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Color Rendering Index, CRI or Ra

The color rendering index (CRI) (sometimes called color rendition index), is a quantitative measure of the ability of a light source to reproduce the colors of various objects.

The CRI is calculated by comparing the color rendering of the test source to that of a "perfect" source which is a black body radiator for sources with correlated color temperatures under 5000 K, and a phase of daylight otherwise (e.g. D65). Chromatic adaptation should be performed so that like quantities are compared. Specified in (Nickerson & Jerome 1965) and republished in (CIE 1995), the Test Method (also called Test Sample Method or Test Color Method) needs only colorimetric, rather than spectrophotometric, information.

An incandescent light bulb has Ra of almost 100 so it has a excelent color reproduction. Light scources with a small wavelength like low pressure sodium lamps are can't render colors so they have a low or negative CRI

CRI of some light sources

Light scourceIndex Ra
Incandescent light bulbbis 100
FARA-LED, cold white90-95
FARA-LED, neutral white80-85
FARA-LED, warm white70-75
Fluorescent tube, white de Luxe85-100
Fluorescent tube, white70-84
Fluorescent tube50-90
Metal halide lamp60-95
High pressure sodium (HPS), warm white80-85
High pressure Mercury-vapor45
High pressure sodium (HPS)18-30
Low pressure sodium (LPS)- 44

Used test color samples (TCS)

#Appearance under daylight 
TCS01Light greyish red         
TCS02Dark greyish yellow         
TCS03Strong yellow green         
TCS04Moderate yellowish green         
TCS05Light bluish green         
TCS06Light blue         
TCS07Light violet         
TCS08Light reddish purple         
TCS09Strong red         
TCS10Strong yellow         
TCS11Strong green         
TCS12Strong blue         
TCS13Light yellowish pink (skin)         
TCS14Moderate olive green (leaf)