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Chapter 11 ∣ Color
Copyright Goodheart-Willcox Co., Inc.
Densitometers
Densitometers compute optical density, the
light-stopping or light absorption ability of an image
or surface material. The fundamental difference
between a densitometer and a spectrophotometer
is the bandwidth of light used. A densitometer
uses red, green, and blue filters to isolate broad
bands of light, which are about 55 nm wide. A
spectrophotometer uses a prism or diffraction
grating to spread the light, and a slit to isolate
narrow bands of light between 1 nm and 10 nm.
The densitometer uses numbers, not curves, to
express density. Depending on the type being
used, densitometers can be used on negative or
positive transparencies, photographs, or printed
images.
• Reflection densitometers measure the
amount of light that bounces off a photographic
print or printed sheet at a 90° angle. It can be
configured to make direct measurements of
halftone values on printing plates.
• Transmission densitometers measure the
fraction of incident light conveyed through a
negative or positive transparency, without being
absorbed or scattered.
• Combination densitometers measure both
reflection and transmission densities.
Goodheart-Willcox Publisher
Figure 11-25. Spectrophotometric curves of an image.
400 nm 500 nm 600 nm 700 nm
10%
30%
50%
70%
90%
20%
40%
60%
80%
100%
Re
f
lectance
Percenta
ge
Wavelength
a
b b
c
©George Deal
Figure 11-26. Strip reading spectrophotometer.