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. The density of a photographic positive or negative is a result of the amount of silver dye developed in the film or photographic paper. In printed copy, density is caused by the light-stopping ability of the pigments in the printing ink that are deposited on the paper by the printing process. Densitometers are widely used in the graphics industry to help control color in each step of the printing process.

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.

Density and opacity are the same thing but they are measured differently. Density is measured with readings that run on a scale from 0 to 4.0, with the highest number having the greatest density. Opacity is measured in decimal numbers ranging from 1 to 100. The higher the number, the great the opacity. 

Densitometer readings are different for different types of substrates. For example, for optimum print reproduction on newsprint, the recommended densitometer reading is 1.4 to 1.8 for a black and white print and 2.5 to 2.8 for a color transparency.

To measure color, a test strip across the edge of the proof or press sheet is used. The test strips are called color bars, print control strips, color control bars, color control strips, or proofing bars. The different strips are available commercially from several vendors. 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.