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
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
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.