The laser system that exposes the plate in a CtP system has an impact on the consistency of the halftone dots that make up the image on the plate and therefore the consistency and integrity of the final presswork. The more well focussed, and hence sharper, the beam of exposing energy the more consistent the resulting halftones will be.
The four main ways that laser energy is focussed on a CtP device in order from basic to most sophisticated:
1) Depth of field. This is the method most often seen used in internal drum CtP machines and film imagesetters. It is similar in concept to how depth of field works in a camera. Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on each side of the focused distance, so that within the depth of field, the unsharpness is imperceptible under normal conditions. Typically requires constant calibration to maintain imaging consistency.
2) Hard focus. The laser is focussed for the particular plate when the CtP system is initially set up. This method is not able to cope with issues of variations in plate media thickness or as plate surface to exposure focus point changes. Hot spots and/or banding may appear.
3) Auto focus. The laser does an auto-focus for the particular plate each time just before actual imaging begins. This method is not able to cope with issues of variations in plate media thickness or as the plate surface to exposure focus point changes. Hot spots may appear.
4) Dynamic auto focus. The laser continuously adjusts focus for the particular plate during actual imaging. This method is able to cope with issues of variations in plate media thickness or as the plate surface to exposure focus point changes. Hot spots are unlikely.
The "tape test"
To get an idea as to how well the focussing system is on a particular CtP device, simply make a large "X" using Scotch tape/Sellotape on the back of the plate. Image a 50% halftone screen across the entire plate. Remove the tape and process the plate.If, after processing the plate, the tape is visible in the 50% flat tone patch then you know that any variations in plate thickness, or plate surface to exposure focus point, that is equal or greater than the thickness of the Scotch tape/Sellotape will result in variation in halftone screen tones and therefore presswork. It's unlikely that in day to day production that you'd stick tape to the back of a plate before exposing it, however it is quite possible for a small piece of debris to land on the CtP drum or back of the plate during pre-exposure handling.
Ideally, the imaging integrity of the best system would not be compromised by the tape on the back of the plate and all you will see is a large flat 50% tone - no visible "X" pattern.
Imagine a grid wrapped around the CtP's imaging drum.
In a perfect world the grid would have perfect integrity.
In reality - the drums are never perfect cylinders.
For those CtP devices that have this capability, geometric correction distorts the bitmap that will be imaged on the plate in order to compensate for differences in the geometry of the imaging drum on the CtP device. Geometric correction also provides CtP device to CtP device integrity so that the plates from different machines will be as identical as possible.
Aluminum plates expand and contract with temperature change as much as film does - about 0.5mm across a 1m plate (typical 8-page size) for every 5°C (9°F) temperature change.
That 5°C change in temperature will result in a half row of dots @ 175 lpi misregistration plate to plate which can result in the need for the shop to reimage all four plates in a process job even if only one is actually needed.
Thermal compensation, for those devices that have this capability, corrects for ambient temperature variations by scaling the bitmap in such a way that it is as if the plates were always imaged at a single temperature.