Shadow casting, thickness measurement

Radiation transmission methods, in which thickness is determined by (measured as) the shadow cast from a radioactive source, are limited to... 

Insoluble surface films can be studied by electron microscopy. The films are transferred from the substrate on to a collodion support and shadow-cast by a beam of metal atoms directed at an angle a (about 15°) to the surface (Figure 4.21). If the width x of the uncoated region is measured, the thickness of the film, x tan a, can be calculated for example, a /i-C36H73COOH film has been shown to be about 5 nm thick - i.e. consistent with a vertically orientated monomolecular layer. The technique has also been used for following the state of the surface as a film is compressed. 

Radiation transmission methods, in which thickness is determined by (measured as) the shadow cast from a radioactive source, are limited to pieces of equipment small enough to be illuminated by small radioactive sources. There are several schemes for highlighting cracks. If the metal is appropriate, magnetic particles can be used to accentuate cracks. Magnetic particles will congregate along... 

It is thus absolutely necessary to measure the sample thickness at the point of interest. One such method is shadow-casting [21]. The thickness is particularly difficult to measure because grinding has to be avoided. Sample thickness cannot be evaluated by visual control of its transparency. Transparency increases as voltage increases, so a 100-A-thick particle is not detectable at 400 kV. 

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