Spatial modulation of electromagnetic radiation is crucial for many optical systems. We are focused on the design of a biological instrument that relies on spatially varying infrared laser radiation for heating of aqueous solutions within an array of 1.5 μL microchambers. Such a radiative heating method is preferable to conductive and convective heating because it allows for fast heating rates at absorption wavelengths, there are no risks of incompatible materials coming into contact with the reaction, and it allows for the possibility of a disposable reaction environment. A single radiation source directed to multiple reaction chambers can reduce complexity of the overall device and also cost. We have developed an optical shutter designed to partially block radiation to a select chamber. This allows for two adjacent chambers to achieve different temperatures. Testing of the device has resulted in a direct, linear relationship between the shutter's duty cycle and the amount of radiation passing through. This device will allow us to control the temperature in two chambers heated by one source of radiation.