"The way the CRI is computed may in extreme cases return negative CRI values, something that is totally without meaning"
Television Lighting Consistency Index, TLCI-2012
The acronym TLCI-2012 stands for "Television Lighting Consistency Index" and is a tool that helps identifying how well suited any light source is for illuminating a scene that is reproduced by a television camera.
Naturally, the aim is to reproduce the contrast range and colours faithfully on a television display. (The number 2012 refers to the year of adoption by the EBU).
For a good many years the CIE CRI (Colour Rendering Index) has been used in datasheets as a quality 'figure of merit' for light sources. When using the CRI it is a condition that the illuminated scene is viewed (and assessed) by human vision. For TV use this condition no longer holds true since the 'observer' is a TV camera. In this case the use of CRI can be misleading and in particular with today's light sources that often may have a very 'spiky' and irregular spectral power distribution. The way the CRI is computed may in extreme cases return negative CRI values, something that is totally without meaning.
In order to be a more meaningful and easy-to-use tool for assessment of light sources for today's electronic cameras the TLCI-2012 comprises a computer program that completely mimics the TV camera and display. Dragging a very precisely 'calibrated' TV camera and display around every time a new piece of lighting equipment comes up for test is not very practical, and can now be replaced by a small lap-top PC and a small spectro(radio)meter. The simulation of the camera and display carefully follows the specifications laid down in the ITU Rec.709 (HDTV). Also the spectral sensitivities of cameras are taken into account and is taken from an extensive measurement campaign on a large number of broadcast type HDTV cameras that are in worldwide use today. This includes not only 3-chip TV cameras but also the later single-chip larger format electronic cameras. Changing system parameters to those laid down in ITU Rec.2020 (UDHTV, extended colour gamut) have only a marginal effect on the computed TLCI-2012 Q-value (for Q-value, see below).
As indicated above a small instrument called a 'spectrometer' is needed. This instrument is capable of measuring the spectral content of light sources, or in other words it tells how much energy (power) there is in every colour in the visible spectrum. In order to do this faithfully the instrument has to be accurately calibrated regarding both wavelength and amplitude (magnitude) over the entire range of visible light. All spectrometers used to record the spectral data from the published TLCI-2012 results (ZERB Magazine and EBU web site) have been carefully calibrated to traceable international standards for both wavelength and amplitude.
The computer program uses these spectral data to compute the Correlated Colour Temperature (CCT) of the light source under test and then the reproduced colour coordinates of a known test chart (ColorChecker). Differences in the reproduced colours are used to compute a single number (TLCI-2012 Q-value) on a scale from 0 to 100. A value of 100 indicates that the light source under test is 'identical' to an CIE reference light source having the same CCT as the source under test. The Q-value is computed on a basis of the rms error of all colours in the ColorChecker chart.
As mentioned above the TLCI-2012 was adopted by the EBU Technical Committee in 2012 as EBU Recommendation R-137 followed by three accompanying documents and a .zip file:
EBU Tech 3353, Tech 3354, Tech 3355 and Tech 3355.zip
The .zip document contains the TLCI-2012 software program and can be downloaded for free together with all related documents from the EBU web site (https://tech.ebu.ch/home).
The recommendation R-137 has been recently revised and now includes a note it is worthwhile to quote here:
TLCI-2012 has become widely accepted by the industry with many vendors now including the TLCI-2012 Qa-factor in their datasheets. However in order to clarify any potential confusion it should be noted that the differences between three sensor cameras which employ dichroic colour filters and single sensor cameras where organic dyes work independently to share light over a wider range of wavelengths was carefully considered and included in the work behind the TLCI-2012.
It is a matter of course that high-end TV/Film productions use extensive colour processing during post production in order to realise the desired ‘colour expression’ and ‘sentiment’ or ‘look-and-feel’ of the production. Any remaining and very small imperfections related to differences between 3-chip/1-chip cameras are insignificant and easily corrected in this process."