45° is always used for the black because of it's "harmony" with the normal horizontal and vertical edges of halftoned art. While even a rosette is a moiré pattern, this (45/90) angle is the least objectionable of all the moiré patterns. For the same reason that square-out halftone and 4/C pictures frame best at 45°, logo art edges will be MUCH more noticeable at 90°. There only has to be a minor slant in a line to create a horrible appearance.
My personal opinion is that God gave us a natural "level" with the horizontal placement of our eyes in our head. We are hyper-sensitive to things not straight- this is why we naturally want to straighten out crooked pictures on the wall. Putting black on anything but 45° may technically work out OK, but aesthetically, not so much.
Herb
I don't think that moiré is involved.
There are two main reasons that the halftone ancients decided that black should be angled at 45°.
First is a phenomenon called "visual integration of tone."
The human visual system is limited in the detail that it can resolve. At a certain size threshold you can no longer sense individual objects but instead
integrate tiny objects together into an average tone. Halftoning relies on this integration phenomenon which allows the viewer to average millions of halftone dots into areas of perceived tone. You don't see the dots - you see the average tone they represent.
There is a directional component to tonal integration. Within an area of tone, fine diagonal patterns are perceived slightly less well than horizontal or vertical patterns. This may be because in nature, most of the significant visual edges are horizontal or vertical - trees, roads, the horizon - and features of interest tend to interrupt these edges. Bottom line, halftone screens look smoother at 45°, where they are less apparent than at other angles - especially 0°.
The second reason is what happens if the graphic does not allow for a complete dot to be formed at the edges of the image. Here's an example:
The upper graphic is screened at 0° and the lower is screened at 45°
Note the micro dots on the top and left side of the upper graphic. They will have two possible impacts. 1) they may make the graphic appear smaller that it should be. 2) Due to their small size they may not print which would make the graphic smaller than it should be.
However the bottom graphic where the screen is at 45° does not have this problem. Also, if there were micro dots around the edges they would be averaged out when lost while there would still be large dots at the perimeter to maintain the size integrity of the graphic.
BTW, If you walk a few feet away from your display and look at the above graphic you'll probably see the phenomenon of directional visual integration of tone. I.e. the pattern in the lower graphic should look smoother than the top one.
best, gordon p
