定义：

将功率或者能量分解成不同波长或者频率。

光源或者一些光束的光谱（或辐射谱）或者包含了能量和功率在不同波长中分布的信息。通常它是用图表的形式表示，给出了功率谱密度随波长或者光功率之间的关系曲线。图1就是一个例子，给出了超连续光源的数值模拟光谱。单频激光器的光谱不同于图中的宽谱，是由非常窄的线表征，极限情况下线宽仅为1Hz量级，对应的波长范围只有约≈ 3 •10−12 nm（中心波长为1微米）。其它的激光器包含多条线，有的具有很大的带宽（尤其是超短脉冲的锁模激光器），约为100 nm，具有频率梳结构。 
 

图1：数值模拟超连续光源。参阅词条超连续产生得到更多细节。 

可以采用不同类型的光谱仪来接收光谱，它们适用的光谱范围及光谱分辨率都是不同的。 
光谱与光的时间相干特性密切相关。例如，由时间相干方程可以得到光谱。光谱还与电场的傅里叶变化有关，但是后者大多数情况下不能直接得到。因此，也称为光场的傅里叶光谱。 

光学带宽 
光学带宽就是指光谱的宽度。具有不同的定义，常用的是半高全宽（FWHM）。 

具有线结构的光谱 
有些光源的光谱非常平坦，例如白炽灯，光二极管或者超发光光源。而有些光源的谱中包含相距很近的窄线，只能从具有足够高光谱分辨率（小的分辨率带宽）的光谱仪中能够看到。例如，连续光激光器辐射多模光束，并且只辐射基本谐振腔模式时，光谱中的线是几乎是等间距的，并且间距等于谐振腔往返时间的倒数，在MHz或者GHz范围。如果激光器同时辐射高阶横模，那么光谱中存在附加的线，这时光谱会更加紧密并且是不等间距的。但是任意的锁模激光器都会产生等间距的频率梳光谱，同时光谱中还存在较弱的激光器噪声。 

Definition: the decomposition of the power or energy of light according to different wavelengths or optical frequencies

The optical spectrum (or emission spectrum) of a light source or some beam contains information on how the optical energy or power is distributed over different wavelengths. Usually, it is presented in the form of a diagram where some spectral quantity is plotted as a function of the wavelength or optical frequency. The plotted quantity may e.g. be a spectral flux (in units of W/Hz or W/nm), a spectral intensity (in W sr⁻¹ Hz⁻¹ or W sr⁻¹ nm⁻¹) or spectral radiance (in W sr⁻¹ m⁻² Hz⁻¹ or W sr⁻¹ m⁻² nm⁻¹), but in many cases optical spectra are presented without an absolute scale. In some cases, one uses a (calibrated or uncalibrated) logarithmic scale, e.g. with units of dBm/nm.

As an example, Figure 1 shows the numerically simulated optical spectrum of a supercontinuum source, which is a highly polychromatic light source. In contrast to that very broad spectrum, the optical spectrum of a single-frequency laser source is often characterized by a very narrow spectral line – in extreme cases, with a linewidth of the order of 1 Hz, corresponding to only ≈ 3 ·10⁻¹² nm (for 1 μm center wavelength). Other lasers have a spectrum consisting of multiple lines, and some (particularly mode-locked lasers for ultrashort pulses) can have a large spectral width of 100 nm or more with a frequency comb structure.

Figure 1: Numerically simulated broadband supercontinuum spectrum. See the article on supercontinuum generation for details.

Optical spectra can be recorded with different types of spectrometers (e.g. spectrographs), which greatly differ in terms of the covered spectral range and the spectral resolution.

The optical spectrum is intimately related to the temporal coherence properties of the light. For example, the temporal coherence function fully determines the spectrum. The optical spectrum is also related to the Fourier transform of the electric field (although the latter is in most cases not directly accessible). Therefore, it is also called the Fourier spectrum of the optical field.

Optical Bandwidth

The optical bandwidth is essentially the width of the optical spectrum. There are different definitions, but the full width at half maximum (FWHM) is often used.

Optical Spectra with Line Structures

Some optical sources such as incandescent lamps, light-emitting diodes or superluminescent sources have a very smooth optical spectrum. However, other sources can have spectra consisting of closely spaced narrow lines, which can be resolved only with a spectrometer having a sufficiently high spectral resolution (small resolution bandwidth). In case of a continuous-wave laser with multimode emission, but only on fundamental resonator modes, the lines in the spectrum are approximately (but not precisely) equidistant, with a spacing which typically corresponds to the inverse resonator round-trip time and is in the megahertz or gigahertz region. (The round-trip time can be frequency-dependent due to chromatic dispersion, and nonlinearities may also affect the mode frequencies.) If the laser also emits on higher-order transverse modes, there are additional intermediate lines (related to the Gouy phase shift), so that the line spectrum is denser and not equidistant. Any mode-locked laser, however, produces a frequency comb spectrum with exactly equidistant lines, apart from some laser noise, which is often very weak.

Measuring Optical Spectra

Optical spectra can be recorded with instruments which are called optical spectrum analyzers. They are often a kind of spectrographs, but there are other kinds of spectrum analyzers based on completely different operation principles. For example, there are interferometric devices, using either Michelson interferometers (→ Fourier transform spectroscopy) or on Fabry–Pérot interferometers.

It is also possible to combine spectral analysis with imaging – see the articles on multispectral imaging and hyperspectral imaging.