A thermochromic optical filter incorporating quantum confinement devices is formed as multilayered composite film of semiconducting materials. A quantum well adjacent a barrier layer ensures proper confinement of charge carriers within the well. A transition wavelength (i.e., the energy/wavelength at which the filter becomes transparent) is established by selecting a quantum well material with a bandgap near the desired energy and a barrier layer material with a higher bandgap. For a given reference temperature (e.g., room temperature), the exact transition wavelength is fixed by the thickness of the quantum well. The quantum confinement energy is added to the bandgap energy to yield the transition energy. A thermal control system varies the temperature of the thermochromic filter to adjust the transition wavelength. Temperature changes affect both the bandgap and the quantum confinement energy, and thus the optical properties of the thermochromic filter. The thermochromic filter may function as a longpass filter.