Managing Secondary Metamerism Index Values under Store Fluorescent Light Sources
Secondary metamerism under store fluorescents is controlled by constraining dye reflectance crossover spikes to keep the index below 0.5 Delta E.
Diffuse Illumination
Geometry Specification
Light distribution methods in spectrophotometry scatter radiant energy uniformly across a sample surface to minimize directionality effects. Diffuse illumination exposes a textile specimen to indirect light coming from all angles inside an integrating sphere. Color measurement instruments use this light geometry to evaluate fabric reflectance without interference from surface texture or directional nap.
Integrating Sphere
Internal cavity walls coated with highly reflective, non-selective barium sulfate or highly diffuse polymers scatter incoming lamp light in every direction. When diffuse illumination lights a specimen surface, reflected light passes to detectors set at eight degrees or zero degrees from the surface normal. Integrating sphere spectrophotometers allow operators to include or exclude specular reflectance using a motorized specular port flap.
Including specular reflectance measures total colorant absorption regardless of surface gloss or nap orientation. Excluding specular reflectance isolates surface gloss variations, enabling technicians to evaluate how fabric structure and surface finish alter visual shade perception under retail lighting.
Surface Texture
Multidirectional lighting eliminates shadows created by raised yarns or heavy pile constructions. Sueded fabrics and heavy knits present uniform colorimetric values across repeated rotational measurements.
Measurement Boundary
Uniform optical scattering minimizes surface structure interference during color matching on woven and knitted substrates. However, diffuse illumination cannot isolate micro-roughness variations caused by mechanical finishing or calendering processes. Technical laboratories must supplement sphere geometry with directional forty-five degree illumination instruments when assessing surface sheen or directional pile orientation across velvet and satin fabrics.