Editing Film speed (section)

===Emulsion speed rating criteria===
Five criteria for the rating of emulsion speed have been used since the late 19th century, listed here by name and date, these criteria are: threshold (1880), inertia (1890), fixed density (1934), minimum useful gradient (1939) and fractional gradient (1939). <ref>{{citation
 | last = Allbright | first = G S
 | journal = The Journal of Photographic Science
 | doi = 10.1080/00223638.1991.11737126
 | title = Emulsion Speed Rating Systems
 | year = 1991| volume = 39
 | issue = 2
 | pages = 95–99
 }}
</ref> 
====Threshold====

The threshold criterion is the point on the [[Sensitometry|characteristic curve]] corresponding to just perceptible density above fog.

====Inertia====

The inertia speed point of an emulsion is determined on the Hurter and Driffield [[Sensitometry|characteristic curve]] by the intercept between the gradient of the straight line part of the curve and the line representing the base&nbsp;+&nbsp;fog (B+F) on the density axis. 

====Fixed density====

The fixed density speed point is determined by defining a fixed minimum density as the basis the emulsion speed (e.g. 0.1 above B+F). 

====Minimum useful gradient====

The minimum useful gradient criterion places the speed point where the gradient first reaches an agreed value (e.g. tan <big>𝜃</big> = 0.2).

====Fractional gradient====
  
The fractional gradient is defined as the speed point at which the slope of the characteristic curve first reaches a fixed fraction (e.g. 0.3) of the average gradient over a range (e.g. 1.5) of the characteristic curve.<ref>{{citation
 | last = Jacobson | first = Ralph E
 | publisher = Focal Press 
 | title = The Manual of Photography (Seventh Edition)
 | page = 410
 | year = 1978}}
</ref>