LED characterization
Today, LEDs are used in a wide range of applications, including home, street, and business lighting, where LEDs are beginning to replace more traditional light sources. Despite the many advantages of LEDs, the measurement techniques for power, flux (equivalent to power, but measured in lumens) and spectrum are not much different from those of traditional light sources. The main reason is that these light sources emit incoherent light uniformly in all directions. These light sources produce highly divergent light emissions. Accurate measurement of the power of these light sources using sensors with fixed aperture dimensions requires complex geometrical elements that collect light. The system used by MKS to characterize the LED output is the subject of this section.
Integrating spheres
A power sensor with a finite aperture, one of the photodiodes, cannot measure the complete beam of a highly divergent light source. In addition, estimating the measured portion of the power through the aperture is not as intuitive as a laser with a well-defined beam size. Instead, an optical harvesting device, known as an integrating sphere, can be used in combination with a sensitive photodiode to determine the power of a highly divergent light source such as an LED. As shown in Figure 1, the inner surface of the integrating sphere is coated with a highly reflective coating. When a divergent beam hits the inner wall of the integrating sphere, the light is reflected and scattered several times until the light hitting any position on the wall of the sphere has the same intensity. The sensor placed on the sphere thus acquires the same intensity as any other position; The power detected by the sensor is proportional to the total incident power and is independent of beam divergence. In order to achieve this, the sensor must be placed in a position where only the scattered light can be seen, not the incident beam. Since the integrating sphere distributes the beam evenly across the sphere, the photodiode can measure the power passing through its aperture section to determine the total beam power by the ratio of the known aperture area to the entire sphere area. The integrating sphere also reduces the power entering the photodiode, so higher input beam power can be measured.
Figure 1:Inside the integrating sphere, multiple diffuse reflections of light from a divergent beam (left). Spectral reflectance of the material coated on the inner surface of the integrating sphere (right).
Figure 2 shows the MKS integrating sphere, which has been designed to be used in a variety of ways for many applications. Combined with a large number of available accessories, a single sphere is capable of performing a wide range of integrating sphere tasks, such as uniform illumination, light measurement, and reflectance measurement, with reasonable accuracy. The spheres have a highly reflective, diffuse white coating for increased efficiency and readings independent of beam size, position, and divergence. The sphere can accommodate light emission in the UV, VIS, and NIR spectral regions while handling input power of up to 30 W. The spheres are also available in different sizes, as the sphere size must match the input beam aperture to ensure uniform scattering. Larger diameter spheres can be used for both divergent beams (e.g., LEDs, laser diodes, fiber optic outputs) and collimated laser beams (see Figure 2). This is achieved by setting the optical geometry so that the input beam never hits the sensor directly and the sensor can only see the light reflected from the wall. In order to maintain accuracy, it is recommended to perform an integrating sphere sensor calibration annually. Finally, many thermopile or photodiode sensors can be combined with integrating spheres as a suitable choice for LED measurements.
Figure 2For divergent beams with inner diameters of 1", 15" and 53" ophir integrating sphere (from left to right). Divergent or collimated beams can be configured with larger diameter integrating spheres, as shown in the far right figure.
fluxgage™
The integrating sphere is the standard instrument for measuring the optical power, flux, and color of LEDs. However, the integrating sphere size must be at least three times the size of the device under test (DUT). This ensures that the light from the DUT is scattered evenly within the sphere before reaching the sensor, enabling accurate power estimation. For large LED light-emitting devices or lamp assemblies that may be as close as 2-3 feet, an integrating sphere diameter of 6-10 feet is required for proper characterization. In addition, the integrating sphere must be recalibrated for each use, taking into account the absorption of the DUT itself. MKS has developed the FluxGage measurement system (see Figure 3) to solve these problems in a unique way: the detection surface with absorbing, detecting light. By eliminating the need for diffuse reflection, the characterization device can be the size of a DUT, such as a rectangular device, and calibrated only once. The probe surface should be able to detect light effectively, regardless of the angle of incidence, and absorb all light with very little reflection. The detection surface of the fluxgage system (see Figure 3) is made of solar panels for detecting light, sheet diffusers that make the solar panels insensitive to angles, and a dense array of small holes in the astigmatism sheet marked with black ink. This design ensures that all three requirements are met.
Figure 3The chassis of the FluxGage measurement system (left) and the internal solar panels with an array of astigmatists and small holes (right).
As a result, the FluxGage system is a compact measuring system for LED light-emitting devices. The system uses solar panels to measure total flux or power, and this complete system with spectrometer and fast photodiode provides both color and flicker (power output oscillation) measurements. The benefits of using the fluxgage system are its small size, robust construction, good production stability, ease of use, and low calibration frequency. The increased demand for LED light-emitting devices requires manufacturing processes to adapt to improved quality testing methods that can provide high-quality measurements without slowing down the production line. The value of using the FluxGage system lies not only in the ability to make critical measurements quickly, but also in the ability to test the entire luminescence device as a well-performing, operational lamp in the manufacturing process.