In the field of lighting, people recognize objects and perceive colors through light source lighting, so the color rendering evaluation of light sources is very important. Now the most widely known and most widely used to describe the color rendering of light sources is the color rendering index Ra proposed by CIE (formal in 1974). become the evaluation standard of light source color rendering).
So, since there is a unified color rendering evaluation index, why is the color display of each COB product different from the color saturation evaluation index of the light source announced to the outside world? This has to start with the rapid development of LED technology.
With the development of LED technology, many studies and experimental results show that the general color rendering index Ra has limitations in describing people's perception of color. For example, Ra emphasizes the difference between the measurement light source and the reference light source. The reference light source has a maximum color rendering index of 100, which is regarded as the light source with the best color rendering effect. During the color rendering calculation process, any color difference will cause the color rendering index. It is considered that the color rendering effect is not as good as the reference light source. This is different from people's daily psychological perception. For example, the light-emitting mechanism of incandescent lamps is considered to be the closest to the reference light source, and the color rendering index is generally above 98. However, in practical applications, using incandescent lamps to illuminate dark blue and black socks, It is difficult for the human eye to accurately distinguish between colors.
Therefore, based on this, when the general color rendering index Ra has limitations in describing the color rendering characteristics of LED light sources, the industry has begun to propose various new light source color rendering evaluation methods. For example, in order to improve the light quality of products, some LED manufacturers claim that by adjusting the light source spectrum (the budding stage of full-spectrum COB), appropriately increasing the color saturation of the illuminated object can improve visual clarity, thereby enhancing the psychological preference of object color, etc. .
However, this evaluation method is difficult to describe with the color rendering index Ra, so new marketing tools are needed to measure color saturation, so that customers can understand the color rendering effect of this product through data, which can provide a more comfortable vision. experience.
So, what are the more representative evaluation methods? The following will introduce the color saturation evaluation indicators commonly used in the market one by one.
R9 special color rendering index
The special color rendering index R9 is the color rendering ability of saturated red samples. Because LED products generally lack red light components, R9 is generally used in the industry as an important supplement to the general color rendering index Ra , which is used to describe the restoration of saturated red by light sources. capability, R9>0 becomes the industry standard requirement.
GAI color gamut index
The GAI color gamut index was proposed by Rea et al. in 2007. It uses the 8 color samples TCS01-TCS08 used in the color rendering index as the standard color samples, and calculates the 8 color samples formed by the reflection of the light source under test in the standard color sample in the CIE1976 chromaticity space. The ratio of the color gamut area enclosed by each color point to the area of the reference light source. The reference light source used by GAI defined by Rea is iso-energy spectrum, and it is recommended that the light source satisfy the color rendering index of at least 80 and the GAI color gamut index between 80 and 100, which will make the color rendering of the light source have good naturalness and vividness at the same time. Spend. In actual calculation, it is found that GAI is positively correlated with color temperature. Some manufacturers change the reference light source to a black body radiation light source and express it by GAIBB.
CQS Color Quality Index, Qg
The CQS color quality index was proposed by Wendy and Yoshi in 2010. CQS also uses the colorimetric method to calculate their color difference by illuminating the same set of standard color samples with the measured light source and the reference light source. Desaturated colors have good color rendering, it may have poor color rendering for saturated colors, but the reverse is not true. As long as some saturated colors are used as a new set of color samples, the measurement of color rendering can be more representative, so CQS selects 15 saturated colors that are evenly distributed throughout the visible spectrum, and optimizes the algorithm at the same time. The CQS system indicators include Qf, Qg, Qa, etc. Qf is used to describe the fidelity of the color, it describes the color difference between the measured light source and the reference light source, and Qg describes the relative color gamut formed by 15 standard color samples in CIELAB Area, that is, color saturation, Qa allows a certain degree of supersaturated color when calculating color difference].
TM-30-15 Standard, Rg
The Illuminating Society of North America (IES) approved the standard named IES Method for Evaluating Light Source Color Rendition, numbered TM-30-15 in May 2015 . This standard provides an accurate and robust calculation method for fidelity Rf and saturation Rg to characterize the color rendering of light sources. TM-30-15 also adopts colorimetric method, but in terms of calculation methods such as reference light source, standard color sample, and index description, many optimizations have been carried out on the basis of summarizing predecessors.
The above four are relatively representative evaluation indicators of color saturation. It is precisely because each manufacturer adopts different evaluation indicators, so different manufacturers have different standards when producing COB products, and they will also have different focuses when publicizing them. It became popular in the Asian exhibition, but it is difficult to see one of the reasons for the differences.
So how effective are these metrics for evaluating color saturation or visual experience?
In order to evaluate the effect of these indicators used to describe the color rendering saturation of light sources, I once participated in an experiment - selecting several products on the market that advertised with improved color rendering saturation for index calculation and effect evaluation. In the experiment, 4 models with the same illuminance (500lux) and the same nominal color temperature (3000K) were selected for the experiment, respectively illuminating the color card, the simulated fruit and the miniature model, and 24 observers with normal vision were selected for subjective visual evaluation in the dark room.
The following conclusions are drawn from the above experiments:
1. The correlation between the four saturation evaluation indicators is weak, and it is difficult to make a horizontal comparison;
2. The four indicators are difficult to effectively express the observer's preference for color;
3. The improvement of saturation needs to be moderate, and too high saturation cannot bring a better visual effect experience;
4. Deliberately declaring how much a certain indicator achieves is of little significance, and further research is needed on the evaluation method of the color saturation of the light source.
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