Methods to Measure Optical Density using RGB Cameras
Optical density is a common quantity for controlling the layer thickness of glazing CMYK inks. Typical measurement devices are densitometers that measure the CMYK ink absorption utilizing standardized narrow band filters. Usually, optical density is measured offline, which means outside the press. The ink layer thickness is directly controlled by the results of those measurements.
Unfortunately, it is very difficult in practice to realize an automatic control system of the press using offline measurements. For this purpose we have to measure the optical density inline, which means inside the press for every printed sheet. To implement an automatic control of optical density inline densitometers are used. Such devices are expensive and less accurate.
A new idea for inline measurement of optical density is to use standard RGB-cameras that are very cost-effective. However, there are some drawbacks of RGB-cameras related to this special application:
- The filters of typical RGB-cameras are optimized to emulate the color matching functions of human observers. These sensitivities are different from the standardized density filters used by densitometers.
- The measurement geometry prescribed by the density measurement standard cannot be realized by a camera setup (unidirectional illumination with directed light)
- Polarisation filters cannot be used due to the limited amount of light.
Generally, an inline measurement is performed at a stage where the ink has not dried yet. The drying process changes the optical density of the ink and needs to be considered by an automatic control system.
One aim of this research project is to develop a RGB-camera-based measurement setup for optical density that yields to similar results as standard densitometers. A second aim is to develop a model that describes the drying process in terms of optical density variation based on camera estimates.