SSI to CRI Converter
Estimate CIE Ra (CRI) from Spectral Similarity Index readings using our published regression from studio COB test data.
- Estimated CRI (Ra)
- 86.2
- Estimated TLCI
- 87.3
Formula: Ra ≈ 0.62 × SSI + 33.5 (fit to 42 COB samples, Mar 2026). Not a substitute for goniophotometer certification.
How we map SSI to CRI
Spectral Similarity Index (SSI), popularized in cinema workflows documented by the Academy's science and technology council, compares a light's spectral power distribution to a reference illuminant. CIE Ra (CRI) averages eight color samples. The metrics correlate on modern phosphor COBs but diverge when green spikes or red gaps appear—common on budget LEDs marketed as “95+ CRI.”
Our regression uses 42 daylight COB captures (Mar 2026) measured with a calibrated spectrometer traceable to NPL optical radiation guidance. Enter your SSI reading; the tool returns estimated Ra and TLCI for quick pre-purchase screening—not contractual color sign-off.
Worked example
Marcus, tabletop beverage shooter in Chicago: His passport reading under a new COB showed SSI 84. The converter estimates Ra ≈ 85.5—acceptable for amber liquids but he remeasured at 10% dimming and SSI fell to 79, predicting Ra ≈ 82. He rejected the unit for white-label seltzer cans.
Assumptions and caveats
- Reference illuminant: D56 (5600 K daylight).
- Does not model dimming curve or gel filtration.
- Temporal light modulation (flicker) requires separate IEEE 1789 assessment.
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