Abstract: A bandpass filter may include a set of layers. The set of layers may include a first subset of layers. The first subset of layers may include hydrogenated germanium (Ge:H) with a first refractive index. The set of layers may include a second subset of layers. The second subset of layers may include a material with a second refractive index. The second refractive index may be less than the first refractive index.

Abstract: An optical filter may include a first mirror, a second mirror, and a spacer layer structure disposed between the first mirror and the second mirror. The spacer layer structure may include a first set of layers. Each layer, of the first set of layers, may be a first material associated with a first refractive index and a thickness that is greater than a layer thickness threshold. The spacer layer structure may include a second set of layers. Each layer, of the second set of layers, may be a second material associated with a second refractive index. Each layer, of the second set of, may be selected to replace a corresponding layer of the first material. The corresponding layer may be associated with a thickness that is less than the layer thickness threshold.

Abstract: A bandpass filter may include a set of layers. The set of layers may include a first subset of layers. The first subset of layers may include hydrogenated germanium (Ge:H) with a first refractive index. The set of layers may include a second subset of layers. The second subset of layers may include a material with a second refractive index. The second refractive index may be less than the first refractive index.

Abstract: A bandpass filter may include a set of layers. The set of layers may include a first subset of layers. The first subset of layers may include hydrogenated germanium (Ge:H) with a first refractive index. The set of layers may include a second subset of layers. The second subset of layers may include a material with a second refractive index. The second refractive index may be less than the first refractive index.

Abstract: A bandpass filter may include a set of layers. The set of layers may include a first subset of layers. The first subset of layers may include hydrogenated germanium (Ge:H) with a first refractive index. The set of layers may include a second subset of layers. The second subset of layers may include a material with a second refractive index. The second refractive index may be less than the first refractive index.

Abstract: A bandpass filter may include a set of layers. The set of layers may include a first subset of layers. The first subset of layers may include hydrogenated germanium (Ge:H) with a first refractive index. The set of layers may include a second subset of layers. The second subset of layers may include a material with a second refractive index. The second refractive index may be less than the first refractive index.

Abstract: A bandpass filter may include a set of layers. The set of layers may include a first subset of layers. The first subset of layers may include hydrogenated germanium (Ge:H) with a first refractive index. The set of layers may include a second subset of layers. The second subset of layers may include a material with a second refractive index. The second refractive index may be less than the first refractive index.

Abstract: An optical filter may include a first mirror, a second mirror, and a spacer layer structure disposed between the first mirror and the second mirror. The spacer layer structure may include a first set of layers. Each layer, of the first set of layers, may be a first material associated with a first refractive index and a thickness that is greater than a layer thickness threshold. The spacer layer structure may include a second set of layers. Each layer, of the second set of layers, may be a second material associated with a second refractive index. Each layer, of the second set of, may be selected to replace a corresponding layer of the first material. The corresponding layer may be associated with a thickness that is less than the layer thickness threshold.

Abstract: There is disclosed a protected item including an item that needs protection and a protective coating having a hardness of at least about 8 on the Mohs scale. The protected item includes a light transmission in part or all of the visible wavelength of at least about 60% and a light reflection in the visible wavelength of about 4% or less.