Densitometry transmittance

3 DIRECT MEASUREMENTS FROM SOLID SURFACES 2.3.1 Densitometry (transmittance) 

The theoretical basis of thin-layer densitometry by transmission is the same as that for absorption in solution. The medium provides the main differences between the two processes. For example, in solution the effects of reflected and scattered light are at a minimum, whereas such effects are significant with samples adsorbed on layers. Also, the distribution of a compound in solution is homogeneous, while that of a spot on an adsorbent is not, neither vertically nor horizontally in the layer. Shibata [42] described the phenomenon of light striking a translucent material as 

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Thin layers of powdered cellulose, silica gel, alumina, ion-exchange or gel-permeation material supported on glass plates, plastic sheets or aluminium foil development tanks components sometimes examined by reflectance or transmittance densitometry or removed for spectrometric analysis. 

Figure 12.14 X-ray densitometry - examples of percentage transmittance for two films made of light elements. A 7 0,m film of beryllium is often used for making windows of energy detectors. For a radiation of 1 keV, (e.g. Na Ka) it is noteworthy that the attenuation achieved by this film is still of 50 per cent.

Gray scale standards of reflectance are the equivalent of neutral-density filters in transmittance spectroscopy. These materials are used in determining the linearity of detector systems for reflectance spectrophotometers. Although these materials are common in the field of photography and densitometry in the form of gray scale cards produced by Kodak and other film manufacturers, the equivalent in diffuse reflectance standards has been, until recently, difficult to produce or obtain. 

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