Temperature non-uniformity in a heated window can result in a significant distortion in the transmitted wavefront. Aberrations are introduced by actual physical distortion of the window due to differential thermal expansion and by localized optical path variations due to the change in index with temperature (dn/dT) of the substrate material. Typically, the second factor is the more pronounced. This effect represents a significant limitation in the performance of windows with non-symmetric geometries made from materials that exhibit combinations of high dn/dT and low thermal conductivity.
LightWorks Optical Systems (LWOS) has recently developed a software tool capable of quantitatively modeling the thermal distribution of a heated window in operation. This capability allows the design team to optimize the heater layer sheet resistance (whether the layer is a metallic grid or a transparent conducting oxide thin film) and the configuration of the bus-bar (electrode) connections prior to any hardware fabrication. Consideration of both of these factors is critical to achieving a uniform thermal distribution at the specified temperature across a given window.
This presentation will describe the recent efforts of LWOS to establish the capability for quantitatively modeling the temperature homogeneity across a heated window based on window material and dimensions, heater layer characteristics and bus-bar configuration. Data will be presented that demonstrates the validity of these models via comparison to actual heated windows observed under heated conditions.