Due to many misconceptions regarding the wrong interpretation of the CRI – color rendering index, we are presenting information regarding CRI, how it is determined, and why the CIR index is limited in assessing the spectrum of the LED light sources, and what the TLCI-2012 index says about the LED fixture.
The basic, natural light sources, used also in the film and television industry, are the incandescent and sun light. These sources emit white light, which is a balanced mixture of all existing colors. Thanks to that, it is possible to differentiate colors as well as register a colorful picture reflecting the colors of particular elements of a film frame, through the electronic camera sensors or photographic emulsion. The differences in the color temperature of light are eliminated by the eye thanks to the eye’s ability to adapt to the existing lighting. This does not apply to camera or the photographic emulsion, as these need to be adjusted to the color temperature of lighting.
The white light or rather the impression of the white light is obtained also by mixing two or more lights with complementary colors. The spectrum of such light is the amount of the spectral components creating the impression of the white light, and most often it is not a continuous spectrum. The white fluorescent lamps and the white LEDs work this way.
In order to use these sources to illuminate filming sets, they have to be adjusted to the same color temperature as the natural sources. However, the registered colors may differ from the real ones due to the incomplete spectrum or its distortion – from distorted hue, through the changed color, to the complete lack of registration of some of the colors. Neither adjusting the camera nor using the correction filters helps in such a case.
In order to define the difference between the spectrum of the particular light from the spectrum of the natural light, a parameter was created, called the color rendering index (CRI).
Way of defining this factor by the organization CIE is associated with the concept of “standard observer” and how the human eye is seeing the colors. Defining this index required creating a special table of 14 sample colors (TCS – Test Color Samples). Those colors were marked with the symbols from TCS1 to TCS14.
The individual test colors TCS1 – TCS14 are illuminated by the tested light, and the extent to which the light reflected from the test colors is similar to the reflection obtained through the reference light is determined. The color rendering indexes R1 and R14, which represent particular colors TCS1 to TCS14, are created this way.
The R1 – R14 indexes for the reference light are equal to 100. The larger the difference between the reflections of the tested light and the reflections of the reference light for the particular TCS color, the lower the adequate index.
The Ra factor usually stated by fixture manufacturers is an arithmetic mean of the first eight factors, namely R1 through R8, And – as such – does not reflect the values of R9 and R12 factors at all. At the same time, it is those factors that represent the outermost regions of spectrum, which correspond to deep reds and blues, and are most troublesome for white LEDs. This significantly reduces the practical reliability of Ra as a parameter describing the light spectrum quality of LEDs intended for illuminating filming locations and sets. Besides, that factor reflects neither colour temperature nor green–purple balance, and these parameters are fundamental to capturing colour images with motion picture cameras.
Accordingly, in 2012 the European Broadcasting Union (EBU) defined and approved for general use a new measure of the quality of light fixtures intended for filming, called TLCI-2012, which stands for Television Lighting Consistency Index 2012. Establishing TLCI-2012 started with defining “standard camera” as a term different from “standard observer”. TLCI determination is based on spectral characteristics of a light source, measured using a spectrometer. Creators of the new method provided a software tool for obtaining the factor from this information. A TLCI equal to 100 means that there is no need at all for colour equalisation, which would be otherwise required due to lighting issues. The greater any deviations in balance and of spectrum curve, the lower the TLCI. Below you will find the TLCI-2012 and CRI of warm diodes used in our V-WHITE fixtures.