The straight-line principle applies to both black and white and colour intermediates.
Setting up a duplication system can only be done using sensitometry. This can be done in one of two ways:
A full set up procedure involves the printing of a test negative or control strip, sometimes called the 2-point method, or the Gale and Kisner Method [1959] [see references].
The 2-point method requires that full characteristic curves of the duplicating stocks are plotted and the positions of shadow and highlight from the original image be followed from stock to stock to ensure that they fall on the useful part of the curve. It can be time consuming but always produces the optimum result, and is particularly necessary when the process or the materials are being handled in non-standard ways.
A shorter, simpler method uses a single density step as a guide, called the Laboratory Aim Density [LAD] method. This is less precise for some archive routes, as it depends on Kodak having provided an LAD for the duplication route.
The LAD method utilises a single mid-tone density (the LAD) on a standard Kodak supplied negative spliced into and printed along with the camera original material during the duplication stages. If the LAD patch prints through onto the duplicating stock at certain pre-set density levels in interpositives, duplicate negatives and prints, then the correct exposure of the duplicate materials has been achieved. The procedure relies on the fact that processing is standardised and on the principle that most subjects have a fairly repeatable brightness range.
For this reason the LAD values can only be used where the processing method is exactly as specified by Kodak for that particular stock. Where the procedure used closely resembles the procedures used in modern commercial laboratories there will be a recommended LAD value available from Eastman Kodak, but LAD's do not exist for many archive techniques. This is especially true where a film is being developed or flashed to achieve a non-standard contrast value.