do the screen angles of 2 trapping spot colors have to be different?

[lanctk]

I am having a debate with a co-worker. I was under the impression that when you have a 2 colour job, say Black and Reflex Blue, and there are greyscale images touching a screened blue background that the screen angles need to be different. She says no because they are only trapping, not a duotone. I have searched extensively online and can find nothing to support either arguement. I have attached a screen cap. Help!
Thanks,
Kim

 

[dgerharter]

No, not really

No, not really

Lanctk-

I don't mean to be condescending, but for the benefit of the entire readership - the reason for differing screen angles in two colors combining into a duotone is to create a rosette, avoid moire and color shifts due to fluctuations in register. Because the only area actually overlapping between the 2 colors in question is the trap thickness (and thus subject to the above mentioned phenomena), the need for differing screen angles is dependent on the trap thickness. If your traps are wide enough to actually see a difference in color or a moire with a slight register difference, you would want to use a complementary set of angles. Since that's not normally the case in offset printing, I would say you can safely run both at the same angle without issue.

 

[gordo]

She is correct. No need to change the angles because the trapping area is too small to show the moiré which results from halftone screen angle conflict.

gordo

 

[lanctk]

dgerharter:
Yes, I am aware of the reason for the differing screen angles, I know all about moire and rosette patterns. The reason I was asking was to confirm whether or not having the trap area at the same angle would create a noticable moire or not. Thanks.

 

[lanctk]

Gordo:
Thanks for your reply. Good to know, I stand corrected. Will pass it on.