Optical Filtration
Optical filter assemblies block specific ultraviolet wavelength bands in spectrophotometer light paths to isolate non-fluorescent spectral reflectance from optical brightener fluorescence excitation. A UV cutoff filter consists of an optical glass or gel element placed between the illumination source and the sample, absorbing light below specified wavelengths such as four hundred nanometers or four hundred twenty nanometers. Installing this filter prevents ultraviolet radiation from striking the substrate, allowing measurement of pure base fabric color without fluorescent emission interference.
The filter is disengaged when assessing total radiant reflectance or calculating fluorescent whiteness indices.
Spectral Exclusion
Evaluating white and light-tinted fabrics treated with fluorescent whitening agents requires distinguishing between intrinsic fibre reflectance and optical brightener emission. Inserting a UV cutoff filter into the spectrophotometer illumination beam eliminates ultraviolet wavelengths that induce fluorescence. The resulting spectral reflectance curve represents the base substrate color profile alone.
Comparing reflectance scans captured with and without the UV filter quantifies the precise fluorescent contribution to total lightness and chromaticity. This dual-measurement method allows colorists to formulate dye recipes independently of brightener loading levels. Spectrophotometers feature automated filter positioning wheels to switch between UV-included and UV-excluded illumination modes during standard measurement routines.
Measurement Standardization
Standard color difference testing for non-fluorescent dyed shades mandates UV-excluded illumination to eliminate ambient fluorescent interference. Standardizing UV cutoff filter position ensures consistent colorimetric comparison between mills and testing laboratories.
Filter Inspection
Filter inspection protocols require periodic examination for surface scratches and solarization aging. Degraded filters leak unwanted ultraviolet light, introducing measurement error into UV-excluded spectral reflectance readings.
