A thermally switched optical downconverting (TSOD) filter is a self-regulating device including a downconverter that converts incoming light at a variety of wavelengths into longer-wavelength radiation and then directs it using one or more bandblock filters in either the inward or outward direction, depending on the temperature of the device. This control over the flow of radiant energy occurs independently of the thermal conductivity or insulating properties of the device and may or may not preserve the image and color properties of incoming visible light. The TSOD filter has energy-efficiency implications, as it can be used to regulate the internal temperature and illumination of buildings, vehicles, and other structures without the need for an external power supply or operator signals. The TSOD filter also has aesthetic implications, since the device has unique optical properties that are not found in traditional windows, skylights, stained glass, light fixtures, glass blocks, bricks, or walls. The TSOD filter has particular, but not exclusive, application as a building material.