It is here necessary to respect a rather simple rule :

Whatever are your halftone and your fabrics, the relation between the number of threads of the tensioned fabric and the number of lines of the halftone ruling should never be even numbers, or even relation in an even decimal.

This rule is formal: it is absolutely necessary to avoid even relations of 2, 4, 6... Or especially 2,6 - 2,8 - 4,2 - 4,4 - 4,6 - 4,8, etc. even relations of 3,2 - 3,6... Are not very good, because they are " perfect multiples ".

So, relations as 3,5 - 3,7 - 3,9 - 4,1... uneven at the first decimal are good a priori.

But, by determining them, it is necessary to take into account the " coefficient of elongation" of the tensioned fabric: the number of threads / cm can be very significantly modified by the tension: in theory, before tension, the relation was good; after tension, it can become bad... So take care!

Joël Garnaud's SerilorLog allows a very good approach of this problem, at least up to the halftone of 150 lines / inch (60 / cm) :
- Thanks to " Mesh Calc " one can measure at one thread near, the real number of threads of the screen after tension,

First rule, you do not too much trust in the purely mathematical and theoretical calculations, nor in the advices of fabric " pre--angulation " given by some manufacturers of fabrics, films or emulsions...

The tension of the fabric distorts completely these figures and it is much more important that one imagines himself it, this problem is absolutely crucial.

Please , remind yourselves that you should determine your own criteria for halftone printing - 120 parameters to be mastered in fine line halftone -. But it is not the object of this conference: in moiré also, it is necessary to define those of these parameters which are implied and they are numerous!

How to proceed?

It is necessary to reduce the number of variables - so to simplify the problem of the elimination of the moiré among others - and it is one of the reasons for which I work with only two fabrics (semi-calendered because I print only in U.V.): the 180 T and the 140 T

As it is not all the same necessary to throw at once to the basket all the mathematical data - they allow to rough-hew to make an approximation to the range - the problem.

It can be much worse when more or less respected relations in enlargements give incredible variations in the number of lines of enlarged films (projection on film) or direct projection on the screen of course) : I mean you can have 22, or 34 or 47, etc.. lines / inch as a final result… Or absolutely anything else of course !

In these systems, by combining them, one can limit the problems - it is the principle of Moiré Guard - which allows to approach the good combination fabric / halftone ruling.

The Angle Calc will allow, in the same order of idea to see if the pre--angulation chosen of some fabric on the screen (for an automatic press) was well respected.