Measurement of opacity

The main technique that has been used for the measurement of opacity has been to prepare a standard disc of AB cement 1-0 mm thick and aged for 24 hours at 37 °C. This disc, contained in a small trough of water to prevent desiccation, is placed in a reflectometer on a black background. It is then illuminated with diffuse light and the amount of light reflected from it, is measured. The disc is then placed on a white background of 70% reflectivity, and the new amount of reflected light, measured. The contrast ratio 7 o/ o is defined as the Cp., opacity (Crisp et al., 1979). 

Using this technique, it has been shown that the opacity of glass-ionomer cements decreases as they age in other words, their translucency increases over this time. This change has been found to be rapid in the first hour after mixing, but becomes much slower after this time (Wilson McLean, 1988). 

Colour and opacity have been foimd to be connected for glass-ionomer cements (Crisp et al., 1979 Asmussen, 1983), with darker shades giving increased opacity. However, this is merely a consequence of the underlying physical relationships, and is not thought to be a clinical problem (Wilson McLean, 1988), mainly because the stained tooth material for which the darker shades are necessary for colour match is itself of reduced translucency. 

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The in vitro battery would ideally include measures of opacity, cytotoxicity, and inflammation. The actual test method(s) will vary depending upon the experience of the laboratory, types of compounds to be tested, and so on. If the measured endpoint(s) indicates that the test material is approximately equipotent with known irritants, one would presume the unknown to be an irritant and further testing would not generally be required. One should keep in mind, however, that in many cases in vitro assays are more sensitive than whole-animal testing, so a positive response in vitro may not always indicate an in vivo irritant. If the assays give equivocal results or responses similar to those seen with non- or mild irritants, some type of animal testing may be indicated as confirmation. 

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