LED light source optical parameter detection

1. Photometry

The intensity of light, that is, the intensity of light, is the amount of light emitted at a certain Angle. The CIE127 standard stipulates that the inverse square law is not applicable to the luminous second tube because the light is relatively concentrated, and CIE127 puts forward two conditions for measuring the average normal light intensity of the measurement condition A (far field condition) and the measurement condition B (near field condition), both of which are 1cm2. In general, the luminous intensity is measured using standard condition B.

2, luminous flux and luminous efficiency detection

Optical flow is the sum of the amount of light emitted by the light source, that is, the amount of light emitted. There are two main testing methods:

(1) Comprehensive method

The standard lamp and the measured lamp are successively lit in the integrating sphere, and the photoelectric converter reads Es and ED respectively. When the standard light source flux Φs is known, the lamp flux ΦD = ED× Φs /Es. The integral method adopts the principle of "point light source", which is simple to operate, but the measurement error is large due to the color temperature deviation between the standard and the measured lamp.

(2) spectroscopy

The luminous flux is calculated using the spectral energy P (λ) distribution. A monochromator is used to measure the 380 nm~780 nm spectrum of the bulb in the integrating sphere, and then the measured spectrum is measured under the same conditions, and the luminous flux of the measured lamp is calculated by comparison.

Luminous efficiency is a ratio of the luminous flux emitted by the light source to the power consumed, and the luminous efficiency of the LED is often measured by the constant current method.

3, spectrum characteristics detection

The detection of spectral characteristics includes spectral power distribution, color coordinate, color temperature, color rendering index and so on.

The spectral power distribution represents that the light of the light source is composed of a variety of wavelength color radiation groups, and the radiation power of each wavelength is also different, and this different order with the wavelength is called the spectral power distribution of the light source. It is obtained by comparing the light source with a spectrophotometer (monochromator) and a standard lamp.

Color coordinates are the number of luminous colors of a light source digitally represented on a coordinate diagram. Color coordinate map has many kinds of coordinate system, commonly X, Y coordinate system.

Color temperature is a measure of the amount of light that a person sees on the color table (appearance color). The light emitted by a light source, at a certain temperature, is the same color as the light emitted by the black body, it is the color temperature. Color temperature in light is an important parameter to describe the optical properties of light sources. The theory of color temperature is derived from blackbody radiation, and the color coordinates of the light source can be obtained from the color coordinates containing the blackbody trajectory.

The color rendering index represents the amount of light emitted by the light source that correctly reflects the color of the illuminated object, usually expressed by Ra, Ra is the arithmetic average of the color rendering index of the light source for 8 color samples. The color rendering index is an important indicator of the quality of the light source, which determines the application range of the light source, and improving the color rendering index of the white LED is an important topic in the research and development of the LED.

4, strength distribution measurement

The relationship between the light intensity and the spatial Angle (direction) is called the false light intensity distribution, and the closed curve connected by this distribution is called the light intensity distribution curve. Because there are more measurement points, each point must be processed by data, and automatic distribution photometer is generally used for measurement.

5, the impact of temperature on LED optical characteristics

Temperature has an effect on the optical properties of leds. Many experiments have shown that temperature has an effect on the emission spectrum and color coordinates of leds.

6, the ground brightness measurement

The brightness of a light source in a certain direction is the luminous intensity of the unit projected area in the direction. Generally, surface luminance meter and aiming luminance meter are used to measure the surface brightness, and there are two parts: aiming light path and measuring light path.

Part 2

Determination of other performance parameters of LED lights

1, LED lamp electrical parameters measurement

The electrical parameters of the LED lamp mainly include positive, inverse and inverse three electrical parameters, which is related to whether the LED lamp can work normally, and is one of the bases for judging the basic performance of the LED lamp. There are two ways to measure the electrical parameters of LED lights: one is that the current is certain, and the other is to measure the voltage parameters; The second is to measure the current parameters when there is a certain voltage. Take the following measures:

(1) Forward voltage

Applying a positive current to the detected LED light will generate a voltage drop at both ends of the LED light. By adjusting the current value to determine the power supply, record the corresponding reading on the DC voltmeter, that is, the forward voltage of the LED lamp. According to relevant common sense, when the LED guide is passed, the resistance is small, and the ammeter external connection method is more.

(2) Countercurrent

The reverse voltage is applied to the detection LED tube, the voltage regulator is adjusted, and the ammeter reads the reverse current of the LED lamp under test. Because the resistance of the LED is large when the LED is switched back, the forward voltage of the LED is measured by the ammeter connection method.

2, LED tube thermal characteristics test

The thermal characteristics of the second tube have an important impact on the optical and electrical properties of LED. Thermal resistance and junction temperature are the main thermal characteristics of LED2. Thermal resistance refers to the thermal resistance of PN junction to the surface of the shell, that is, the ratio of the temperature difference in the thermal flow channel to the dissipated power on the channel, and the junction temperature refers to the joint temperature of the LED.

The measurement methods of LED junction temperature and thermal resistance are generally: infrared microimaging, spectrum, electrical parameters, photothermal resistance and so on. The infrared temperature measuring microscope or micro-thermocouple is used to measure the surface temperature of the LED chip, and the accuracy is not enough.

The current widely used electrical parameter method is to use the positive pressure drop and PN junction temperature linear relationship method, using LEDPN junction forward pressure drop difference to obtain LED junction temperature.