A Technique for Inspecting and Measuring Wall Thicknesses of Transparent Articles

JSGT 1956 V40 T319-T337

When an article made of glass, or other transparent material, is viewed in air, the observed image of its inside shape is not true, because refraction of the light as it leaves the outer glass surface causes distortion of the images of shapes inside the article. This refraction, and the resulting distortion, can be prevented by immersing the article in a liquid which has the same refractive index as the glass, thus enabling accurate measurements of the cavity dimensions and of wall thickness to be made by projecting the image on to a screen. Liquids are described which are suitable for workshop use. When a single liquid does not match a particular glass, mixtures of liquids can be employed. Simple tests can be used when mixing, to ensure that a match is obtained. Small differences of refractive index between the glass and the liquid are always present, and will cause distortion. It is shown, however, that the accuracy obtained is satisfactory for the purposes required in glass container manufacture, and simple formulae are given to determine the errors and to enable corrections to be applied when necessary. These formulae also establish the sizes of glassware which may be examined without introducing errors exceeding specified limits. Other applications of the immersion technique are briefly described.

W. R. B. Frank & L. Green