Photometric head for measuring the diameters of hardness indentations

Claim 1

A photometric head for automatic measurement of the indentation diameter obtained on a metal sample by Brinell hardness ball testing and optically projected with reference to a fixed centre, which head comprises: a. an optically opaque screen located to intercept a projected image of said indentation, said screen being interrupted by at least two similar windows therethrough, one said window being at a distance to one side of said centre substantially equal to the tolerated minimum indentation image radius and another said window being at a distance to the other side of said centre substantially equal to the tolerated maximum indentation image radius, said screen having two further identical windows aligned with the first said two windows and disposed symmetrically with respect to said fixed centre at a common distance therefrom substantially greater than the tolerated maximum indentation image radius whereby said further windows transmit in all cases light from the metal sample surface situated outside the indentation; b. photoelectric means associated with each window and adapted to generate electrical signals which vary according to the luminous flux passing through each respective window, said photoelectric means comprising a photoelectric cell associated with each of the four windows, the circuits of the two cells associated with each pair of windows on the same side of said fixed centre forming two of the arms of a Wheatstone bridge; c. detection means for detecting the levels of said electrical signals, said detection means includes a logic circuit connected to the diagonal measuring arms of the two Wheatstone bridges and adapted to supply one of three different signals representing respectively a state of simultaneous equilibrium of the two bridges, a state of simultaneous disequilibrium of the two bridges and a state of dissimilar equilibrium of the two bridges; d. means coupled to said detection means for responding to said levels in accord with compliance of the metal sample with respect to the tolerated minimum and maximum indentation radius.