LED car headlights are the most high-end applications for LEDs in the automotive lighting industry and are still in their infancy. On the one hand, as a new light source for LED headlamps, there are many basic difficulties that need to be overcome by itself. Therefore, the current development efforts focus on achieving a good distance and near light distribution, total luminous flux output performance, and lamp cooling solutions. , control technology, appearance modeling.
On the other hand, the standards for LED headlamps at home and abroad only relate to the color range, color stability and color rendering index, and there is little research on the visual comfort during use. The human eye is observing things for a long time at the mixed color temperature, which easily causes discomfort such as visual fatigue. Studies have shown that the detection rate of the hybrid color temperature lamp on the target is lower than that of a single color temperature lamp, so the inhomogeneity of the color temperature will be Driving safety has an impact.
In addition, LED headlights with poor color temperature uniformity will occupy significant disadvantages in subjective evaluation, and it will also affect the aesthetics and comfort of the lights.
Figure 1 LED lamp color temperature uniformity test
In evaluating the color temperature uniformity of lamps, the first thing we think of is the use of a distributed photometer to test the spatial color temperature distribution of LED lights. We have selected an LED headlamp as a test sample. The test results show that the lamp performs poorly in terms of color temperature uniformity regardless of whether it is low beam or high beam. First of all, the distance between the maximum color temperature and the minimum color temperature of far and near lights is greater than 1600K and 1300K, respectively. Some studies have suggested that the color temperature of the human eye can be extremely low resolution up to 50-100K, we measured the color temperature difference data is 10 times as much, such serious color temperature dispersion is unexpected.
It can be observed from the spatial color temperature distribution map of far and near light that the average color temperature in the range of 0 degrees to 30 degrees is higher than the color temperature in the range of 0 to -30 degrees in the direction of Gamma angle, and in this area, the range is 0 to 10 degrees. The color temperature inside is significantly higher than in other areas. This is related to the angle of light output from the headlights. The projection target of the headlight is the road surface, so the center light intensity will deviate horizontally downwards by about 10°.
Because the headlamp beam angle is very narrow, the color temperature other than the main beam angle is often not used. Based on the above phenomenon, we believe that the color temperature distribution of the headlamp measured on a distributed photometer cannot be intuitively reflected in the color temperature distribution. Thus, we turned to observe the color temperature distribution of the luminaire on the test screen.
When evaluating the uniformity of the color temperature of the headlight, we refer to the uniformity test method of the backlight, that is, the light pattern is projected onto the test screen at a certain distance, and the color temperature value at the test point is collected at a certain interval. The results showed that the maximum difference in the color temperature of the low beam lamp is 725K and that of the high beam lamp is 642K. Both of these values ​​are less than half the published difference in spatial color temperature of the previous test. Similarly, the standard deviation of the color temperature is also half of the previous test value. This is related to the test valid data and the sample size of the two. Different color temperature rays superimposed on the test screen will also reduce the occurrence of extreme color temperature values. Although the inhomogeneity is not so serious, it reflects the presence of headlamp color temperature non-uniformity from another perspective.
Figure 2 The average color temperature distribution of LED lights
Some phenomena are worth our attention. The maximum color temperature point and the minimum color temperature point are at the edge of the spot. If the color temperature distribution on the test screen is divided into longitudinal and vertical blocks, and their average color temperature is calculated, the color temperature in the middle zone will be high. In the edge area, the color temperature uniformity in the middle area is also higher than in the edge area.
Both of the above tests reflect the uneven color temperature of the LED headlamps. We speculate that LED light sources are the most likely causes of these phenomena. After investigation, we have selected several LED device models that are currently applied in the mainstream market. The former two are single-device integrated packages, the third sample is a headlamp specifically developed COB device, and the fourth sample is EMC package. The single device needs to be soldered through the circuit board. This device is also the light source used for the entire lamp in the above test. The near-field optical test system was used to test the color temperature distribution of the light emitting surfaces of the above four samples, and about 2,000 points were collected on each sample. From the color temperature maps of the four samples, we can clearly see that the edge of the device is yellow, and the color temperature in the middle is relatively cold.
From the data point of view: the average color temperature of the four samples is very close, all around 5500K. The difference between the maximum and minimum of their color temperature is greater than 2500K. The maximum value of the color temperature of samples 1 and 2 is close to the average color temperature, but their minimum color temperature is different from the average color temperature by more than 2000K. The maximum and minimum values ​​of samples No. 3 and No. 4 are significantly different from their average value. These data allow us to feel that the device itself has much higher color temperature inhomogeneity than the entire lamp.
Figure 3 The color temperature distribution on the light output surface of the device
The color temperature distribution of the light exit surface of the device is similar to that of the lamp distribution:
The color temperature in the middle is higher than the edge color temperature, which is consistent with the phenomenon of the light-emitting surface observed by the naked eye above, and is similar to the color temperature distribution of the entire lamp tested previously.
The color temperatures of these LED devices on the circumferences of different radii centered on their geometric center are very close. The larger the radius, the lower the color temperature.
Look at the device's color temperature dispersion, mainly reflected by the standard deviation of color temperature:
If we calculate the dispersion of color temperature within a certain circle, we will find that the standard deviation of the color temperature of four devices gradually increases with the expansion of the circle radius, which is approximately linear. Explain that the color temperature worsens as the test range increases.
The standard deviation curve of the color temperature shows a sharp increase when the test range is extended to the edge of the light-emitting surface, indicating that there is a clear color temperature dispersion at the edge of the device. The No. 3 device has the largest degree of dispersion, and it belongs to the COB package. It is difficult to apply phosphor uniformity.
We know that the use of the car environment is much more severe than the average consumer electronics. The temperature used can be 20-30 degrees Celsius, or it can be 40 degrees Celsius or more. The direct temperature of the sun can reach more than 70 degrees Celsius. Under different conditions of use, will the LED device's color temperature uniformity change? We change the input current of the device while other conditions are unchanged, that is, change its power.
As a whole observation, as the power increases, the overall color temperature of the device will increase. The change trend of the color temperature at the edges of the two devices is relatively uniform, while the color temperature trend near the center changes greatly, and the third device is particularly noticeable. There are two possible reasons for the above phenomenon: First, when the power is increased, the luminous efficiency is decreased, more heat is emitted, the phosphor's ability to excite yellow-green light is reduced, and the overall color temperature becomes large. In general, the temperature of the device PN junction is the highest point of the LED temperature, and the chip is generally located near the geometric center of the package structure. Therefore, the change of the color temperature in the central area of ​​the light emitting surface is more significantly affected by the temperature. Secondly, the output wavelength of the LED chip will change with the influence of the injection current, temperature, and time. A "blue shift" occurs, and the change of the excitation wavelength leads to a change in the overall color temperature.
The overall color temperature dispersion has also changed, with the increase of power, the dispersion becomes larger and the color temperature uniformity becomes worse.
Therefore, when we are developing LED headlamps, we need to take into account the relationship between the power and color temperature, do a good thermal design to ensure the uniformity of color temperature and its stability.
Figure 4 The result of the device under the headlight reflector model
In order to simulate the color temperature distribution on the road surface, we imported 3# and 4# devices into the same headlight reflector model. The simulation results show that.
First, as the power increases, the overall color temperature on the road surface and the test screen rises as the device's color temperature rises.
Secondly, the color temperature distribution pattern on the test screen is very close to the illumination distribution pattern, with a significant horizontal cutoff and a 15 degree cutoff. The non-uniformity of the color temperature is mainly concentrated on the edge of the pattern, and is generally expressed as a "yellow outline". The color temperature in the middle is high, and the phenomenon that the edge color temperature is low coincides with the color temperature unevenness of the LED device. The unevenness of the color temperature on the road surface is more serious than that on the test screen, which is manifested as a more pronounced “spotâ€. The color temperature of the beam at different angles will have more obvious differences. Different color temperature intervals will appear, and sometimes there will be obvious points. Boundary line. Mainly appear in the front 1-5 meters area, 5 meters after the color temperature uniformity is relatively better.
We can consider improving the uniformity of the color temperature of the LED headlamp from the following aspects. For example, LED devices with better color temperature uniformity are used, with particular attention to the edge color temperature. As a packaging plant, improving the phosphor coating process ensures that the optical path of blue light is consistent at all angles. Use various possible cooling measures to improve the thermal performance of the device and ensure the stability of its light output. For optical designers, how to hide or remove the inhomogeneous marginal rays is also one of the topics worth thinking about.
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