Theory of application: Dot type vs. laser path

[Drexus]

In the many discussions I've read regarding the perfect dot shape for a given production method, a lot of weight was placed on conflicting channels, single channel moire, and matching angles to a specific tonal application.

My question rests on how the CTP device writes the plate. The CTP we have is fairly traditional in how the image is written. A narrow beam hits a traversing/spinning mirror. However, the "tail-stock" traverses to write the whole plate, effectively drawing the image cork-screw style.

Is there anything built into these systems to correct for this, or is it so insignificant that we all accept the corkscrew image? Are there smaller drums out there that may have a challenge with this? Are there systems that use dual laser - effectively multiplying the concern?

 

[SteveAgfa]

Drexus:

With single beam imagers, this was of little concern.

However, now that swaths can be 1/2 of an inch wide,
this is indeed something to be taken into account. For
some applications such as lenticular lenses, there can
be a "step" mode. Most often however, the spiral
effect is pre-compensated digitally.

regards

 

[KevinC@EFI]

<snip> ... effectively drawing the image cork-screw style.

Is there anything built into these systems to correct for this, or is it so insignificant that we all accept the corkscrew image? ...<snip>

I used to work with a company that made such internal drum CTP systems, and originally had the same question. However, if you think about it, the effect is negligible to begin with:

The offset from one line to the next is 1/2400 of an inch (in most cases). However, the plate is only covering a portion of the 360 degree imaging arc, so the offset is a fraction of this amount... maybe 1/5000th of an inch skew across the height of a plate.

Secondly, even that offset could be theoretically corrected by skewing the axis of the spinning mirror in the opposite direction, or tilting the plate ever so slightly in the drum.

Given the other shortfalls of gausian-dot imaging systems, I can honestly say that this is a complete non-issue - even for lenticular imaging (technically you're absolutely right Steve, but the margin of error of everything else in the system is well beyond the 1/5000" maximum we're talking about here).

As Steve mentions, the "corkscrew imaging" process is more pronounced in multi-beam systems, but that's completely compensated for in the design of the device (such as intentionally skewing the plate in the opposite direction on the drum to make it parallel to the imaging path).

Kevin.

 

[Drexus]

Just what I thought.

I only pose the question after examining a plate I was looking over. For what appeared to be a perfectly straight border, I saw a small step in the line. It must be something else causing the step on what is interpreted as a perfect square in postscript.

 

[KevinC@EFI]

Just what I thought.

I only pose the question after examining a plate I was looking over. For what appeared to be a perfectly straight border, I saw a small step in the line. It must be something else causing the step on what is interpreted as a perfect square in postscript.

Depending on what device/manufacturer you have, it certainly could be a software correction for geometry. I'm very surprised you could see a clear 1-pixel correction on a gausian device though.

For example, Kodak devices do this for several reasons:
1) to perfectly map the geometry of the drum to an "ideal standard" - so we can perfectly register plates from different CTP devices (a standard feature with our Quantum machines)

2) to adjust for temperature variations in prepress. Aluminum plates can expand 0.5mm for every 5C temperature change - and our CTP automatically corrects for that, so you don't have mis-registration on press if you remake a plate on a cold morning versus a hot afternoon.

Even if you can see a 1-pixel correction on the plate with a loupe, it won't be noticable on the printed sheet due to the printing process itself.

Kevin.

Kevin.