將復色光分離成光譜的光學儀器。光譜儀有多種類型，除在可見光波段使用的光譜儀外，還有紅外光譜儀和紫外光譜儀。按色散元件的不同可分為棱鏡光譜儀、光柵光譜儀和干涉光譜儀等。按探測方法分，有直接用眼觀察的分光鏡，用感光片記錄的攝譜儀，以及用光電或熱電元件探測光譜的分光光度計等。單色儀是通過狹縫只輸出單色譜線的光譜儀器，常與其他分析儀器配合使用。
The polychromatic light is separated into optical spectrum instrument. There are many types of spectrometers, in addition to the spectrometer in the visible band, as well as infrared spectroscopy and ultraviolet spectroscopy. According to the difference of dispersive elements, it can be divided into prism spectrometer, grating spectrometer and interference spectrometer. According to the detection method, a direct observation of eye plate records with a spectroscope, spectrograph, and photoelectric detection or thermoelectric element spectrum spectrophotometer. The monochromator spectrometer is only through slit output monochromatic spectral line, often used in conjunction with other analytical instruments.

圖中所示是三棱鏡攝譜儀的基本結構。狹縫S與棱鏡的主截面垂直，放置在透鏡L的物方焦面內，感光片放置在透鏡L的像方焦面內。用光源照明狹縫S， S的像成在感光片上成為光譜線，由于棱鏡的色散作用，不同波長的譜線彼此分開，就得入射光的光譜。棱鏡攝譜儀能觀察的光譜范圍決定于棱鏡等光學元件對光譜的吸收。普通光學玻璃只適用于可見光波段，用石英可擴展到紫外區，在紅外區一般使用氯化鈉、溴化鉀和氟化鈣等晶體。目前普遍使用的反射式光柵光譜儀有較寬的光譜范圍。

Shown in Figure three is the basic structure of prism spectrograph. The slit S is perpendicular to the main section of the prism and is placed in the focal plane of the lens L, and the photosensitive film is placed in the image plane of the lens L. Using the light source to slit the S, the image of the S becomes a spectral line on the photosensitive film. Because of the dispersion of the prism, the spectral lines of different wavelengths are separated from each other. The spectral range of prism spectrograph can observe depends on prism optical elements on the absorption spectrum. Ordinary optical glass is only applicable to the visible light band, quartz can be extended to the UV region in infrared region using sodium chloride, potassium bromide and calcium fluoride crystal. The widely used reflection grating spectrometer has a wide spectral range.