How CtP plate readers read plates

Dedicated plate readers take a different approach to measuring dot area than densitometers. What they do it use an internal CCD camera to take a continuous tone photograph of the plate area at high resolution and magnification:

Depending on the instrument's software and display technology, this original image may or may not be shown to the user.

In order to calculate the dot area, virtually all plate readers use a thresholding algorithm to determine what is the non-printing plate and what is the printing dot.* Put another way, the software decides that a pixel of X tone level and lighter in the captured image is the plate while levels darker in the captured image than X tone level are identified as ink carrying dots.

However, because the halftone dots in the photo will have a slight softness to them, the result is a slight ambiguity as to where the transition from non-image plate to ink carrying dot occurs. Change the threshold and you change the size of the dot the instrument "sees" and hence the tone the instrument reports.

For example at a threshold value of 139 (from a range of 0-255), the software sees this as the printing dots:
and reports a dot area value of 50%

If the threshold is a tone value of 175 the software sees this as the printing dots:
and now reports a dot area value of 55%

So which is best, or more accurate, for measuring halftone dots on plate? Well, in a GATF study of products for measuring the dot area on CTP printing plates (RTR 27: Plate Reading Technologies and Their Performance on CTP Plates) a spectrodensitometer gave very similar overall results compared to the best CCD-based solution for plates.

In their defense, dedicated plate readers can provide more information, such as halftone frequency and screen angle, than a densitometer when measuring plates.

* The extremely sophisticated and expensive Jandel planimeter requires the user to trace where they believe the dot edges are in order to differentiate between plate and dot areas.