Focal modulation microscopy (FMM) is an emerging microscopy technique for fluorescence imaging of thick biological tissue in vivo. A spatial phase modulator is a critical component whose characteristics have a significant impact on the performance of a FMM system. We have designed a simple spatial phase modulator based on a tilting glass plate that provides superb modulation stability. Image quality has been improved remarkably after integrating such a modulator into a FMM system. (C) 2009 Optical Society of America