AS ISO 12232:2019 Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index
The value of H in Formulae (4) and (5) may be determined using the procedure described in Annex A.
NOTE 1  The value of K that appears in the numerator of Formulae (4) and (5) places the exposures associated with the respective signal-to-noise ratios at a mid-tone exposure, which is approximately equal to the exposure that would be obtained from an 18 % reflectance value in a 160:1 contrast-ratio scene.
NOTE 2  If there is not a significant effect on the resulting S/N, a smaller or larger area is allowable.
NOTE 3  The S/N values of 40 for the “first excellent” image and 10 for the “first acceptable” image were determined using subjective experiments performed during the development of this document. These incremental signal-to-noise ratios were judged to provide “excellent” and “acceptable” quality prints of typical pictorial images using a high quality printer at approximately 70 sensor pixels/cm on the print (just small enough to be visually imperceptible) using normal tone reproduction. Note that 70 pixels/cm at a standard viewing distance of 25 cm corresponds to 30 pixels per degree of visual subtense. For prints made using significantly higher sensor pixels per centimetre values, lower S/N values can still yield acceptably low noise prints, while for prints made using significantly lower sensor pixels per centimetre values, higher S/N values can be required to provide acceptably low noise prints. In these cases, the S/N value for “excellent” quality prints is approximately equal to (70/P) times the S/N values listed, where P is the actual number of sensor pixels per centimetre on the print.
6.3.3  Colour cameras
The noise of the luminance and colour difference signals shall be determined from CRT display output- referred RGB colour signals based on the ITU-R BT.709 RGB primaries and white point, such as the sRGB and sYCC signals defined in IEC 61966-2-1, which are used as DSC image signals in many DSCs.
For colour cameras using a single exposure process, σ(D) shall be determined using the linearized DSC image signals. If the DSC provides CRT display output-referred RGB colour signals based on the ITU R BT.709 primaries and white point, these signals shall be converted to linearized RGB signals in accordance with ISO 14524. If the DSC encodes these RGB signals as Y, Cr, Cb image signals, the signals shall be decoded to provide RGB image signals using the inverse of the matrix used to encode the signals. The decoded RGB image signals shall then be converted to linearized RGB signals in accordance with ISO 14524.