Most infrared systems and instruments require mechanical chopping or scanning mechanisms for modulating the radiation signal. For some applications, particularly in space, it is desirable to eliminate sliding or rotating mechanical components. One means of accomplishing this is by means of frustrated internal reflection. Spacings and motions of the order of 0.05 wavelength are necessary which are difficult to control in the visible spectral region, but, in the 8–12 μ infrared region, the scale is twenty times greater, and a modulation cell based on this principle becomes quite practical. The characteristics of frustrated internal reflection are briefly summarized, and a modulation cell constructed on these principles is described. This cell has a clear aperture of 12.7 mm and produces 80% modulation of 10-μ radiation. A lead zirconate titanate piezoelectric stack is used for moving the frustrating element, which gives a frequency response higher than 6 kc/sec.