Ball photometer is a fast and effective LED product luminous flux and luminous efficiency measurement device, it involves the luminous flux measurement of LED products, product testing and enterprise quality control department, is currently the main equipment for LED luminous flux and luminous efficiency detection. How to use spherical photometer reasonably to obtain accurate luminous flux is a common concern. This paper attempts to discuss the main technical points of measuring the total luminous flux of LED by spherical photometer.

Principle of spherical light meter

Spherical photometer is a photometer that uses an integrating sphere to measure the total luminous flux of a light source. The basic structure is shown in Figure 1. The light source S is placed inside the integrating sphere (the ordinary position is 4π at the center of the sphere, or 2π near the spherical wall), and the optical probe D is affixed with a detection hole on the spherical wall. The hole is located on the ball wall and a scatterer is placed. Detector D is a photometric probe, and its output electricity is converted into voltage through I/V, and then displayed on the reading meter R; If the probe D is the optical fiber collecting surface, it collects the light and connects the spectroradiometer through the optical fiber. According to the measured spectral power of the visible light region, the total luminous flux is calculated using the spectral optical apparent efficiency function. There are also two methods used in combination, that is, using a photometric probe and a spectroradiometer at the same time, with the photometric probe measuring its flux and the spectroradiometer measuring its spectral power distribution.

1a, spherical photometer (light meter detection system) structure diagram

1b, spherical photometer (detection system for spectroradiometer) structure diagram

White diffuse reflective material is used for coating in the accumulated sphere layer, and the material requires good diffuse reflection characteristics and no spectral selectivity. The total luminous flux emitted by the light source S is φ, the reflectivity is ρ, and the specific spherical radius is R. Based on the integral sphere theory, the illuminance generated by the light source S on the sphere wall is fixed for a certain integral sphere, R and ρ. Therefore, the above formula indicates that due to the multiple diffuse reflections of the inner wall of the integrating sphere, the indirect illuminance value generated by the light source at each point on the inner wall of the sphere is equal and proportional to the total luminous flux of the light source. In Figure 1, the function of baffle B is to prevent light source S from shining directly onto detector D. If the baffle B is removed from the integrating sphere, the light from the light source will shine directly on the probe D, and the illumination of the probe D position depends on the luminous intensity of the light source in this direction. The illuminance value of each point on the inner wall of the sphere is not equal, and is not proportional to the total luminous flux of the light source.

According to formula (1) (1), the total luminous flux of the measured lamp can be measured by comparing the luminous flux between the lamp and the standard lamp.

Where Rtest is the photoelectric reading of the standard lamp, Rstd is the photoelectric reading of the standard lamp, φstd is the total luminous flux value of the standard lamp, and C is the luminous flux constant.

LED characteristics

The second light emitting tube is a new generation of energy-saving light source, which has the characteristics of energy, long life, rich color, wide dynamic adjustment range, etc. It is a high-tech product to realize energy saving and emission reduction. In general, compared with traditional incandescent lamps, LED lamps have the following characteristics:

(1) Great difference in color temperature spectrum. The classic white LED is stimulated by blue light yellow-green phosphor, mixed light to get white. The characteristic of the luminous spectrum of typical LED and incandescent lamps makes it necessary to correct the spectral apparent efficiency function mismatch when measuring with a spherical photometer.

(2) Uneven spatial luminescence. A single LED has a strong directivity, and the spatial luminous intensity distribution curve is obviously different from that of incandescent lamps. This characteristic makes it necessary to introduce the spatial response function of integrating sphere when measuring spherical photometer.

(3) The temperature of the junction has a great influence. Because the LED's light-emitting chip depends on the temperature of the PN junction, the luminous flux of the LED is affected by the ambient temperature, heat dissipation conditions and preheating time. Incandescent lamps take only about 5 minutes to achieve thermal equilibrium, and are not sensitive to the surrounding temperature. This characteristic makes the measurement of LED luminous flux need to be fully preheated, and can maintain the specified ignition attitude and ambient temperature.

Key technical points of LED total luminous flux measurement

(1) The standard lamp value used is reliable, and the verification or calibration certificate is within the validity period.

The standard lamps currently used in our country are BDT and BDP standard incandescent lamps, which are stable and reliable and can measure the flux value. In addition, the radiation spectrum in a spherical photometer can be effectively calibrated by covering the visible region with a high color temperature standard light source with a distributed temperature standard.

(2) Good laboratory environment and sufficient preheating time to improve the repeatability of LED quantity.

Because leds are susceptible to ambient temperature, clean and constant temperature laboratory conditions are required. In the daily LED light source test, in order to achieve thermal balance, the preheating time of LED bulbs and other light sources is 5 minutes, and 0.5-1 hours is appropriate. After selection, the repeatability of the LED light source after full aging can reach more than 0.5%.

(3) Excellent equipment performance.

In order to make the actual integrating sphere close to the ideal state, the diameter of the integrating sphere should be as large as possible under the condition that the sensitivity of photometric detection is met. The coating material in the integrating sphere must be clean, uniform, and reflective to meet The measurement of luminous flux requirements of CIE 84-1989; Internal brackets and clamps must not form a barrier on the lamp and must be sprayed with a diffuse reflective coating. The position of the partition is appropriate, and the area is taken when the measured light source is not directly irradiating the detector. The photodetector performs well, and if a V (λ) modified photometric probe is used, f1 'must be small enough (laboratory grade); If the photometric detector is a radiation spectrometer, its stray light is small, the wavelength position error is small, the dynamic range is large, and the linearity is good.

(4) Mismatch correction of spectral apparent efficiency function.

When measured by photometric probe, the actual response curve is different from the apparent efficiency function V (λ) of ideal spectrum. In addition, the reflectance of the spectrum of the diffused layer inside the integrating sphere and the ground glass on the window is not an ideal plane curve. The above factors cause the response curve of the spherical photometer to deviate from V (λ), resulting in V (λ) mismatch error. The measurement error can be estimated based on the following calculation and mismatch correction:

Where Ptest (λ) is the relative optical power distribution of the measured light source, Pstd (λ) represents the relative spectral power distribution of the standard bulb, and S (λ) is the spectral diffuse reflection ratio of the inner wall of the integrating sphere. For example, if BDP is used to calibrate a spherical photometer, the correction factor is 1.01; If you want to measure blue LED lights, the coefficient can reach 1.10 or higher.

(5) Correction of the whole sphere space response function.

When there is a significant difference between the normalized light intensity spatial distribution curve of the test LED and the standard lamp, the correction coefficient needs special attention. Because of the uneven internal coating of the spherical photometer and the presence of internal components (such as the baffle screen), the illumination value generated by the detector in different areas of the spherical wall of the integrating sphere is not equal, that is, the Spatial response distribution function (SRDF) of the integrating sphere:

K (θ, φ) represents the reading of the detector after a quantitative narrow beam is projected to the inner wall of the sphere at the center position of the sphere (θ, φ) in the integrating sphere and after multiple diffuse reflections. You can usually use K (0,0) as the unit 1.

The correction coefficient of the spatial response distribution function of the integrating sphere can be written as:

Itest (θ, φ) and Istd (θ, φ) represent the spatial distribution curves of the standardized light intensity of the measured LED and the standard lamp, respectively. According to formula (5), if the internal spatial response of the integral sphere is obviously uneven (such as yellow coating, ash accumulation at the bottom, large area of the light baffle, etc.), and the spatial distribution curve of the light intensity of the measured LED lamp and the standard lamp is significantly different. For example, if BDP is used to calibrate a spherical photometer and then an LED spot light is measured, the correction factor can reach 1.10.

(6) Self-absorption correction

When the size difference between the detected LED and the standard lamp is large or the state is significantly different, the absorption correction factor should be calculated. Light a stable auxiliary lamp at an appropriate position in the ball (generally on the ball wall) to block the light emitted by it to the window and the lamp under test. Place the standard lamp in the position where the normal light source is installed. When the detector reading is Astd, remove the standard lamp position, place the measured LED in the same position, and the detector reading is Atest, then the calculation formula is as follows:

(7) The reference lamp uses LED quantity transmission to avoid the above correction, making the measurement process simple and reliable.

If the parameters of the total luminous flux, normalized spatial distribution curve of light intensity and luminous spectrum of the light source and the standard light source are close to each other, the above three correction coefficients are close to 1 and can be ignored. It is recommended to choose a standard light emitting two-tube transmission lamp with stable quantity value, and directly apply formula (2) to calculate the luminous flux.

outline

The spherical photometer meets the above hardware requirements, the operator can keep the integrating sphere clean, and pay attention to the optical characteristics close to the measured lamp and the standard lamp (or LED value transfer reference lamp), the total luminous flux error of the measured LED lamp is completely controllable. When there is a large difference between the measured lamp and the standard lamp (or the light-emitting two-tube transfer reference lamp) in the spectrum, spatial distribution of light intensity or shape, the corresponding correction should be made to reduce the measurement uncertainty.