Samples within the range 1 to 20 μg are suited for transmission analysis ( Smith, 1996). However, transmission technique is limited by the sample thickness. It is suited for analyzing solid, liquid, and gaseous samples. It provides spectra with high SNR and is relatively cost effective. It involves passing the IR radiation directly through the sample and detecting on the other side ( Figure 7.10a). Transmittance is probably the simplest of all sampling techniques. Ultraviolet-visible spectrophotometers can also be used to measure light transmittance over a wavelength range of 200–2,000 nm when samples are cast as films. A typical test sample consists of a disk 50 mm or 100 mm in diameter although almost any flat sample can be used. Gardner Co., and Hunter Associates Laboratories. 18 Instruments that can measure transmittance, haze, and clarity are available from BYK Gardner, Paul W. The standard test method for measuring both transmittance and haze is ASTM D1003-00, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics. The total light transmittance through a material is equal to the total incident light less the light that is absorbed and light that is scattered.Ī related property is haze or the scattering of light as it passes through a transparent material. Adhesives used in electro-optical interconnects must have a high transmittance. Luminous transmittance is a measure of the amount of light that passes through a transparent material. Swanson, in Adhesives Technology for Electronic Applications (Second Edition), 2011 7.1.6 Luminous transmittance But, since no material exist that approximates this behavior, we generally do not speak of a transmittance factor. Perhaps the best one could imagine would be to have a material that reflected half of the incident radiant flux uniformly in the reflection half-space and transmitted the other half of the incident radiant flux uniformly in the transmitting half-space. We do not define transmissive analogs to the reflectance factor or the radiance factor, because it is impossible to have a perfectly transmitting diffuser, or even an approximation to one. When the incident flux is scattered via its travel through the sample, for example, due to surface roughness or due to volume scattering, so that Snell's law no longer applies, the transmittance is referred to as diffuse. If the incident radiant flux travels through the sample under test in such a way that the exit angle can be predicted from the entry angle according to Snell's law of refraction, the transmittance is referred to as regular.
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