1 Principle and purpose
Physically, it is assumed that an ideal complete radiator is called a black body, and the standard black body is heated. When the temperature is raised to a certain extent, the surface will emit light, and the color begins to change gradually from deep red-light red-orange-white-blue. Using this characteristic of light color change, when the color of a light source is the same as that of a black body, we refer to the absolute temperature of the black body at that time as the color temperature (CT) of the light source. The absolute temperature standard K (Kelvin, or Kelvin temperature scale) is the unit; the relationship between absolute temperature and Celsius temperature is: TK = (t + 273.15) oC. The law of black body changing from red to blue with increasing temperature is shown in Fig. 1.
Figure 1 shows the position of the different black body color temperatures on the chromaticity diagram, on a curved curve (called the black body trajectory also known as the Planck trajectory). Since the spectrum emitted by the actual light source is different from the black body spectrum, the color temperature of the black body that is similar to the color coordinate of the light source is defined as the color temperature of the light source, and the black color is called the correlated color temperature (that is, the color temperature of the light source usually referred to). The color coordinates of the light source of the same color temperature in the figure are represented as a short straight line intersecting the color temperature curve, which is called "isochromatic temperature line".
Since most of the light emitted by the light source is called white light, the color temperature reflects the difference in the amount of light of various colors contained in the white light that constitutes different color temperatures. For example, a whiteboard placed under an incandescent lamp or under the sun, although the incandescent lamp and the sun emit different spectra, due to the automatic adjustment of the human eye, people see white. But with the same camera shooting the whiteboard in these two kinds of light, the picture presented on the display is significantly different. The camera can be adjusted to suit different high and low color temperature scenes. When the camera is turned to a high color temperature, the whiteboard under the sunlight is normal, but the whiteboard under the incandescent light is yellow; when the camera is turned to a low color temperature, the camera is photographed. The whiteboard under the incandescent lamp is normal, but the whiteboard under the sunlight is blue. This is the characteristic of the camera. Therefore, in order to obtain a high-quality image with color balance, the TV lighting should adapt to these characteristics of the camera, and the color temperature of the lamp must have certain stability and consistency. A consistent and stable color temperature environment can be balanced by adjusting the camera. If the color temperature of the various lamps used is poor and the stability cannot be guaranteed, the camera can't do anything to face such a scene.
LED luminaires use LEDs as the light source. Today's white LEDs are made of blue LEDs with yellow phosphors. When the light is turned on, part of the blue light emitted by the LED is absorbed by the phosphor into yellow light, which is emitted together with the remaining part of the blue light, and is mixed to form white light. The color temperature of the white light of the LED is controlled by the adjustment of the ratio of blue light and yellow light, the ratio is different, and the color temperature is also different.
Since the white LED is composed of two main color lights, if the ratio of the two components is constant, the color temperature does not change. However, as the semiconductor emits light, the wavelength of blue light will change correspondingly with the influence of temperature and other factors. Generally, the wavelength of the LED chip emits light with the increase of temperature (the change is called “red shiftâ€), while the commonly used The yellow phosphor (YAGé’‡ aluminum garnet) has an optimum peak wavelength for the absorption and conversion efficiency of blue light. At normal temperature, the wavelength of the blue light of the LED and the optimum peak wavelength of the YAG phosphor always have a slight deviation. The peak conversion wavelength of YAG may be longer than the blue wavelength or shorter than the blue wavelength. When the temperature rises and the spectral wavelength of the LED is red-shifted, the amount of yellow light converted may increase or decrease. The ratio of the two will change, so that the color temperature of the mixed white light may increase or decrease. This change exists objectively, but in the studio, it will affect the recording quality of the program. Whether this change is within the allowable range requires testing and experimentation to verify.
LED production process is to use tens of thousands of LED dies on a large substrate, and then cut into individual LED dies. In these tens of thousands of LED dies, their characteristics are not exactly the same, the center Part of the die is quite different from the parameters of the peripheral die. The die segmentation and packaging manufacturers will perform testing and screening grading. However, the grading standards of each chip manufacturer are different. The manufacturer of the luminaires has different screening standards for each LED tube. Under the current conditions, the parameters such as the color temperature of each luminaire are also different. Moreover, the color temperature of different manufacturers' products is different, the color temperature of different models of the same brand is different, and the same model and different batches of products also have differences. The extent of the various differences, whether it is within the allowable range of recording, is also the subject of this test study.
In addition, in the case of dimming commonly used in the studio, whether the color temperature of the LED light source changes; after a long time of use, the attenuation of the LED light effect and the attenuation rate of the YAG phosphor performance will not be the same, how does this affect the color temperature; different brands The difference in color temperature of LED lamps; the difference in color temperature between spotlights and soft lamps, etc., has yet to be tested, and conclusions are drawn through experiments.
2 experiment
2.1 Experimental conditions
The color temperature of the luminaire can be measured by a colorimeter, and the spectral distribution of the luminaire can be measured by a spectrum analyzer, as shown in Fig. 2, and the chromaticity coordinates are obtained on the chromaticity diagram by computer analysis, and the black body trajectory is compared. The difference between the color temperature of the lamp and the standard black body is obtained, and the small change of the color temperature is more accurately grasped.
2.1.1 Instruments
This experiment mainly uses the following equipment:
(1) A PMS-50 spectral analysis system was used. As shown in Figure 3. Including sampling integrating sphere, spectrometer, computer digital processing system software.
(2) Distributed photometer. As shown in Figure 4. Due to the large size of the luminaire, the effective illumination and light distribution characteristics of the luminaire were tested with a GO1900 distributed photometer.
(3) CL–200 CHROM METER illuminance meter. The illuminometer has many functions, not only can measure the illuminance, but also can measure the excellent temperature, and the (x, y) value or (u, v) value of the excellent coordinates.
(4) Thermostat box. In order to measure the change of color temperature of lamps at different ambient temperatures, this experiment uses SDH4020 low temperature constant temperature and humidity chamber, which is an incubator with a light window. The temperature range of the device is controlled from -40 °C to 100 °C.
(5) AC regulated power supply. Multi-function AC regulated power supply APC AFC–500W.
