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How to measure led flux and radiant power? Introduction of common methods and measurement suggestions

Aug 29, 2017

In general, luminous flux and radiation power are the most important optical parameters of LEDs, but sometimes they also refer to the spatial distribution of light intensity. For smaller devices, the average led strength is still very common. In reality, some led luminous flux is just an increasing number, but has not been widely measured. For solid-state lighting sources, photometric and colorimetric properties are important.

The two main methods of measuring total radiant power and luminous flux are the use of integral sphere or side angle photometer/Spectroradiometer. The next two sections describe both methods of measurement and the challenges of measurement.

Integral sphere method and measurement geometrical dimensions

Luminous flux is sometimes called total luminous flux, which emphasizes that it is the sum of all directions. It is also called 4π flux because a complete sphere has a 4π stereo angle. To collect all the light from the 4π stereo angle, the light source must be in the center of the sphere. Fig. 1a is a conventional 4π geometric structure for measuring luminous flux. The radiation emitted in all directions is captured and the total luminous flux is measured.

Figure 1. International Lighting Commission The spherical geometry recommended for all light sources (a) and for light source (b) not having a rear radiation

For light sources that can be neglected or without radiation, the total flux can be measured in a more convenient forward flux or 2π geometry space. In Figure 1b, the light source is located at the port of the ball wall. Only light radiation emitted from the front hemisphere is used for measurement. This forward radiation is a typical feature of most LED products. The integral ball must be calibrated according to the measurement geometry and the substitution principle. The substitution principle indicates that the test light source should be measured by comparison with the standard source of similar space and spectral distribution.

Choose the right size

Testing samples should always be less than the diameter of the sphere, the purpose is to allow the sample itself caused by the interference factor as low as possible. However, as the spheres become larger, the intensity of the incident light on the detector decreases. According to experience, the luminous flux of the integrating sphere is inversely proportional to the square radius of the sphere. Therefore, selecting the size of the test object and the size of the sphere is critical to the effective balance between high-precision measurements and good traffic (see Figure 2).

Figure 2. The diameter 1m sphere (left) is ideal for measuring most LEDs and modules in the 4π and 2π geometry structures. The diameter 2m sphere (right) is suitable for large-scale lamps and solid-state lighting products.

For a given size test sample, there are some criteria for selecting the correct size of the sphere. Using 4π geometry, the total surface of the sample should be less than 2% of the surface of the sphere. The length of the linear lamp should be less than 2/3 of the sphere diameter. Using 2π geometry, the diameter of the metering port and the maximum elongation of the test sample shall not exceed 1/3 of the sphere diameter.

The error and correcting method of self-absorption production

The detection object itself absorbs the light radiation in the integrating sphere. This form of interference, known as self absorption, can cause significant attenuation of light radiation and results in measurement deviations. The larger and darker the sample, the more obvious the attenuation. Figure 3 shows two samples and the resulting transmission and wavelength. Self-absorption can result in up to 10% error.

Fig. 3. Self-absorption spectra of two units to be tested

Therefore, the self absorption modification needs the appropriate auxiliary light source to carry on the accurate measurement. Full Spectrum Halogen Lamps are meeting this requirement. The auxiliary light source must be positioned behind the bezel to avoid direct exposure to the sample and should be operated by a stable power supply. The light source is used to determine the spectral absorption characteristics of the tested equipment, the sample frame and the connecting cable, and then offset by the actual measured values. As the reflectivity of the coating increases, the ratio of sphere area to the specimen decreases and the self absorption effect increases.

Near-field absorption

Any object in the vicinity of the light source, such as an outlet, absorbs light significantly and may cause greater error. This so-called near-field absorption cannot be corrected by the self absorption measurement. This effect should therefore be avoided. The object should be as far away from the lamp as possible to avoid forming cavities. In addition, a high reflectivity material is recommended to cover the surface of an object. Figure 4 shows a good solution for a linear tube rack.

Figure 4. An example of avoiding near-field absorption effects. The stent of the linear tube is placed where possible away from the light source and coated with a high reflectivity material.

Burning position

For passive cooled solid-state lighting sources, measurements should be made at the manufacturer's defined combustion position. When measured with 4 pi geometries, it is convenient to use an internal lamp post that can be mounted up and down to achieve the burning position of the light source. In the case of 2π geometries, a rotatable sphere is preferred (see for example Figure 5). The entire sphere can be rotated within its mounting frame. Therefore, the measurement port is located on the side, top or bottom.

Figure 5. rotatable 1 metre sphere. A position-sensitive light source can be measured in its designed work position.

Consider measurement error

The factors that cause measurement error are manifold. The wide range radiation characteristic of LED can easily cause calibration error when measuring luminous flux. For parts with distributed ejection, there will be a 5% change, but with a narrow angle led, more than 10% deviations may occur.

As mentioned above, it is important to select the correct sphere size, to perform the self absorption correction, to avoid near-field absorption and to measure the position of the light source in the design of a high precision measurement.

A large part of the error is measured before the light source is thermally stabilized. In addition, the ambient temperature of the 25°c is recommended when testing on the basis of CIE S 025 or en 13032-4. The ambient temperature (the temperature in the sphere) will rise and be different from the "normal" operating temperature by placing a heat source into the integration ball. When measuring with a 4 pi configuration, it is recommended that the hemisphere of the sphere be opened to stabilize the heat source. Before measuring, you should close the sphere carefully to avoid air movement. In this way, the environment conditions in normal operation can be best met.

Method of measuring Angle photometer

Although measuring the luminous flux or radiant power using a measuring angle photometer is more time-consuming than using an integral ball, it is more accurate. The measurement process does not require the luminous flux standard lamp as the reference value. If you must measure different luminous intensity distribution of the lamp, it is the preferred method, is to calibrate the luminous flux standard lamp benchmark, for other test procedures to provide reference values. Another notable feature of the photometric method is the ability to measure part of the luminous flux and the half intensity angle. These values need to be determined when measuring properties related to energy efficiency or if they conform to the Zhaga specification.

The method can be described by an imaginary sphere around the LED. The a cosine correction detectors move on the surface of the sphere at a specific path at the distance of R (Sphere radius). The function of the detector is to determine the irradiance of E. The calculation formula is shown below: (Da represents detector area, dφ represents part of radiation flux)

To determine the total radiant power, the detector moves incrementally with angle θ. Angle φ from 0 ° to 360° change, the corresponding record angle θ value, according to the sphere's constant latitude, scanning each area. Total radiant power φ is:

Alternatively, you can use a fixed detector to scan the end of the LED. However, this may not be applicable for modules and luminaires with convection cooling.

Fig. 6. Measuring angle photometer with compact shielding chamber. The LED moves and the detector does not move. Angle φ is adjusted by rotating the mechanical axis of the LED, and the angle theta is adjusted by rotating its end. The detector is located on the photoconductive rail and can be measured at different distances.

The distance is the requirement of luminous intensity distribution to satisfy the far-field condition. Measuring the total flux using a measuring angle photometer does not require a long distance. Assuming that the detector has good cosine response, the irradiance can be accurately measured at all angles. Irradiance is not the property of the lamp, but the light that falls on the surface. By measuring the irradiance around the virtual ball in the appropriate position, the total flux can be calculated by integral. Assuming that no interaction occurs between the light source and the detector, the size of the light source is almost the size of the virtual sphere.