Spectral Similarity Index presents at SMPTE 2016

October, SMPTE – At the first day of Centennial SMPTE ATC conference three speakers: Paul Debevec, Co-Chairman of Science and Technology Council,  George Joblove, Co-Chairman and Jack Holm, Co-Author of Science and Technology Council SSL Project Committee presented the challenges and issues in color rendering using the solid state lighting (SSL) along with a solution – a Cinematographic Spectral Similarity Index. The new color index is based upon the similarity of a luminaire’s spectrum to a reference spectrum that eliminates the need for any assumption of the specific observer or camera spectral sensitivity.

The Academy of Motion Picture Arts and Sciences’ Science and Technology Council has investigated the issue the last few years. The AMPAS project was presented at SMPTE Tech Conference in 2009 (“State of Solid State Lighting”), at NAB conference in 2011 (“Chromatic Chaos: Implications of Newly Introduced Forms of Stagelight”) and again at SMPTE in 2013 (“Color Predictor Tool”). “The Experiment” conducted in 2010 showed that luminaire spectral power distribution affects skin tone, makeup, costumes, props, and sets. It showed differences in color for 4 different sources despite all of them being at the same color temperature.

The characteristics of Spectral Similarity Index (SSI) that is a solution for that issue are following:

  • it defines how close a test spectrum is to a reference spectrum (e.g. tungsten),
  • does not assume a spectral sensitivity for the camera/film/eye,
  • single value representing quality of curve fit,
  • index is easy to understand
  • easy to include on luminaire/lamp packaging and literature.

The presenters explained the technical aspects of SSI but the final message is that the index yields a confidence factor, where a high score implies predictable color rendition for cinematography, and a moderate score implies possible color rendition challenges. The look of the image and the color of the things on the large screen for cinema, and now with the addition of HDR (high dynamic range) and WCG (wide color gamut) being available on home television screens, means color rendition for projects is once again at the forefront of the content creators vision.

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