Cut-off depth refers to the maximum transmittance of light through the cut-off band. For different applications on the cut-off depth requirements of different systems, such as the use of fluorescence in the excitation of the occasion, the general requirements of the cut-off depth in the T <0.001% of the following, in the general monitoring and identification system, the cut-off depth of T <0.5% is sometimes enough. The detailed cut-off depth depends on the intensity of the light source, the size of the disturbing light and the demand for signal-to-noise ratio to determine. Similarly, we can not confirm the depth of cut-off indicators, "in addition to the passband, the cut-off band of all light transmittance is zero!"
For the sake of simplicity, the OD value is often used to indicate the cut-off depth, and the connection between the OD value and the transmittance is shown below:
For example, if T＜0.01%, the transmittance is 10-4, which corresponds to an OD value of 4, and if T＜1%, the corresponding OD value is 2.

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