Detection of Optical parameters of LED Lamps
Luminous intensity testing:
The intensity of light, that is, the intensity of light, refers to the amount of light emitted at a particular angle. Because the light of LED is concentrated, the inverse square ratio law is not applicable at close range. CIE127 standard stipulates that the measuring conditions A (far field condition) and B (near field condition) are put forward to measure the average normal light intensity. The detector area of the two conditions is 1cm2. In general, the luminous intensity is measured using standard condition B.
Luminous flux and light efficiency detection: luminous flux is the sum of the amount of light emitted by the light source, that is, the amount of light emitted. The detection methods mainly include the following two methods. Integral method: ignite the standard lamp and the measured lamp in turn in the integral sphere, and record their readings in the photoelectric converter as Es and ED, respectively. If the standard light flux is known Φ s, the luminous flux Φ D=ED × Φ s/Es of the measured lamp. The integral method makes use of the principle of "point light source", which is simple to operate, but it is affected by the color temperature deviation between the standard lamp and the measured lamp, and the measurement error is large. Light separation method: the luminous flux is calculated by spectral energy P (λ) distribution. The 380nm~780nm spectrum of the standard lamp is measured in the integrating sphere by using the monochromator, and then the spectrum of the measured lamp is measured under the same conditions, and the luminous flux of the measured lamp is compared and calculated. The light efficiency is the ratio of the luminous flux emitted by the light source to the power consumed by the light source, and the light efficiency of LED is usually measured by constant current method.
Intensity distribution test: the relationship between light intensity and spatial angle (direction) is called false light intensity distribution, and the closed curve connected by this distribution is called light intensity distribution curve. Because there are many measuring points and each point is processed by data, automatic distribution photometer is usually used to measure.
The effect of temperature effect on the optical properties of LED: temperature will affect the optical properties of LED. A large number of experiments can show that temperature affects the emission spectrum and color coordinates of LED.
Spectral characteristic detection:
The spectral characteristics of LED include spectral power distribution, color coordinates, color temperature, color index and so on.
The spectral power distribution indicates that the light of the light source is composed of many different wavelengths of color radiation, and the radiation power of each wavelength is also different, which is called the spectral power distribution of the light source in order with the wavelength. The light source is compared and measured by spectrophotometer (monochromator) and standard lamp.
Color coordinates are the amount of luminous colors that represent the light source digitally on the coordinate diagram. The coordinate diagram that represents the color has a variety of coordinate systems, usually using X and Y coordinate systems. Color temperature is the amount of light source color table (appearance color representation) seen by the human eye. When the light emitted by the light source is the same color as the light emitted by the absolute blackbody at a certain temperature, the temperature is the color temperature. In the field of lighting, color temperature is an important parameter to describe the optical properties of light sources. The theory of color temperature originates from blackbody radiation and can be obtained from the color coordinates containing blackbody trajectories through the color coordinates of the light source. The color index indicates the amount of light emitted by the light source that correctly reflects the color of the illuminated object, usually expressed by the general color index Ra. Ra is the arithmetic average of the color index of the light source to eight color samples. Color index is an important parameter of light source quality, which determines the application range of light source. Improving the color index of white light LED is one of the important tasks of LED research and development. Surface brightness measurement: the brightness of the light source in a certain direction is the luminous intensity of the light source on the unit projection area of the light source in that direction. Generally, the surface brightness is measured by the surface brightness meter and the aiming brightness meter, which has two parts: the aiming light path and the measuring light path.
Measurement of other performance parameters of LED Lamps
Measurement of electrical parameters of LED lamps: electrical parameters mainly include forward voltage, reverse voltage and reverse current, which is related to whether LED lamps can work properly, and is one of the basis to determine the basic performance of LED lamps and lanterns. There are two kinds of electrical parameters to measure LED lamps and lanterns: when the current is constant, the voltage parameter is tested, and when the voltage is constant, the current parameter is tested. The specific methods are as follows:
Forward voltage: applying forward current to the LED lamp to be detected produces a voltage drop at both ends of the voltage drop. Adjust the power supply determined by the current value and record the relevant readings on the DC voltmeter, that is, the forward voltage of the LED lamp. According to the relevant common sense, when LED is forward conduction, the resistance is small, and the external connection method of current table is more accurate.
Reverse current: apply the reverse voltage to the detected LED lamp and adjust the voltage stabilizing power supply. The reading of the ammeter is the reverse current of the measured LED lamp. It is the same as the measurement of forward voltage, because the resistance of LED reverse conduction is large, and the ammeter internal connection method is adopted.
Thermal properties of LED lamps and lanterns: the thermal properties of LED have an important influence 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 between the PN junction and the shell surface, that is, the ratio of the temperature difference along the heat flow channel to the power dissipated on the channel, and the junction temperature refers to the temperature of the LED PN junction.
The methods of measuring LED junction temperature and thermal resistance are generally infrared microimager, spectroscopy, electrical parameter method, photothermal scanning method and so on. Using infrared temperature microscope or micro thermocouple to measure the surface temperature of LED chip as the junction temperature of LED, the accuracy is not enough.
At present, the electric parameter method is widely used to make use of the linear relationship between the forward voltage drop of LEDPN junction and the temperature of PN junction, and the junction temperature of LED is obtained by measuring the difference of forward voltage drop at different temperatures.
