TLCI analysis of color rendering evaluation standard for LED light source

Abstract: Usually we use CRI to evaluate the color reproduction ability of a light source. However, due to the spectral discontinuity of the LED light source, this evaluation method has some problems, especially the white light illumination environment mixed with color light sources, due to the spectrum and camera. The sensor chip does not match, showing the deviation of color reproduction, and the TLCI evaluation system is more accurate. This paper compares CRI and TLCI evaluation system.

Keywords: TLCI CRI LED spectrum camera

1. Generation of TLCI standards

The International Commission on Illumination (CIE) defines color rendering as the effect of a light source on the color appearance of an object compared to a standard reference source. For many years, people have been using the Color Rendering Index (CRI) as a way to judge the ability of a light source to restore true colors. CRI is determined for the human eye and is a measure of color recognition in comparison to a standard source. It is still a widely accepted standard for measuring the color rendering capabilities of light sources. In this evaluation system, the tungsten lamp has the best CRI performance, almost 100, and other sources have lower spectral discontinuities than tungsten lamps CRI.

In recent years, the use of LED light sources has become very common in the lighting environment of film and television shooting. Due to the special principle of LED light source illumination and spectral distribution, sometimes a light source with a high CRI index, when used in a film and television shooting environment, will also show a mismatch between the spectral components and the color characteristics of the camera's sensor chip. The deviation in the color of the output screen. Although these deviations can be appropriately corrected by post-grading, they usually take a lot of time and cost, and are often not satisfactory.

Therefore, this requires a new metric that not only correctly reflects the human eye's perception of the light source color, but also reflects the camera's acceptance of the light color and the degree of color reduction. To this end, the former BBC engineer, optical expert Alan Roberts and the European Broadcasting Union (EBU) worked together to propose the TV light source consistency parameter in November 2011 (Television Lighting) The Consistency Index, abbreviated as TLCI, is named TLCI-2012. It is to test the spectral energy distribution of the measured light source by the spectral radiation tester, and then calculate with the analysis software to obtain the test result of the TLCI.

2. Main contents of the TLCI standard

TLCI-2012 is a standard recommended by the European Broadcasting Union (EBU) for lighting manufacturers and film and television production units. It is not an internationally accepted technical standard. This standard was established and improved under the strong support and active promotion of the BBC TV station. It has become the most accepted and universal standard. It is a standard for assessing the quality of studio lighting color performance.

TLCI is a measurement and calculation of the spectral energy distribution emitted by a light source using a spectroradiometer. It uses data analysis to represent the degree of performance of the color reproduction of the light source in a television lighting environment with values from 0 to 100. The meaning of the values is basically the same as CRI, but it is also very different. For CRI, the color rendering index of a light source is greater than or equal to 85, which is considered to be suitable for TV shooting, and the value of TLCI-2012 is more detailed, as shown in Table 1.

Table 1 Relationship between the value of light source TLCI-2012 and post-gradation

As we all know, the measured value of CRI is determined by the difference between the appearance of the specified 14 color samples under the measured light source and the appearance of the standard reference light source. These samples were selected from Munsell color samples. Figure 1 shows 14 standard CRI color samples. The first 8 blocks are usually used to determine the general color rendering index Ra (usually the CRI value mentioned is the general color rendering index). The selected TCS01-TCS08 has medium saturation and Roughly the same brightness, and the range of colors covers the entire visible spectrum. The last 6 blocks are special color samples. TCS09-TCS14 is rarely used. In addition to imitating European skin color and leaf green, it also includes higher saturation primary colors.

Figure 1 14 standard color samples specified by CRI

The TLCI standard test is somewhat similar to CRI and is determined by a color contrast icon showing the alignment results (as shown in Figure 2). There are 24 test color blocks in TLCI. The test results are based on the standard light source (the standard light source refers to the light source whose color and color temperature are both falling on the Planck curve), and the TV camera output screen is based on each standard contrast color block. The color performance obtained. This color performance refers to the color performance that can be seen on a standard level HDTV monitor.

Figure 2 TLCI standard test patch table

A small color block is inserted in the center of each test patch in the test results of the TLCI standard. This small color block is the color performance of the tested light source on the camera monitor in the same test environment. At the time of testing, the camera was color balanced for each color, so the gray level was accurate. Figures 3 and 4 show the results of testing two different light sources. It is obvious that the TLCI test result of the first light source is lower than that of the second light source. Figure 3 shows a white LED spotlight randomly sampled on the market. Figure 4 shows the test results of an ARI (ARRI) L7-C spotlight.

Figure 3 General LED white spotlight TLCI test results

Figure 4 Alai (ARRI) L7-C spotlight TLCI test results

Figure 5 shows a chart of test results containing complete TLCI data. In addition to the patch map, the test results include a color correction suggestion table and a spectral energy distribution map. The first line of the top text of the figure describes the name of the light source being measured and its associated color temperature value (CCT).

The value "-0.6" in parentheses refers to the deviation of the color temperature value of the light source from the standard color temperature curve, "+" means the side of the magenta direction, and "-" means the side of the green direction. When the deviation value is greater than 1, the actual test significance is not great. The red text in the second line indicates the value of the TLCI test result. The test result value of this light source is 90.

Figure 5 TLCI complete data test results chart

Figure 6 Color correction suggestion table and spectral distribution curve

On the right side of the colorimetric block, there is also a color correction suggestion table and a spectral distribution curve, and FIG. 6 is a partial enlarged view. The actual light source TLCI has reached 90, has reached the broadcast standard, but the test results still give the direction of calibration if you want a very accurate color reproduction. The color correction suggestion table gives the direction and degree of color correction in the later stage of the image. For 12 different color regions, "+" and "-" are marked according to the degree, and "+" indicates enhancement, "- "It means weakening, and the number of "+" and "-" indicates the degree of strength. For each color area, correction suggestions for three parameters of lightness, chroma, and hue are indicated. For example, the first line "red (R)" and the second line "red / yellow (R / Y)" three parameters do not need to be corrected. The third line "yellow (Y)" brightness and saturation do not need to be corrected, the hue needs a little weakening. The sixth line "Green/Cyan (G/C)" brightness does not need to be corrected, the hue needs to be slightly weakened, and the saturation needs to be enhanced to a large extent.

The lower right corner of the TLCI complete data test chart is the spectral energy distribution of the visible light region (wavelength 380nm to 750nm) of the measured light source. The light blue curve in the figure is the spectral curve.

3. Comparison of TLCI and CRI test results

Through the above analysis, it is not difficult to see that the purpose and method of TLCI and CRI testing are not the same, TLCI is for cameras and CRI is for human eyes. But they all use the value of the number 0 to 100 to indicate the color performance of the light source. What is the relationship between the two? Let's compare the two values of the light source.

Figure 7 shows the relationship between the TLCI and CRI test results of the tested light source. Each black dot represents the coordinate position of a light source test result. The horizontal coordinate corresponds to the CRI value and the ordinate is the TLCI value.

From this we can see that the two are not necessarily linear. That is to say, a light source with a high CRI value does not necessarily have a high TLCI value.

The right side of the pink column in the figure indicates a light source with a CRI value higher than 80. Among these light sources, the TLCI value is high or low. From the experience of use, it is the light that looks bright to the naked eye, and its color rendering performance in the camera lens is good or bad.

We select the test results of four different light sources to illustrate: The light source represented by point A has a CRI value of 92, but the TLCI value is only 50, indicating that the light source is very good in color reduction, but in the camera. The output is poor in color reproduction. This is a light source that is not suitable for broadcast. The C-point and TLCI values of point B are higher than 90, which is a good color performance for both the naked eye and the camera output. The light source is also suitable for the broadcast environment; the C-point value of point C is 70 is not very high but the TLCI value is 90, which means that it has a general color reproduction degree under the naked eye, but it is suitable for broadcast; D point represents a CRI and TLCI. A light source of less than 50, which has poor color reproduction at the output of the naked eye and at the output of the camera, is a light source that is not suitable for use in a broadcast environment.

Figure 7 TLCI and CRI test results of various different LED sources

The reason is that the CRI test and calculation is based on the human eye's ability to distinguish colors, and the TLCI calculation also refers to the camera's ability to distinguish colors. The principle of resolution of the two is similar, but there are differences. Figure 8 and Figure 9 show the sensitivity curve of the human eye to color and the sensitivity of the general TV camera to color. The sensitive peaks of the two are not completely coincident.

Figure 8 Human eye sensitivity to color

Figure 9 Camera sensitivity to color

In this case, the color displayed in the human eye may not be the same as the color displayed on the camera output. Similarly, the 5600K white light that looks the same in the human eye, in the camera lens, the color reproduction ability is even different. Because the LED light source is different in white light, the spectrum of the resulting white light may vary greatly. Figure 10 shows a conventional RGB hybrid to produce a white LED spectrum.

Figure 10 Traditional RGB mixed white LED spectrum

Figure 11 Spectrogram of a conventional tungsten lamp

This RGB mixed white light spectrum is quite different from the white light spectrum of a conventional tungsten lamp (shown in Figure 11). It is a poorly continuous spectrum. It has a TLCI value of only 20 and a CRI value of 75. In order to get better TLCI values, LED lighting manufacturers are also using different methods to design LED lamps with better spectral distribution and more suitable for studio broadcast environment.

4 Conclusion

How to choose high-quality LED luminaires suitable for film and television lighting environment requires practice to verify and standard specifications. TLCI-2012 reflects the lighting environment requirements of TV cameras, improves the application level of LED light sources in film and television lighting, and lays a scientific foundation for the production and use of LED lamps. It is currently the most accepted and universal standard.

references:

1. Practical Spectroradiometry - An introduction for users of the Television Lighting Consistency Index, Alan Roberts, 2013.8

British Television Photography Conference Conference Article

2.Lighing Product Introduction by ARRI product management

Florian Bloch, ARRI China Open Day Technology Exchange 2014.10

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