Colour Rendering Index CRI

CIE 1960 UCS. Planckian locus

Colouring Rendering

An object looks whitish under the fluorescent lamp and meanwhile, the same object looks yellowish under the incandescent lamp. The Same object looks different in (or under?) different light sources. You can see light source change the colour of objects by comparing under fluorescent lamp. Incandescent lamp, daylight and etc.
Colour Rendering is the property of the light sources that affects the colour of the object.

Colour Rendering Index (CRI)
CRI is a numerical representation to evaluate the Color Rendering Properties of the light source.
CRI Shows how accurately a sample light source reproduces on object colour compare to The *reference Light Source.

The CRI value of 100 means the sample light sources reproduces the same colour a the reference Light Source does. Therefore the higher the value of CRI, the more it resembles to the reference light sources.

When evaluating CRI, the reference Light Source is chosen to have the same (or within the range of 5 mireds) correlated colour temperature as the same light source.

* Reference Light Source
a) Lower than 5,000 Kelvin: Planckian Radiator
b) 5,000 Kelvin and higher : CIE Daylight