Note, however, that noise measurements at high optical power levels often require optical attenuation, which raises the shot noise level of the relative intensity (→ relative intensity noise). In such situations, the detector setup (including the attenuator) is substantially responsible for increased shot noise.

Intensity noise at the shot noise level is obtained e.g. for a so-called coherent state, which may be approximated by the output of a laser at high noise frequencies. At lower noise frequencies, laser noise is normally much higher due to relaxation oscillations, mode hopping, excess pump noise, and other phenomena. The intensity noise of a simple incandescent lamp is close to the shot noise level. Noise below the shot noise level is obtained for amplitude-squeezed light, which can be obtained e.g. by transforming an original coherent state with the help of nonlinear interactions.

Linear absorption of light also pulls the noise level closer to the shot noise level. Therefore, the noise registered with a photodetector having a low quantum efficiency may be close to shot noise even if the incident light is well below the shot noise level.

Note also that background light often introduces not only just a constant addition to an actual signal, but also the corresponding shot noise. That makes it more difficult, for example, to detect a weak signal if the detector is at the same time affected by substantially more intense sunlight.