Density is the level of darkness in a negative or positive film or print. The measurement of density is called densitometry. An instrument called a densitometer is used to measure the density. Any printing process from the most simple black and white work to complex color work will benefit from the use of a densitometer.
The densitometer provides the element of control, providing you with information that will guide the decisions that you make as you print a job.
There are three types of densitometers:
Transmission – A transmission densitometer measures the amount of light transmitted through a transparent material. It determines the opacity for different areas of an original transparency or of the processed film negative or positive.
Reflection – A reflection densitometer measures the amount of light being reflected from a surface, such as a reflection original. The reflection readings are also used to calculate total dot gain, hue error, grayness, and other characteristics in printed pieces.
Combination – A densitometer that measures both reflection and transmission densities.
Transmission densitometers are most commonly used in the prepress area for controlling the quality and processing of film material. Transmission densitometry is used to measure the amount of light that passes through the film. This will give you readings for Dmin, the clear or unexposed film area, and Dmax, the darkest exposed black area of the film. These, with measurements between the two extremes, assure that correct exposure times were used, as well as good chemical processing.
Reflection densitometers are most often used in the pressroom area. Their primary function is to measure the SID or Solid Ink Densities of the inks being used. All other measurements and the quality of the print job can be traced back to the proper control of the SID.
The most practical way to make these measurements is to use a colorbar
This is an image that is printed across the press sheet, usually on the trailing edge, that contains targets to provide measurement areas for the solid ink densities, dot measurements and other control elements. These colorbars are provided as film and exposed onto the plates along with the image or they may be digitally derived in CTP (computer to plate) systems.
The usual targets on a test strip are the solid ink density, dot area/gain of the quarter, half, and three-quarter tints, contrast, and the trapping of ink overprints.