Apparatus for determining the thickness of material

#The present invention relates to a method and apparatus for the non-contact measurement of the thickness of a material such as glass plate or tube, which is inaccessible during production by virtue of its high temperature (say 500.degree. C.) thus precluding the use of a conventional contact method of measurement.

#In the manufacture of glass, the constituent materials required to form the glass are raised to a high temperature in a furnace. The materials are combined, in a molten state, and the resultant glass product is, in effect, a super cooled liquid.

#In the molten state, the glass temperature is in the range of 1,000.degree. to 1,500.degree. C. and flows fairly freely. It is in this state that it may be formed into bottles, vials, and tubes for use in fluorescent lighting, medical pipettes, surgical instruments, rods or lenses for optical instruments as well as glass sheets or panes for domestic, industrial and automotive applications. Specialist glass may be formed and ground into optical elements for use in telescopes, cameras and other types of photographic equipment.

#Other features and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention taken with reference to the accompanying drawings.

#FIG. 1 illustrates how a beam of light directed at incidence onto a glass plate is split to provide a first reflected beam from the front surface and a secondary reflected beam from the back surface of the plate;

#FIG. 2 is a schematic diagram of one embodiment of thickness measuring instrument according to the invention;

#Before entering into specific detail concerning preferred ways and means of putting the invention into effect, it would be helpful to consider some basic optical theory with reference to FIG. 1.

#If a ray of light 2 falls on the face 3 of a glass plate 1 of thickness `d` at an angle of incidence .alpha. to the normal N, then a little of the light will be reflected as ray 4, and a portion will be refracted in the glass as ray 5. The refracted rays will undergo internal reflection 7 from the opposite face 6 of the glass 1 and emerge from face 3 as ray 8 parallel to and spaced a distance S from ray 4.

#In effect the glass plate 1 acts to split the incoming beam 2 resulting in a double reflected image 4 and 8 of the incoming beam 2.