SSI (Spectral Similarity Index) measures the similarity between a light’s spectral power distribution and a chosen reference spectrum. Developed by the Academy, it is standardized by SMPTE as ST 2122.
Always read the reference
A score is meaningful only with its reference. SSI[P3200] compares against a 3200 K Planckian reference; SSI[D55] uses standard daylight D55. A score against one reference is not interchangeable with a score against another.
What it tells you
A closer spectral match reduces one source of uncertainty when recording colours. SSI compares spectra directly rather than assuming one particular camera response. It does not describe exposure, light output, beam shape or the direction of a possible colour error.
Practical example
When replacing a tungsten source with an LED, check the LED’s tungsten-reference SSI at the intended setting. If you instead compare two LEDs against each other, a close match means they are similar to each other; it does not automatically mean either matches daylight.
Related terms
CRI (Colour Rendering Index) · TLCI (Television Lighting Consistency Index) · CCT (Correlated Colour Temperature) · TM-30 · Duv
References
Academy: Spectral Similarity Index overview · SMPTE: SSI standard