There is provided a diffractive waveguide device comprising: a light source, at least one light detector, an SBG device comprising a multiplicity of separately switchable SBG elements sandwiched between transparent substrate to which transparent electrodes have been applied. The substrates function as a light guide. Each SBG element encodes image information to be projected on an image surface. Each SBG element when in a diffracting state diffracts light out of the light guide to form an image region on an image surface. The light detector detects light scattered from an object disposed in proximity to the image surface and illuminated by said image region.