The fundamental practical element of sensitometry and control is the Step Wedge or Control Strip.
It is a precisely exposed length of film with, in practice, usually 11 or 21 steps of exposure.
The exposure will vary such that the strip will have blocks of image from clear film to totally black film.
The diagram shows a typical wedge.
A Sensitometer produces these step wedges or strips.
A Sensitometer gives a precise, controllable and repeatable exposure.
There are two types of sensitometer; intensity scale and time scale sensitometers.
Both types of sensitometer require lamps that have been calibrated to give a known intensity of light at a known voltage, usually less than the rated voltage of the lamp to give increased life. The lamps are run from a constant power source with an accurate voltmeter.
In most sensitometers for motion picture applications a fixed exposure time is used which represents the exposure time typically used in practice with the particular film in question, and therefore the sensitometer is of the intensity scale type. A strip of film is exposed through a step wedge or step filter giving varying intensities of the light, which cover the normal usable exposure range of the film. Because of the difficulty in producing accurate step wedges the manufacturer always calibrates these. For black and white work a photographic (silver image) wedge can be used, but for colour work where the image must be neutral the wedge is normally made from a dispersion of graphite particles in gelatine.
Control strips can be exposed in a sensitometer to provide a repeatable strip that is exposed with known increments between the steps but no precise value for the Log E scale. Such strips are widely used to control print and duplicating material processes.
However for controlling camera negative film processes a precise or absolute value for Log E is used by the use of a calibrated lamp in the sensitometer and calculating the value of Log E for one of the steps.
This degree of precision is rarely, if ever, needed in the restoration of archival film.
Some film stock manufacturers sell control strips already prepared and in foil packs for freezer storage. In general these are for colour processes, especially for reversal processes.
The control strips have relative, not absolute Log E values but the pack includes a "correctly" processed strip that can be used as a reference against which to compare the process.
Unfortunately precision made control strips are limited to Kodak colour camera negative, positive and camera reversal films.