Radiometric Characterization
An optical curve describes the radiant energy emitted by a light source across each specific wavelength of the visible electromagnetic spectrum. Within the graphic arts and paper converting sectors, spectral power distribution defines the illumination conditions required for consistent visual evaluation and instrumental colorimetry of substrates, inks, and protective coatings. Standard industrial illuminants, such as CIE D50 and D65, assign precise relative energy values from 380 to 730 nanometres.
The metric applies strictly to emitted illumination characteristics and does not measure the reflectance of the substrate itself.
Color Evaluation
Visual color perception shifts dramatically when viewed under light sources with divergent spectral curves. Standard viewing booths for packaging appraisal comply with ISO 3664 standards, which prescribe D50 lighting that balances blue, green, and red spectral energy levels. Inadequate lighting sources, including typical cool white fluorescent tubes, exhibit pronounced spikes in narrow mercury emission bands while lacking broader red wavelengths.
These spectral disparities distort the perceived colour density of printed cyan and magenta inks on white packaging boards. Spectrophotometers minimize this problem by projecting internal reference beams of known spectral composition onto test samples, calculating absolute reflectance factors at ten-nanometre wavelength intervals.
Substrate Metamerism
Uneven spectral distribution triggers metameric failure, where two printed packaging panels match identically under pressroom inspection lighting but diverge sharply under commercial supermarket fixtures. Optical brightening agents blended into paper coatings absorb non-visible ultraviolet radiation below 400 nanometres and re-emit that energy as visible blue light around 440 nanometres. When viewing booths lack adequate ultraviolet output in their spectral curve, the brightness added by optical brighteners disappears, masking shade variations between virgin kraft and recycled board stocks.
Sourcing specifications therefore specify the required ultraviolet component along with the primary colour temperature rating.
