Optical brighteners respond to UV, so a lamp with the wrong UV content gives wrong brightness and whiteness on brightened papers even when non-fluorescent tiles read perfectly. The operator measures a fluorescent reference standard and moves a UV-adjusting filter, or lets the software tune it, until the reading matches the certified value for illuminant C or D65. It is separate from the ordinary white calibration and must be redone whenever the lamp or filter changes.
New lamp went in Monday and nobody redid the UV calibration, so every OBA sheet this week reads two points hot.
A non-fluorescent tile cannot reveal a UV calibration problem; only a fluorescent standard will.
Also heard as
- UV adjustment
- UV content adjustment
- fluorescence calibration
Related terms
UV cut filter
Filter that blocks wavelengths below about 420 nm so a sample can be measured without its fluorescence.
Reference standard
Certified material with assigned reflectance values that transfers a measurement scale to a working instrument.
Fluorescence
Light a sample emits after absorbing UV, measured in paper as the gap between UV-included and UV-excluded readings.
Illuminant D65
Standard CIE daylight at about 6500 K, with enough ultraviolet to excite optical brighteners strongly.
Illuminant C
Standard CIE light distribution approximating average daylight, with relatively little ultraviolet.