The interpretation of printer control data is best demonstrated by an example of the control procedure used in a specialist archive restoration laboratory that has a number of very different printers.
The photographic control of a printer is by means of a test negative strip consisting of a neutral patch with a density of 0.8. This density was chosen as representing the mid point of a well-exposed negative.
When this image is printed onto positive stock the aim is to obtain a density of 1.3. This relates to a position on the sensitometric curve from a negative density of 0.8 onto the print stock.
The density obtained on the positive film will depend on the activity of the process.
If the process is off aim then the print density will be equally off aim. A process control strip of the same film stock is processed at the same time as the printer strip and the reading for the positive strip corrected if necessary.
For example, if the process strip is reading 0.05 high at the 1.30 step, 0.05 is subtracted from the reading of the printer strip. If this is not done the correction deduced for the printer will, in part be due to process variation instead!
Laboratory technicians call this "chasing the process"! Once the printer strip is corrected for any process variation any other variation will be due to the printer and will need correction.
It is wise to be conservative about any corrections, and look for a trend rather than a single aberrant reading. Only if the technician is satisfied that an alteration is necessary, should one be made.
The alteration to the light intensity to make the correction will depend on the contrast of the print stock.
Contrast = cb/ab = 2.60 (for B/W
Positive stock)
cb = actual density - aim density = 1.42- 1.30 = 0.12
ab = log exposure = cb/2.60 = .12/2.60 = 0.046
In order to make the adjustment it is necessary to alter the Log Exposure by 0.46.
The actual density is higher than the aim so the exposure must be reduced.
Adjustment can be achieved by adding a neutral density of 0.05 (the nearest to 0.046).
Alternatively the trims can be reduced by 0.046/0.025 (the log E of 1 printer point) approximately = 2.
Alternatively the lamp voltage can be reduced by 4 volts [an alteration known by previous test to produce this reduction]. The choice of which of these techniques to use to reduce the exposure will depend on the printer and what alternatives are possible.
Once calculated we can apply this correction to all the stocks we use on this particular printer.