Method and apparatus for phase evaluation of pattern images used in optical measurement

An apparatus for optical measurement comprising:

• means for directing light to an object to be measured;
• means for generating a pattern image from said light directed to said object and distorted by said object, the pattern image having light intensities representing a phase modulated carrier wave being a superposition of a carrier wave and a modulation wave, the modulation resulting from distortion by said object;
• an array of individual detectors arranged for producing respective individual electrical output signals corresponding to the intensities of said pattern image along a minimum of five different phase angles of the carrier wave, the differences between said phase angles defining nominal phase steps;
• first and second convolution circuits receiving said electrical output signals of said array of individual detectors for convolving the output signals by convolution values and for assigning values to expressions Z and N and forming electrical signal outputs indicative of said expressions Z and N which represent, respectively, sine and cosine functions of said phase angles of said phase modulated wave, whereby the convolution values are selected so that a phase error, which is the amplitude of error that results when phase steps deviate from said nominal phase steps, as a function of said phase steps, has at least three zero positions;
• and,
• an arctan circuit receiving the electrical signal outputs of said convolution circuits for computing said phase value of the phase modulated wave by using the ratio Z/N.