Detection of optical parameters of LED lamps

Update:01 Nov 2019

1.  Luminous intensity detection
Light intensity, the intensity of light, refers to the amount of light emitted at a particular angle. Due to the concentrated light of the LED, the inverse square law is not applicable in the close range. The CIE127 standard specifies two measurement averaging methods: measurement condition A (far field condition) and measurement condition B (near field condition) for measurement of light intensity. In the case of light intensity, the detector area of ​​both conditions is 1 cm 2 . Normally, the standard light condition B is used to measure the light intensity.
2. Luminous flux and light efficiency detection
The luminous flux is the sum of the amount of light emitted by the light source, that is, the amount of luminescence. The detection methods mainly include the following two types:
(1) Integration method. The standard lamp and the lamp under test are sequentially ignited in the integrating sphere, and their readings in the photoelectric converter are recorded as Es and ED, respectively. The standard light flux is known as Φs, and the luminous flux of the lamp under test is ΦD=ED×Φs/Es. The integral method utilizes the principle of "point 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, the spectrum of the standard lamp from 380 nm to 780 nm is measured in the integrating sphere, and then the spectrum of the lamp under test is measured under the same conditions, and the luminous flux of the lamp to be measured is calculated and compared.
The luminous efficiency is the ratio of the luminous flux emitted by the light source to the power consumed by it, and the luminous efficacy of the LED is usually measured by a constant current method.
3. Spectral characteristics detection
The spectral characteristic detection of the LED includes spectral power distribution, color coordinates, color temperature, color rendering index and the like.
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. This difference is sequentially arranged with the wavelength, which is called the spectral power distribution of the light source. The light source is obtained by comparison measurement using a spectrophotometer (monochromator) and a standard lamp.
The color coordinate is a digital representation of the amount of illuminating color of the light source on the graph. The coordinate graph representing the color has multiple coordinate systems, usually in the X and Y coordinate systems.
The color temperature is the amount of the light source color table (appearance color appearance) that the human eye sees. When the light emitted by the light source is the same color as the light emitted by the black body at a certain temperature, the temperature is the color temperature. In the field of illumination, color temperature is an important parameter describing the optical properties of a light source. The theory of color temperature is derived from blackbody radiation, which can be obtained from the color coordinates of the blackbody locus by the color coordinates of the source.
The color rendering index indicates the amount by which the light emitted by the light source correctly reflects the color of the object, which is usually expressed by the general color rendering index Ra, which is the arithmetic mean of the color rendering index of the eight color samples. Color rendering index is an important parameter of light source quality, which determines the application range of light source. Improving the color rendering index of white LED is one of the important tasks of LED research and development.
4. Light intensity distribution test
The relationship between the light intensity and the spatial angle (direction) is called the pseudo-light intensity distribution, and the closed curve formed by such a distribution is called the light intensity distribution curve. Since there are many measuring points and each point is processed by data, it is usually measured by an automatic distribution photometer.
5. Effect of temperature effect on optical characteristics of LED
Temperature affects the optical properties of the LED. A large number of experiments can show that the temperature affects the LED emission spectrum and color coordinates.
6. Eurface brightness measurement
The brightness of the light source in a certain direction is the luminous intensity of the light source in the projected area of ​​the light source. Generally, the surface brightness meter and the aiming brightness meter are used to measure the surface brightness, and there are two parts of the aiming light path and the measuring light path.