is very important.

Two kinds of main methods to measure the total radiated power and luminous flux is the use of integral ball or side angle photometer / spectral radiometer. The next two sections will introduce these two kinds of measurement methods and measurement, which will be the challenge.

Ball method and measurement of geometric dimension integral

The flux is also sometimes referred to as the total luminous flux, in order to emphasize that it is the sum of all directions. It is also known as the 4 PI flux, because a complete sphere has a solid angle of 4 pi. To collect all 4 PI light solid angle, the light source should be in the center of the sphere. Figure 1A for the measurement of the luminous flux of the conventional 4 PI geometric structure. Capture radiation emission in all directions, and to measure the total luminous flux.

Figure 1. the International Commission on illumination for all light source (a) and not with the backward radiation light source (b) sphere geometry recommended

With negligible or no radiation source, can be more convenient to the flux of 2 pi geometric space total flux measurement. In Figure 1b, the port is located in the light of the global wall. Only the light radiation emission was used to measure the hemisphere. The forward radiation is typical of most LED products. The ball must be measured according to integral geometry, and follow the principle of substitution for absolute calibration. The substitution principle that should be compared to the measurement light source with similar spatial and spectral distribution of standard source.

Choose the right size

Samples should always be less than the diameter of the sphere, the purpose is to allow the sample itself caused by interference factors as low as possible. However, as the ball is more and more big, the strength will reduce the incident light detector. According to the experience of the flux integral ball is the inverse square of the sphere radius. Therefore, choose the size of the test object and the sphere for effective balance between flow and critical measurement precision (see Figure 2).

Figure 2. 1m diameter sphere (left) is ideal for measuring the most LED and module 4 and 2 pi pi in geometry. The diameter of the 2m spheres (right) is suitable for large lamps and solid-state lighting products.

For samples of a given size, select the correct size spheres have some guidelines. The use of 4 PI geometry, the total surface samples should be less than 2% of the surface of the sphere. Linear light length should be smaller than the diameter of the sphere 2/3. The use of 2 pi geometry, maximum diameter and sample measuring port extension degree of the sphere diameter should not exceed 1/3.

The error produced by self absorption and correction method

Detection of the object itself will absorb light radiation in the integrating sphere. This form of interference, called self absorption, can lead to significant attenuation of light radiation and lead to measurement error. Samples the more dark, more obvious attenuation. Figure 3 shows the two samples and the transmission wavelength. The self absorption can lead to errors of up to 10%.

Since the absorption spectra of 3. two test equipment

Therefore, the self absorption correction to the auxiliary light source appropriate, accurate measurement can be made. The halogen lamp is in line with the requirements of full spectrum. The auxiliary light source must be located on the back of the baffle, to avoid direct exposure of samples, and shall be stable operation power. The light source is used to determine the device under test, sample holder and cable spectral absorption properties, and then use the actual measurement value offset. With the rise of coating reflectivity, sphere area and sample ratio decreased, increased self absorption effect.

Near field absorption

In any object near the light source (e.g. socket) will significantly absorb light, and may cause large error. The so-called near-field absorption cannot be corrected by self absorption measurement. It should avoid this effect. The object should be far away from the light, to avoid the formation of cavity. In addition, as recommended by the high reflectivity material is covered on the surface of an object. Figure 4 shows a good linear solutions of pipe frame.

Figure 4. example to avoid near-field absorption effect. Support linear tube is placed in as much as possible away from the light source place and coated with high reflectivity material.

The position of combustion

For solid state lighting source of passive cooling, measurement should be carried out in a combustion location manufacturer defined. When the 4 PI geometry measurement, can be installed under the lamppost used to achieve combustion location of light sources is very convenient. In the 2 pi geometry case, rotating sphere is preferred (see Figure 5 for example). The entire sphere can be in its installation frame rotation. Therefore, the measurement port is located on the side, top or bottom.

Figure 5. rotating sphere 1 meters. The sensitive position of the light source can be measured in the design of working position.

Considering the measurement error

The measurement error is caused by factors in many aspects. The radiation characteristics of wide range of LED is very easy to cause the calibration errors in the measurement of luminous flux at. For the decentralized jet components, there will be 5% changes, but the use of narrow angle LED, may occur more than 10% deviation.

As mentioned above, the sphere size, select the correct implementation of the self absorption correction, avoid near-field absorption and light source in the design of combustion location measurement is very important for high precision measurement.

The error caused by a large part of the factors is started before the measurement of heat stable light source. In addition, when the CIE S or EN 13032-4 on the basis of 025 tests, recommend the use of 25 DEG C temperature. A will