Throughout this book, the word density refers specifically to optical density. Optical density measurements and density-measuring devices (densitometers) are important in color imaging for several reasons: most forms of color encoding are based directly or indirectly on density readings; most input and output devices are calibrated using density measurements; and most reflection and transmission scanners essentially are densitometers.
Figure B.1 illustrates the basic concept of optical density measurement. A densitometer consists of a light source, a means for inserting a sample to be measured, a photodetector, an amplifier, and some type of analog or digital density-value indicator. Density readings are made by measuring the amount of light with and without the sample in place.
For transmissive samples, the transmission density is determined from the transmittance factor of the sample. The transmittance factor, T, is the ratio of the amount of light transmitted, It, measured with the sample in place, to the amount of incident light, Ii, measured without the sample (Figure B.2). The transmission density, Dt, of the sample is the negative logarithm of its transmittance factor:
The following examples may help to illustrate the basic relationships of sample transmission characteristics, measured transmittance factor, and computed transmission density: