Absolute Density vs. Relative Density

Densitometers were designed to measure a process. For this reason there are different filter sets or “responses” for various processes. From monitoring a film processor in the photographic industry to measuring the consistency of output from an offset press, densitometers have found wide use in the imaging industry.

When using them to monitor the consistency of a press run, the user can be faced with a choice of how the data is presented. Does the user wish to see the data in terms of “Absolute” density, in which the measured values are traceable to a known white calibration reference, or to see the data in relation to the paper being used in the print job, also known as “Relative” density? Both Absolute and Relative density have their applications in the imaging process, but the reason to choose one over the other is fairly logical.

The mode should be based on how the data will be used.

To maintain the consistency of a press run:

When a job is being printed on a press, little can be done about the paper being used once it has been chosen for the job. Unless it is completely wrong, it will be used.

Within a roll or skid of paper, there will be slight fluctuations in the density characteristics of that paper. The press operator cannot change these characteristics, but the densitometer can measure them. But in measuring them, the data obtained could give a false indication of the conditions on the press. For this reason it is a good idea to use the densitometer in the Relative density mode (white reference, paper). The operator can now “zero” on the unprinted paper base and the measurements will not be subject to the variations of the paper. The data will reflect the true stability of the press.

To determine the overall reproduction characteristics of the printed image:

Sometimes during the reproduction process, you need to determine the overall appearance characteristics of the job. This is usually done after the job has been printed, but can also occur during the run for very critical work. The current job is compared to the proof or a job that was previously run. In these cases, the overall appearance of the work is evaluated. Knowing the density values for the paper for all color channels of the densitometer will be an important parameter in the comparison. For this reason, it is best to use the Absolute Density mode (White Reference, Absolute). Now the characteristics of the paper and the inks can all be compared, one to the other.

The Simple Rule

For process control of the press, use Relative Density (White Reference, paper).

For image evaluation, use the Absolute Density mode (White Reference, Absolute).

Exceptions

There may be some instances in which a substrate is not available or inappropriate for zeroing. An example would be printing on metal, in which a layer of white is printed first, and then all other colors are printed on top. Since the white base layer can change in density, and is a critical part of the image itself, this application would necessitate the use of the Absolute Density mode.

There may be other applications in which the correct procedure is difficult to determine. But if is determined the base color (i.e., white) is critical to the overall reproduction and can be adjusted by the operator, then using Absolute Density mode would be necessary.

To learn how to measure density, read our Measuring Density with a Densitometer blog.