A tabulated spectral power distribution representing average daylight at a correlated color temperature of about 6770 K. It carries less UV than D65, which is why brightened papers read lower under ISO brightness, set to C-level UV, than under D65 methods. It is the basis for ISO brightness, printing opacity and the indoor version of CIE whiteness.
Pulp spec is under illuminant C, so set the UV filter to the C position before you read those pads.
Also heard as
- CIE illuminant C
Do not mix it up with
- Illuminant D65: D65 contains more UV than C, so fluorescent papers measure brighter and whiter under D65 settings.
Related terms
Illuminant D65
Standard CIE daylight at about 6500 K, with enough ultraviolet to excite optical brighteners strongly.
ISO brightness
Diffuse blue reflectance factor at 457 nm, measured with d/0 geometry and UV content matched to illuminant C.
UV calibration
Setting an instrument's ultraviolet content so fluorescent samples read as they would under the specified illuminant.
Printing opacity
Opacity calculated as single-sheet reflectance over black divided by the reflectance of an opaque pad of the same paper.