Tactical missile infrared (IR) windows are exposed to high heating conditions during free-flight trajectories. These high heating rates and resultant non-uniform temperature distributions result in degradation of optical performance as well as structural capability of the IR window. Optically, the wavefront and the signal-to-noise ratio (S/N) can be catastrophically degraded and is strongly dependent on the time varying magnitude and distribution of the heat on the window. Therefore, for each missile application, it is critical to be able to determine the optical effects of the heating on window performance. As analytical techniques for predicting optical heating effects are limited, test results are critical in providing conclusive measured data on performance as well as validating analytical predictions. Unfortunately, conventional techniques for measuring the free flight heating effects, e.g. shock tubes and wind tunnels, are expensive and require complex set-ups for measuring the optical performance. The high speed flow and resultant thermal and vibration environments require that the hardware be robust and nearly form factored. This increases the cost of the hardware and limits the available volume for instrumentation and optical measurement devices. In addition, because of the volume limitations, many times it is necessary to use a form factored, expensive IR seeker to measure the optical performance. A new test technique has been developed by Raytheon to measure the optical performance of a non-form factored IR window while simultaneously exposing it to tactical missile free flight heating levels. The high power, continuous wave carbon-dioxide laser, located at the Laser Hardened Materials Evaluation Laboratory (LHMEL) at Wright Patterson Air Force Base, was used to heat the exterior surface of a flat, internally cooled silicon window. During the application of the laser heating, the optical performance of the cooled window as measured using a 128 by 128 focal plane array (FPA) midwave IR detector. Several weeks of testing was conducted in which significant data on the wavefront distortion, thermal response, and overall optical performance of the internally cooled silicon window was captured. The test technique can be utilized for any IR window material, internally cooled or uncooled. The test set-up and technique are discussed in this paper. The low cost of performing the tests allowed significant data to be collected on the IR performance of the window. The ability to test non- form factored hardware resulted in significant cost savings as available test equipment was utilized in lieu of expensive form factored IR seekers. Also, the ability to measure non- form factored hardware early in the development process resulted in reductions in overall development cost and schedule.