Identifying process variations during product manufacture

A method of identifying process variations during a product manufacturing process comprising:

• at a predetermined stage during manufacture of a first product:
• obtaining, using an imaging system, images of an area of said first product using different values of at least one imaging parameter;
• analyzing said images of said area of said first product to generate a first contrast signature for said first product representing variations of contrast of said images of said area of said first product with said at least one imaging parameter;
• at the predetermined stage during manufacture of a second product:
• obtaining images of an area of said second product corresponding to said area of said first product using different values of said at least one imaging parameter;
• analyzing said images of said area of said second product to generate a second contrast signature for said second product representing variations of contrast of said images of said area of said first product with said at least one imaging parameter;
• and
• comparing said first and second contrast signatures to identify whether a variation in process occurred between manufacture of said first and second products.