Method of calibrating optical coherence tomography apparatus to measure depth

A method of calibrating an apparatus configured to perform optical coherence tomography to measure depth of an article of a prescribed type, the method comprising the steps of:

• a) using an existing measurement system comprising a surface profiler configured to measure depth, measuring a sample of the article of the prescribed type to obtain reference depth measurements, wherein measuring the sample comprises performing a plurality of measurements at different positions on the sample, wherein each measurement is in the form of a profile of the sample at a corresponding one of the positions thereon;
• b) using the apparatus configured to perform optical coherence tomography, capturing sectional images of the sample at the same positions at which the reference depth measurements were obtained;
• c) determining, in each sectional image of the sample captured by the apparatus configured to perform optical coherence tomography, a number of pixels in a depth direction of the sample;
• and
• d) determining, using the reference depth measurements and the numbers of pixels in the depth direction in the sectional images captured by optical coherence tomography, a correlation curve to associate pixels in images captured by optical coherence tomography with physical depth.