Swung into the light path, it removes the ultraviolet that excites optical brighteners and leaves only the sample's ordinary reflectance. Measuring brightness with and without it separates the fluorescent component from the base sheet. Labs use the UV-excluded reading to follow pulp and furnish quality without the masking effect of brightener dosage.
Read it with the UV cut filter in too; if UV-excluded brightness dropped, the problem is pulp, not OBA.
Also heard as
- 420 nm filter
- UV-excluding filter
- UV cutoff filter
Related terms
Fluorescence
Light a sample emits after absorbing UV, measured in paper as the gap between UV-included and UV-excluded readings.
UV calibration
Setting an instrument's ultraviolet content so fluorescent samples read as they would under the specified illuminant.
OBA
Optical brightening agent: a dye that absorbs UV light and re-emits it as blue, making paper look whiter.