This SBIR Phase I proposal describes a novel method to make optical filters based on mesoporous silicon multilayers, for use at cold temperatures in the far infrared spectral region. This type of filter consists of a lattice of different porosity layers formed in monocrystalline silicon by electrochemical means. Due to the nature of the material, mesoporous silicon filters do not suffer from thermal, mechanical, and environmental instabilities. More over, due to the high transparency of the mesoporous silicon throughout a large part of the far IR range, such filters can be made for wavelengths far longer than those that can be addressed with conventional interference filter technology. They will considerably outperform filters based on metal meshes. Such filters are expected to impact astronomical, commercial, military and scientific communities in many filter applications. In Phase I, the feasibility of the method was demonstrated by fabricating porous multilayers with ultrahigh thicknesses and good long wave pass and bandpass characteristics in the mid-to-far IR. In Phase II, optimized filters will be fabricated and their properties compared with design predictions. Phase III will involve product design, fabricating filter structures to meet customers' physical as well as optical needs, and marketing and sales investments.