We report on the first prototype IMOS (integrated mirror optical switch), FIR (far-infrared: 8-12-mu-m) light modulator. The operation of this type of modulator is based on the reflection properties of a high density, thin electron sheet in semiconductors. The present prototype was built to give experimental backing for earlier theoretical predictions. Low electric power (4 W) and low voltage (12 V) operation have been achieved. A modulation depth of about 30% and a 3 dB frequency of 400 kHz were obtained.