COLOUR RENDERING INDEX
Colour rendering describes how a light source makes the colour of an object appear to human eyes and how well subtle variations in colour shades are revealed. The Colour Rendering Index (CRI) is a scale from 0 to 100 percent indicating how accurate a "given" light source is at rendering colour when compared to a "reference" light source.
The higher the CRI, the better the colour rendering ability. Light sources with a CRI of 85 to 90 are considered good at colour rendering. Light sources with a CRI of 90 or higher are excellent at colour rendering and should be used for tasks requiring the most accurate color discrimination.
It is important to note that CRI is independent of color temperature (see discussion of color temperature). Examples: A 2700K ("warm") colour temperature incandescent light source has a CRI of 100. One 5000K ("daylight") color temperature fluorescent light source has a CRI of 75 and another with the same color temperature has a CRI of 90.
Polarizing Filter
A polarizing / polarising filter is often placed in front of the camera lens in photography in order to darken skies, manage reflections, or suppress glare from the surface of lakes or the sea. Since reflections (and sky-light) tend to be at least partially linearly-polarized, a linear polarizer can be used to change the balance of the light in the photograph
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