Camera-based color measurement of DLP projectors using a semi-synchronized projector camera system

Modern DLP projectors often use a time-multiplex approach to generate color: A rotating color wheel is used to project the red, green and blue components (and possibly more) as separate sub-images which are each displayed for a short period of time. Applications like color calibration require high quality measurements of the color output of the projector, which might be acquired with digital cameras. When capturing the output of a DLP projector with a color wheel, the timing of the projector in relation to the exposure time must be taken into account, to avoid deviations introduced by capturing fractional parts of a color wheel rotation. In this work, we demonstrate the feasibility of software-only semi-synchronization between a DSLR camera and a DLP projector, using only a PC, camera with an USB interface and a projector connected via HDMI. We found that a reasonable estimate of the end of the actual exposure can be acquired with millisecond precision. By relating that to the previous vertical blanking interval, we are able to reconstruct the position of the color wheel throughout the exposure. Using a multitude of photos, it is possible to measure the actual color wheel timing of the projector for a specified input color. Furthermore, we show that this data can be used to build a simple model of the image formation process of the projector, which enables the compensation of color deviations introduced by the incomplete rotation. We show that using our compensation technique significantly improves the accuracy of the color measurements for reasonable exposure times.