Angular Configuration
Optical instruments designed with specific illumination and observation angles minimize the influence of surface gloss on color measurements. This optical arrangement, referred to as directional geometry, typically utilizes a forty-five degree illumination angle paired with a zero degree observation angle to analyze paper and board. By excluding the specularly reflected light from the detector, this setup evaluates color in a manner that closely replicates how a human observer views a sheet under typical room lighting.
Standard test methods specify these angles to ensure consistent results across different laboratories. The configuration prevents glossy surface reflections from artificially lightening the measured color value.
Measurement Discrepancy
Discrepancies between spectrophotometers often arise when comparing diffuse instruments with those employing directional geometry on textured substrates. Diffuse instruments illuminate the sample from all directions using an integrating sphere, which includes both specular and diffuse reflectance. When paper has a pronounced surface texture or a pattern, directional geometry registers different values depending on the orientation of the sheets relative to the light beam.
To eliminate this variation, the instrument or the sample holder must rotate during the measurement process to average out the directional effects of the fibers. This rotation ensures that the resulting data remains consistent regardless of whether the paper is measured along or across the machine direction.
Substrate Sensitivity
Physical characteristics of the paper surface, such as calendar marks or wire patterns, affect the repeatability of measurements made with directional geometry. High-resolution color sensors detect minute shadows cast by raised surface fibers when the illumination is unidirectional. This sensitivity is useful for assessing the tactile uniformity of uncoated papers but requires multiple readings at different angles to obtain a reliable mean value.
