The linearization of CTP differs per CMYK colors

[Saulius]

Hello,

Where could be a problem if a linearized CTP (with the one curve for all colors) makes slightly different halftone on the Magenta plate around 70% and 80% ?

Dot shape is round, 175 lpi
Angles:
Cyan 105.000° (Cyan)
Magenta 45.000° (Magenta)
Yellow 90.000° (Yellow)
Black 75.000° (Black)


70% and 80% on C, Y, K plates are as it is expected, except of the Magenta plate, where measurements show +2-3 % more.

See attached screenshots of rendered images and photos of measurements.

Thank you in advance.

 

[gordo]

Hello,

Where could be a problem if a linearized CTP (with the one curve for all colors) makes slightly different halftone on the Magenta plate around 70% and 80% ?

Dot shape is round, 175 lpi
Angles:
Cyan 105.000° (Cyan)
Magenta 45.000° (Magenta)
Yellow 90.000° (Yellow)
Black 75.000° (Black)


70% and 80% on C, Y, K plates are as it is expected, except of the Magenta plate, where measurements show +2-3 % more.

See attached screenshots of rendered images and photos of measurements.

Thank you in advance.

Because of how plate readers read plates (explained here: The Print Guide: How CtP plate readers read plates ) the same error is found for the same tone value in all plates. Can you change the order in which plates are imaged to see if changing the order still results in the M reading different? Or can you image one plate at a time to see if the problem remains? Can you look at the plate using a high powered loupe or microscope to see if what you see with your eye is what your plate readers "sees"? At 80% the dots should just touch each other at 4 points. The M image on the reader shows the dots are larger than the CYK plates - but that could be caused by the plate reader not being in complete contact with the plate when the reading for M happened.

The different lpi readings (except for the Y which should be higher) is probably just instrument error.

 

[Saulius]

Thank you Gordon for the answer. It is honor for me.

Today I have done as you've suggested. Mixed the order of output, measured on the flat table by one plate on it and remeasured with XRite eXact. Results are attached.

eXact hadn't showed a big difference in halftone readings per color, but Techkon still shows the same result. Magenta at 80% is 82-83%. It is not one case, that phenomenon I observe during a week. Maybe it was always, but I've noticed it only recently.

 

[gordo]

Thank you Gordon for the answer. It is honor for me.

Today I have done as you've suggested. Mixed the order of output, measured on the flat table by one plate on it and remeasured with XRite eXact. Results are attached.

eXact hadn't showed a big difference in halftone readings per color, but Techkon still shows the same result. Magenta at 80% is 82-83%. It is not one case, that phenomenon I observe during a week. Maybe it was always, but I've noticed it only recently.

What happens if you do not apply a linearizing curve (or any curve)?
Does the Techkon still show a different value for the M plate?

I'm not surprised that the two instruments don't give the same reading (that's the effect of the different thresholding method each uses to determine what is dot and what is not) but I would have expected them to show the same effect in the M plate if indeed the M plate dot size is different.