Purpose: For transparent materials, gloss is seen in specular reflectance and color is seen primarily in the regular transmittance that passes without deviation through the transparent material, modified by the absorption of the colorants. The presence of internal scattering centers such as scratches, cloudiness, bubbles, or suspended particles within the material or the surface texture can cause the regular transmittance signal to scatter or diffuse. This diffuse transmittance also contains a secondary color component of the material and is responsible for any hazy or cloudy appearance. Clear, transparent materials generally have little or no diffuse transmittance.
Total transmittance is a combination of regular and diffuse transmittance.
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Most sphere instruments are constructed in the inverse d/8° geometry (diffuse illumination/8° viewing). In this configuration, total transmittance (TTRAN), which includes both the regular and the diffuse (scattered) components of the transmitted light, is measured with the sample situated at the sphere side of the transmittance compartment. Regular transmittance (RTRAN) is measured with the sample situated at the lens, including only the transmitted light that comes straight through the sample. Diffuse transmittance is calculated as total transmittance minus regular transmittance.
Diffuse Transmittance = Total Transmittance - Regular Transmittance.
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TTRAN measurements are recommended for samples that are slightly hazy and have some internal light scattering. Brewed tea, fruit juices, and biopharma/chemical solutions are examples of samples that require TTRAN measurements due to scattering. RTRAN measurements are used for clear (non-hazy, non-scattering) samples.
The Hunter Lab instruments that are capable of measuring diffuse, regular, and total transmittance are the UltraScan PRO, UltraScan VIS and Vista.
See attached pdf file for the complete article with illustrations.
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