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在物理学里，单色光（monochromatic light）一般是指波长为单一值的电磁辐射。严格的说，并没有任何光源，能够制造出纯单色光。虽然有些先进的雷射，能够制造出线宽极窄的雷射光。这些雷射光的波长，也有一定的线宽（称为光谱线宽（spectral linewidth））。实际上，经过滤光器过滤的光波，经过绕射光栅分离的光波，与雷射的光波都惯常地被称为单色光。假若，一个光源散发出来的光波的线宽，比另一个光源，更为狭窄，则称此光源更具有「单色光性」。单色光器是一种仪器，可以用来选择特定波长的单色光。

A spectral color is a color that is evoked by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths, also known as monochromatic light. Every wavelength of visible light is perceived as a spectral color, in a continuous spectrum; the colors of sufficiently close wavelengths are indistinguishable.