CRI/Ra test color samples (TCS)
Each of these scores is called Ri, where i represents the TCS number. For example, R9 is a commonly referenced deep red color score and is an important indicator of color quality for many applications.
CRI itself is a single number, and this is both a blessing and a curse. It is a great, convenient metric that is both intuitive and simple to communicate, but at the same time, particular color samples can distort the truth behind whether a light source is truly a high color rendering light source.
Below, we'll go into detail about each of the test color samples that make up the CRI metric.
Overview of the CRI test color samples
The test color samples are used in the calculation of CRI by simulating the reflectance spectrum from a light source under question, and comparing it to a reference source.
What is a reference source? It depends on the color temperature, but in general is a variant of black body radiation or the daylight illuminant.
The spectral files for the reference illuminants can be found here, and the individual TCS spectra are available here.
TCS1 is described as a light grayish red and has RGB values of (242, 185, 158) and HSL values of (13, 183, 188).
TCS1 has a relatively low reflectance across the visible spectrum. TCS1 scores tend to be slightly lower for light sources that lack red content.
TCS2 is described as a dark grayish yellow and has RGB values of (206, 177, 82) and HSL values of (31, 134, 136).
Similar to TCS1, TCS2 has low reflectance, but has additional reflectance in the green portion of the spectrum. It is generally not difficult for light sources to score well on TCS2.
TCS3 is described as a strong yellow green and has RGB values of (128, 186, 76) and HSL values of (61, 106, 123).
TCS3 reflectance is relatively low across the spectrum, and is particularly muted in the red and blue portions of the spectrum. As a result, light sources that have broad emission in the green regions will perform satisfactorily for this color.
TCS4 is described as a moderate yellow green and has RGB values of (0, 168, 166) and HSL values of (120, 240, 79).
TCS4 is very similar to TCS3, but blue-shifted slightly. Likewise, scoring well on this metric is not difficult for most light sources.
TCS5 is described as a light bluish green and has RGB values of (0, 159, 222) and HSL values of (131, 240, 104).
TCS5 is very similar to TCS4 but again, blue-shifted slightly. Although there is a stronger emphasis on blue wavelengths, its low overall reflectance and overall even spectrum keeps this from being a significant challenge for scoring well on CRI.
TCS6 is described as a light bluish green and has RGB values of (0, 134, 205) and HSL values of (134, 240, 96).
TCS6 is a ultimately a blue color, so it requires relatively accurate presence of blue wavelengths. As such, this score can be a bit more challenging for light sources that have a higher color temperature due as these colors naturally have more blue content.
TCS7 is described as a light violet and has RGB values of (165, 148, 198) and HSL values of (174, 73, 163).
We switch gears a bit and emphasize both blues and reds, though not exclusively. LEDs can struggle with this TCS in particular due to the uneven blue spike and lack of red color.
TCS8 is described as a light violet and has RGB values of (233, 155, 193) and HSL values of (221, 153, 183).
TCS8 is essentially the same as TCS7 but with a stronger red presence. As such, LEDs that have weak red components will struggle with this TCS a bit more.
TCS9 is described as a strong red and has RGB values of (230, 0, 54) and HSL values of (231, 240, 108).
With the introduction of the special test color samples, we see a significant shift in the spectral composition of these TCSs. TCS9 is primarily concerned with spectral reflectance at 600 nm and above. Therefore, it is very challenging for light sources to score well on this metric.
Conversely, it can be a very useful metric to better understand the ability of a light source to render red colors accurately, as this is a particularly important color for applications such as photography and retail.
We cover CRI R9 in further depth here.
TCS10 is described as a strong yellow and has RGB values of (255, 255, 0) and HSL values of (35, 240, 120).
TCS10 looks at a wide portion of the visible spectrum past 500 nm. It is a relatively saturated color sample but because of its breadth, most light sources are able to score relatively well on this metric.
TCS11 is described as a strong green and has RGB values of (0, 137, 94) and HSL values of (107, 240, 64).
TCS11 is similar to TCS3 (strong yellow-green), but with a more blue-shifted relfectance peak. It has a low relfectance profile and is not very challenging for most light sources to score well on.
TCS12 is described as a strong blue and has RGB values of (0, 60, 149) and HSL values of (144, 240, 70).
TCS12 has its primary reflectance that peaks at around 460 nm, with 0 reflectance past 580 nm. This makes it especially important for light sources to have accurate rendition in the 430 - 500 nm range.
For LED sources that use blue light as its excitation source, R12 can be an especially challenging metric to score well on. This is particularly true for higher CCT color points due to the additional emphasis of blue.
The reason LED sources do not score well is that the blue spike has a very narrow emission width (typically 10 - 15 nm) that fails to cover the entirety of the TCS12 reflectance bandwidth. For example, a 460 nm blue LED will not provide any emission at 440 nm and below.
TCS13 is described as a light yellowish pink and has RGB values of (244, 232, 219) and HSL values of (21, 128, 218).
TCS13 has strong reflectance at wavelengths past 580 nm, and moderate but diminishing reflectance at shorter wavelengths as well. Because of its relatively unsaturated color, TCS13 is not a particularly challenging color for light sources to render.
TCS14 is described as a moderate olive green and has RGB values of (0, 96, 68) and HSL values of (108, 240, 45).
TCS14 is described as "olive" green due to its slight yellow hue and low reflectance. As such, most light sources will not have difficulty with this color.
TCS15 is described as Asian skin and has RGB values of (245, 204, 165) and HSL values of (19, 192, 193).
TCS15 was developed by Japanese researchers who felt that the original CRI and CRI extended TCS1 through TCS14 were insufficient to capture the ability of a light source to accurately render the color of human skin.
This is of course important in many applications where distinguishing skin tones are critical for the work being done (medical) or aesthetic appearance is important (hospitality, photography).
R15 is not as difficult to score well on compared to R9, but may give a more generally representative result for how a light source would render skin tones.
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