2.5 Spectroscopic Filter

2.5 Spectroscopic Filter

A filter can transmit or reflect a specified range of wavelength. A filter designed for spectroscopy is called a spectroscopic filter.

Filters are classified into three types - long wave pass filters, short wave pass filters and band pass filters from the viewpoint of function, as shown in Figure 2.5.1. A cold mirror which transmits thermal infrared and reflects visible light is a long wave pass filter, while a hot mirror which reflects thermal infrared and transmits visible light is a short wave filter.

Figure 2.5.2 shows the types of filter from the viewpoint of function.

(1) Absorption filter :
a filter which absorbs a specific range of wavelengths, for examples, colored filter glass and gelatin filter.

(2) Interference filter :
a filter which transmits a specific range of wavelengths by utilizing the interference effect of a thin film. When light is incident on a thin film, only a specific range of wavelengths will pass due to the interference by multiple reflection in a thin film as shown in Figure 2.5.3 and 2.5.4. The higher the reflectance of the thin film, the narrower the width of the spectral band becomes. If two of these films, with different refractive indexes, are combined, the reflectance becomes very high which results in a narrow spectral band, for example of the order of several nanometers. In order to obtain a band pass filter which transmits a single spectral band, a short wave pass filter and long wave pass filter should be combined. A dichroic mirror ,which is used for three primary color separation, is a kind of multiple layer interference filter, as shown in Figure 2.5.5 and 2.5.6. It utilizes both functions of transparency and reflection.

(3) Diffraction grating filter :
a reflective long wave pass filter utilizing the diffraction effect of a grating, which reflects all light of wavelengths longer than the wavelength determined by the grating interval and the oblique angle of the incident radiation.

(4) Polarizing interference filter :
a filter with birefringent crystallinity plate between two polarizing plates, which pass a very narrow spectral band, for example less than 0.1 mm. This utilizes the interference by two rays of light ; a light following Snell's law and the other not following Snell's law, which pass a light with a narrow band of wavelength determined by the thickness of the birefringent crystallinity plate .