Luminous intensity detection
Light intensity is the intensity of light, which refers to the amount of light emitted within a certain angle. Due to the concentrated light of LED, the inverse square law is not applicable in the case of close range. The CIE127 standard stipulates two measurement averaging methods: measurement condition A (far field condition) and measurement condition B (near field condition). Under the condition of the light intensity, the detector area under both conditions is 1cm2. Normally, the standard condition B is used to measure the luminous intensity.
Luminous flux and light effect detection
Luminous flux is the total amount of light emitted by the light source, that is, the amount of light emitted. The detection methods mainly include the following 2 types:
(1) Integral method. Light the standard lamp and the lamp under test in turn in the integrating sphere, and record their readings in the photoelectric converter as Es and ED respectively. The standard light flux is known as Φs, then the luminous flux of the lamp under test ΦD=ED×Φs/Es. The integration method uses the principle of 'point light source', and the operation is simple, but it is affected by the deviation of the color temperature of the standard lamp and the lamp under test, and the measurement error is large.
(2) Spectroscopic method. The luminous flux is calculated from the spectral energy P(λ) distribution. Using a monochromator, measure the spectrum of the standard lamp from 380nm to 780nm in the integrating sphere, then measure the spectrum of the lamp under the same conditions, and compare and calculate the luminous flux of the lamp under test.
The light effect is the ratio of the luminous flux emitted by the light source to the power it consumes, and the light effect of the LED is usually measured by a constant current method.
Spectral characteristics detection
The detection of the spectral characteristics of LEDs includes spectral power distribution, color coordinates, color temperature, color rendering index and other content.
Spectral power distribution means that the light of the light source is composed of many colored radiations of different wavelengths, and the radiation power of each wavelength is also different. This difference is called the spectral power distribution of the light source when they are arranged in order of wavelength. Use a spectrophotometer (monochromator) and a standard lamp to compare and measure the light source.
The color coordinate is an amount that represents the light emission color of the light source on a coordinate graph in a digital manner. There are many coordinate systems for color charts, usually X and Y coordinate systems.
The color temperature is an amount that represents the color table (appearance color expression) of the light source seen by the human eye. When the light emitted by the light source is the same color as the light emitted by the absolute black body at a certain temperature, the temperature is the color temperature. In the field of lighting, color temperature is an important parameter that describes the optical characteristics of a light source. The relevant theory of color temperature is derived from black body radiation, which can be obtained from the color coordinates containing the black body locus through the color coordinates of the light source.
The color rendering index indicates the amount that the light emitted by the light source correctly reflects the color of the object. It is usually expressed by the general color rendering index Ra, which is the arithmetic average of the color rendering index of the light source for 8 color samples. The color rendering index is an important parameter of the quality of the light source. It determines the application range of the light source. Improving the color rendering index of white LEDs is one of the important tasks of LED research and development.
Light intensity distribution test
The relationship of light intensity varying with spatial angle (direction) is called false light intensity distribution, and the closed curve formed by this distribution is called light intensity distribution curve. Because there are many measuring points, and each point is processed by data, an automatic distribution photometer is usually used for measurement.
Influence of temperature effect on LED optical characteristics
Temperature affects the optical characteristics of LEDs. A large number of experiments can show that temperature affects the LED emission spectrum and color coordinates.
Surface brightness measurement
The brightness of a light source in a certain direction is the luminous intensity of the light source per unit projected area in that direction. Generally, a surface brightness meter and an aiming brightness meter are used to measure the surface brightness. There are two parts: an aiming optical path and a measuring optical path.
Measurement of electrical parameters of LED lamps
Electrical parameters mainly include forward, reverse voltage and reverse current, which is related to whether the LED lamps can work normally, and is one of the basis for judging the basic performance of LED lamps. There are two types of electrical parameter measurement for LED lamps: namely, voltage parameters are tested when the current is constant; current parameters are tested when the voltage is constant. The specific method is as follows:
(1) Forward voltage. When a forward current is applied to the LED lamp to be detected, a voltage drop will occur at both ends. Adjust the power supply determined by the current value and record the relevant reading on the DC voltmeter, which is the forward voltage of the LED lamp. According to related common sense, when the LED is conducting forward, the resistance is small, and it is more accurate to use the external method of the ammeter.
(2) Reverse current. Apply reverse voltage to the tested LED lamps and adjust the regulated power supply. The reading of the ammeter is the reverse current of the tested LED lamps. It is the same as the measurement of the forward voltage, because the resistance is larger when the LED is reverse conducting, so the internal connection method of the ammeter is used.
LED lamp thermal characteristics test
The thermal characteristics of LEDs have an important influence on the optical and electrical characteristics of LEDs. 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 surface of the case, that is, the ratio of the temperature difference along the heat flow channel to the power dissipated in the channel. The junction temperature refers to the temperature of the LED's PN junction.
The methods of measuring LED junction temperature and thermal resistance are generally: infrared micro-imager method, spectroscopy method, electrical parameter method, photothermal resistance scanning method, etc. The surface temperature of the LED chip measured by an infrared temperature microscope or a miniature thermocouple is used as the junction temperature of the LED, and the accuracy is not enough.
At present, the commonly used electrical parameter method is to use the characteristic that the forward voltage drop of the LEDPN junction has a linear relationship with the PN junction temperature, and the junction temperature of the LED is obtained by measuring the difference of the forward voltage drop at different temperatures.