A controlled large area illumination, provided by a computer screen, is used to induce distinctive chemical images in field-effect devices upon gas exposure. The present measuring method uses the concurrent optical and chemical modulation of the semiconductor surface potential of a single metal–oxide–semiconductor capacitor to generate two-dimensional photocurrent images, displayed in a bias voltage-color space. Selective patterns for hydrogen, ammonia, and propene are demonstrated. The use of a computer screen as a programable light source allows to simplify a normally complex setup, making the technique more attractive for practical applications.