Definition
Noise is constant and depends on the [[Image sensor|camera sensor]], where signal depends on the intensity of the light. Therefore, when shooting dark scenes, the noise is comparatively higher.
SNR 18% compares the signal and the noise, in which the signal is a 18% grey, (0% being pure white and 100% being pure black)
Why is 18% a standard ? Why is it also called L=50 ? cf [[Paper printing]]
On DxOmark, the SNR compares log10 of signal to log10 of noise ( why ? )
Sources
https://photo.stackexchange.com/questions/45175/what-is-the-precise-definition-of-snr-as-used-by-dxomark
https://www.scotty-elmslie.com/uploads/5/6/3/3/56337819/camera_dynamic_range_2.pdf