定义：
衡量时间相干性的量，是表征相干性显著衰减所需的时间。

相干时间可以定量表示光的时间相干度。在相干理论中，定义相干时间为场相干方程衰减所需的时间。对于一个稳定的光场（具有稳定的统计性质），时间相干的相干度定义为： 

其中 E(t)是某一位置处的电场。当时τ = 0，相干度为1，并且随着时间延迟τ 的增大而单调减小。对于任意形状的相干方程，相干时间都可以定义为： 

当相干度指数衰减时（当激光器只受量子噪声影响时会发生），相干时间等于指数衰减时间。 已知相干方程的形状（或傅里叶光谱的形状）时，知道相干时间非常重要。但是，仅知道相干时间（或者线宽）不能给出相干的全部性质。

 相干时间与辐射线宽紧密相关，也就是光谱的宽度。在如上所述的相干指数衰减的情况下，光谱是洛伦兹型，（半高全宽）线宽为： 

方程中的常数因子对于不同形状的相干方程是不同的（例如，高斯型是其两倍）。反过来，已知线宽可以估计相干时间的大小，不过二者之间的转化关系与光谱形状有关。

如果频率噪声谱不是平坦的，而是在小噪声频率范围内上升很高，那么即使当时间延迟大于线宽的倒数时，相干度也不为0；这在自外差线宽测量中非常重要。 
常用的并不是采用相干时间来量化时间相干性，而是采用相干长度，它是相干时间乘以真空时的光速。 

激光器通常具有长的相干时间，尤其是单频固态激光器，可能达到几个毫秒。在许多应用中都需要长的相干时间（参阅相干）。

Definition: a measure of temporal coherence, expressed as the time over which the field correlation decays

Formula symbol: τ_coh

Units: s

The coherence time can be used for quantifying the degree of temporal coherence of light. In coherence theory, it is essentially defined as the time over which the field correlation function decays. For a stationary light field (having constant statistical properties), the complex degree of temporal coherence can be defined as

where E(t) is the complex electric field at a certain location. The complex degree of coherence is 1 for τ = 0 and usually decays monotonically for larger time delays τ. If it decays exponentially (as it occurs e.g. for a laser with quantum noise influences only), one may take the exponential time constant as the coherence time; it is then the value where the coherence decays to e⁻¹ (≈37%) of its value at τ = 0. More generally, the coherence time can be defined by

for an arbitrary shape of the coherence function, and in the case of an exponential coherence decay this is the same as the exponential decay time. A measurement of coherence time may be based on that equation, where the coherence function is obtained via a path length dependent interference contrast.

Instead of the coherence time, it is common to specify the coherence length, which is simply the coherence time times the vacuum velocity of light, and thus also quantifies temporal (rather than spatial) coherence.

Figure 1: Animated illustration of temporal coherence. An ensemble of 100 oscillators with nominally the same frequency but with same phase noise (corresponding to white frequency noise) are started at t = 0. The phase space diagram (upper part) shows how they get increasingly out of phase. The thick point shows the complex average amplitude; its oscillations become weaker with time. (This phenomenon is analogous to a reduced interference contrast.) The lower diagram shows the decay of the coherence function with time.

Knowledge of the coherence time (i.e., a single number) can be useful when the shape of the coherence function (or the shape of the Fourier spectrum) is approximately known. Obviously, however, the specification of the coherence time (or the linewidth) alone does not constitute a full characterization of the coherence.

The coherence time is intimately linked with the linewidth of the radiation, i.e., the width of its spectrum. In the case of an exponential coherence decay as above, the optical spectrum has a Lorentzian shape, and the (full width at half-maximum) linewidth is

The constant factor in this equation (here: 1 / π) is in general different for other shapes of the coherence function (e.g. roughly twice as high for a Gaussian shape). Conversely, the linewidth can be used for estimating the coherence time, but the conversion depends on the spectral shape.

Many authors state that the bandwidth is just the inverse of the coherence time without referring to any concrete definitions of coherence time and bandwidth, nor to a specific spectral shape. This then simply means that the bandwidth is of the order of the inverse coherence time for typical spectral shapes.

In cases where the frequency noise spectrum is not flat but rises strongly at small noise frequencies, there can be a significant degree of coherence even for time delays well above the inverse linewidth; this issue is important, e.g., in the context of self-heterodyne linewidth measurement.

Lasers, particularly single-frequency solid-state lasers, can have long coherence times, compared with the duration of an optical cycle; values of several milliseconds are possible. Even much longer coherence times (as required e.g. in optical frequency metrology) are possible for carefully stabilized lasers; see the article on the stabilization of lasers. A long coherence time is important for many applications (see the article on coherence).

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