The ellipsometric measurement of thickness is demonstrated using a computer screen as a light source and a webcam as a detector, adding imaging off-null ellipsometry to the range of available computer screen photoassisted techniques. The results show good qualitative agreement with a simplified theoretical model and a thickness resolution in the nanometer range is achieved. The presented model can be used to optimize the setup for sensitivity. Since the computer screen serves as a homogeneous large area illumination source, which can be tuned to different intensities for different parts of the sample, a large sensitivity range can be obtained without sacrificing thickness resolution.