The feeling of the sunlight, as we know, is different from morning till night in nature. According to the position where the sun moves in the sky, the color of the sun will turn into red, orange, yellow, and white. The change in the color of the sunlight is mainly caused by the difference of refraction and reflection of light in the atmosphere in a day. In physics, this change in white light is described by color temperature. Every morning and evening, the natural sunlight looks reddish and the color temperature is low, while the natural light at noon is blue-white, which is a high color temperature.

Color temperature is a unit of measurement that characterizes the color of white light. Theoretically, the color temperature refers to the color that the Blackbody appears after being heated from absolute zero (-273 °C). The color of the blackbody gradually turns from black to red, to yellow, to white, and finally to blue light after being heated. At a certain temperature, the light color radiated by the blackbody is characterized by the temperature at that moment, and the unit is “K” (Kelvin). If the light emitted by another light source is the same as the spectral component contained in the light emitted by the blackbody at a certain temperature, the color temperature of the light source could be defined by the temperature of the blackbody. For example, the light color emitted by the 100 W bulb is the same as that of the blackbody at 2527 K, thus the color temperature of the light emitted by this bulb should be 2527 K.

Artificial light sources, such as fluorescent lamps, LED lamps, etc., are not thermally radiated. The light color emitted by the artificial light sources is not exactly the same as that emitted by the blackbody at various temperatures. The concept of “correlated color temperature” was introduced. When the light color emitted by these light sources is closest to the one emitted by the blackbody at a certain temperature, the temperature of the blackbody is referred to as the correlated color temperature of these artificial light sources.

LEDs, nowadays, have gradually replaced traditional bulbs and fluorescent lamps because of their advantages of energy saving, environmental protection, lng lifetime and small size. They are widely used in indoor lighting, signal lights, indicator lights, vehicle lights, display screens, advertising screens, outdoor large-screen and other light-emitting devices, and are praised as the new generation of green energy light-emitting device of energy-saving and environmental-friendly in the 21st century solid-state lighting field. In lighting filed, different phosphor recipes are designed to make LEDs achieve white light with different color temperatures. However, people sometimes feel that LED lighting is uncomfortable, one of the reasons is that the color coordinate points of the light sources deviate too much from the Planckian locus (or the blackbody locus), resulting in color distortion. And “Δuv” is used to characterize the color point deviation.

The color of the light sources is not good or bad, but the light color used for illumination is an exception. The light color has an evaluation index: ±Δuv, named as the color point deviation. The smaller deviation, the better the illumination color, and the best at Δuv=0. Due to meteorological factors, natural sunlight average +Δuv 0.001 is defined as CIE recombination daylight, so color temperature over 4000K requires + Δuv 0.001.

As is well-known that there is the Planckian locus in the chromaticity diagram. Different positions on the Planckian locus mean different color temperature of the illuminants. Deviation of the color coordinate for this light source to the Planckian locus is expressed by Δuv as shown in Fig. 1. The upper deviation is represented by +Δuv, and the lower deviation is represented by -Δuv. There is a standard with the color temperature at 6504 K. As the color temperature gets larger, the light color gradually changes from white → light blue → blue (6500 K~15000 K). On the contrary, the light color gradually changes from white → light yellow → yellow →orange → orange red → red (6500 K~1800 K). Generally, the white turns greenish when the Δuv increases positively, and turns magenta when Δuv increases negatively.

Figure 1. White Light Region Defined by CIE.

We selected two white light sources to show the color appearance at different color deviations which were nature white at 6500 K color temperature and warm white at 3500 K color temperature. The comparison is shown in Fig. 2.

Figure 2. The comparison of color appearance at different color deviation.

Due to the content ratio distribution of red, green and blue phosphors, the color deviation of the LEDs even in the same batch will be inevitably generated during production process. We generally use color tolerance to determine the product criteria of the color consistency. The smaller color tolerance means the higher color consistency.

When evaluating different color temperature of the light sources, the standard light sources for reference are different (generally the detection equipment will automatically determine the standard light sources). In the lighting industry, because the human eyes have different perceptions of chromatic aberrations at different color temperatures, so the color tolerance requirements are different depend on the color temperature are different. The color coordinates would be different for the light sources with the same color temperature but different color deviation.

SDCM is an acronym which stands for Standard Deviation Color Matching. Usually, we could perceive difference of 5-7 SDCM (Fig. 3).

Figure 3. The range of color tolerances can be identified by the human eyes.

In order to ensure color consistency, the LED will be classified by color bin. The color tolerance at different color temperature have been defined by ANSI. The standard requirements are shown in Tab. 1. And the color bins at different color temperature are defined as shown in Fig. 4.

Table 1. The ANSI C78.377-2008 general CCTs and color tolerance for LED.

Note:

1) There are 6 general CCTs corresponding to fluorescent lamps of 2700 K, 3000 K, 3500 K, 4100 K, 5000 K and 6500 K color temperatures;

2) T should be an integer of 100 K (e.g. 2800 K, 2900 K, …, 6400 K), but excludes the 8 rated CCT values listed above;

3) ΔT = 0.0000108×T2＋0.0262×T＋8；

4) Δuv = 57700×(1/T)2－44.6×(1/T)＋0.0085.

As shown Table 1, the ANSI standard defined the central coordinate on the Planckian locus for the color temperatures below 3500 K. But for high color temperatures, there are none zero Δuv which means the central coordinates deviated from the Planckian locus. Especially at 6500 K, the Δuv is 0.003 which is a large value. This means that the light color will be greenish base on this standard.

Figure 4. The color bins requirements at different color temperature by the ANSI standard.

According to ANSI standard, the white light at 4000 K, 5000 K, 5700 K and 6500 K color temperature is greenish, which is disadvantageous for the color reproduction of the illuminants. Since the color deviation is tiny, so it is difficult to perceive the incorrect color appearance by human eyes. However, in the field of photography, film and television lighting, this tiny color deviation would be easily distinguished by camera or any other equipment.

To Figure out the fundamental reason of the color appearance for the light sources with the same color temperature, we fabricated two kinds of LEDs with the same color temperature but difference color coordinates. The color coordinate of one LED is just on the Planckian locus (0.3136, 0.3235), the spectrum is shown in Fig. 5 by orange curve. And the other is centered at (0.3123, 0.3282) which is the color coordinate of 6500 K white according to the ANSI standard, the spectrum is shown in Fig. 5 by blue curve.

Figure 5. Spectral comparison of two different LEDs with the same color temperature. Orange Curve: Spectrum of the LED with the color coordinate of (0.3136, 0.3235) which is on the Planckian locus. Blue Curve: Spectrum of the LED with the color coordinate of (0.3123, 0.3282) which is the central color coordinate of ANSI 6500 K.

As shown Figure 5, in the green light region at the wavelength from 480 nm to 600 nm, there are significantly more green content for the blue curve than the orange curve.  Additionally, in terms of color rendering, the data show that the color rendering index Ra of the LED with orange spectral curve is higher than the other LED.  Typically, for the special color rendering index R9, and in the special color rendering index R9, the value of the LED with orange spectral curve is higher about 10 than the other LED.  These means that the color quality of the LED with the color coordinate on the Planckian locus is better. The differences will be more obvious from camera vison.

According to the above results, from the view of light quality, color bin scheme centered on the Planckian locus will be better than the bin scheme defined by ANSI-based. Therefore, YUJI developed own color bin scheme for photography lighting that the center color coordinate of each color temperature is just on the Planckian locus. In order to further improve the color consistency, YUJI also reduce the color tolerance of each sub-bin to 3SDCM. Fig. 6 shew the details of YUJI’s color bin scheme.

Figure 6. The color bins at different color temperature of YUJI’s LED.