A densitometer is a device that measures the degree of darkness (the optical density) of a photographic or semitransparent material or of a reflecting surface. A densitometer does not measure color but measures density. In print work, 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.
- How it works
Within a densitometer the light passes through the optical system bundled from a stabilized light source on the printed surface. The amount light absorbed depends on the ink density and pigmenting of the ink. The non-absorbed light penetrates the translucent (transparent) ink layer and is weakened. The remainder is re-emitted by the surface of the material, i.e. diffusely reflected or scattered A part of this scattered light passes through the ink layer and is weakened again.
A lens system captures the light rays coming from the ink layer and sends them to a photodiode. The light striking the photodiode is converted into electric energy. The electronics compares this current with a reference value. The difference between the measured current and the reference value forms the basis for calculating the absorption behavior of the measured ink layer.
Densitometric values are relative measurements. Despite the same measuring conditions, the values from different instruments are not the same. This can be due to differences in:
- the spectral transparency of the filters (aging etc.)
- the spectral distribution of the light sources
- the photodiode
- the measurement geometry
- the hues/ friction of the print scale inks
The right handling and calibration can lead satisfactory correspondence within acceptable tolerances.
- See also: