Characteristic curves

Fig 13.6 A typical characteristic curve

A characteristic curve is the graphical representation of an emulsions response to light and processing. The characteristic curve is the basic tool in sensitometry.

A curve is created by plotting the density of points of known exposure against a logarithmic scale of exposure. A logarithmic scale is used to compress the range and enable the features of the curve to be easily seen.

The curve is measured from a test sample of the film that is given a known and repeatable exposure and processing under normal conditions.

To ensure the exposure is repeatable a device called a sensitometer is used. A sensitometer is a well controlled power supply running a calibrated lamp. Between the lamp and the film there is a shutter to control time and a density step wedge to control and provide a range of intensities.

Curves for colour materials are plotted for each colour dye layer.

Base plus fog The density of the unexposed film base plus any chemical fogging that may occur during processing.
Inertia point The point at which the film has absorbed sufficient light energy to start forming a latent image.
Toe A non-linear region where shadow detail is recorded in negative materials (Highlight detail in print material). Compression of the density differences occur and contrast is decreased.
Straight line portion The linear section where most of the information is recorded. This section is used to determine the processed contrast of the film.
Shoulder non-linear section. Compression of density differences occur and contrast is decreased. It is unusual to record information on this region of the curve, however this region may be reached with overexposure or extended development such as push processing to increase effective film speed.

Table 13.1: Characteristic curve terms