Contact image sensor using switchable Bragg gratings

A method of making a contact image measurement, comprising the steps of:

• a) providing an apparatus comprising:
• a light source providing a collimated beam;
• a platen;
• a plurality of detector elements spatially distributed over two dimensions;
• a plurality of waveguiding cores disposed in a layer;
• a first array of switchable grating elements for coupling light from said source towards said platen;
• and a second array of switchable grating elements for coupling light reflected from said platen into a TIR path to said detector elements;
• each said waveguide core overlaying a unique detector element;
• b) coupling light from said source into a total internal reflection path in said first array of switchable grating elements;
• c) switching one grating element in each of said first and second arrays of switchable grating elements into their diffracting state with all other elements remaining in their non-diffracting states;
• d) diffracting said light towards said platen via said first array of switchable grating elements;
• e) transmitting light incident on a region of an external surface of said platen in contact with an external material and reflecting light not incident on said region from said external surface;
• f) coupling light reflected from said platen into said plurality of waveguiding cores via at least one element of said second array of switchable grating elements in said diffracting state;
• g) coupling said light out of said plurality of waveguide cores onto said detector elements;
• h) repeating steps c) to g) with different pairs of switchable grating elements being switched into their diffracting states.