Re: ATM RE: Ronchi and TDE

["Peter John Smith" <pjifl@bigpond.com.au>]

Poul Linnet wrote :-

>>>>
Be aware that the effect of Ronchi multiple images due to diffraction
depends on the lightsource used.
If you use a LED which is almost monocromatic you get a number of sharp
displaced  linesets,  but if you use an incandescent lamp with a continuous
spectrum you get more like one unsharp set of lines.
>>>>

Yes.  But you can still see this effect in white light if you look.

Some might prefer to use white light and it does work well in Ronchi
testing.  But by using a close to mono light it makes the situation more
amenable to analysis.  I prefer to NOT have one unsharp set of lines.  But
the fact that some use white light may explain why they have not noticed
multiple images.  If the source is weaker, they are also hard to see.


It is just so hard to talk of all this without photos on hand and I fear
many are getting wrong impressions and imagining things quite different.


A little more on the impact of diffraction effects on Ronchi.


The main Ronchi image is much brighter than any other overlapping ones.  So
if the image is duller they may not be visible.  But, on each side of the
0th order image are two 1st order images slightly displaced.  The main
reason they are most visible on left and right is because the overlapping is
from left to right and the edge is where the image drops into a black area.
Its all around the edge, but mainly on left and right.


I am beginning to think that some, whenever they see close packed lines on
left and right, simply dismiss them as diffraction lines.  When there is a
severe turned area towards the edge, it is possible to have many closely
packed Ronchi lines there as well as diffraction effects.

Good examples were the recent photographs from Jack Day at
http://www.avatm.com/ronchi.htm where the left and right areas are a
combination of actual edge ronchi lines and multiple imaging.This can be
easily confirmed by a geometrical analysis. In this case there should be
about 2 to 3 closely packed Ronchi bands on each edge.   Of coures, all of
this is more reason to keep away from this region and look at the band ends
maybe 1/3 distance out from the centreline.


 In some positions, the Ronchi bands and the multiple images due to
diffraction are overlaid with linear scales a simple multiple of each other.
Remember, the Ronchi bands spacing depends on grating position changes, but
the multiple imaging spacing is constant.  This explains why in some
positions weaker bands between the normal Ronchi fringes are visible and in
some positions the Ronchigram looks sharp, in others more confused.  This is
a manifestation of the Talbot effect. Using white light makes it less
visible.

The effect is visible in both the slit/grating and grating/grating mode of
operation. In many film made gratings, the light is scattered more by the
emulsion and the surface, which can never quite match polished glass or a
wire grating.  Printer made gratings are much worse.  If gratings which are
somewhat substandard are used in the grating/grating mode where light
traverses them twice the results can vary from passable to quite poor.


When a surface tests out very well and you look very carefully, the ends of
the ronchi bands are slightly streaked around the rim of the mirror.  Most
(but not all) of this is outwards towards the left on the left and the right
on the right.  It does not matter if you are inside or outside COC, the
streak is mainly outwards.  It is very slight and will only be of interest
to the optician chasing perfection.  For anyone with obvious imperfections,
just forget about this and concentrate on whipping the surface into order.


The photographs from Michael Spooner of a superb test result actually shows
this spurious effect very slightly.  I repeat - this slight streaking of the
very edges of the bands is a diffraction effect AND IS NOT A DEGRADATION OF
THE EDGE. It is mainly caused by the multiple imaging of the bands which is
more obvious at the edges where the background is black.

With digital cameras and people often increasing contrast afterwards, then
sitting down and examining the images in minute detail, the effect stands
out and more and more people will wonder if there is an edge problem.

By the way, the multiple image in Michael's photos is clearly visible in the
centre because of the central perforation - a situation not many ATM's
actually see.  It is obvious because in this region there is a 'black hole'
and the adjacent well illuminated area (triple imaged *) contrasts against
the dark background. On the left and right of the perforation, the edges are
multiple imaged displaced perpendicular with the edge so it shows.  At top
and bottom of the perforation, the displacement is parallel to the edge so
is not obvious.

Peter Smith.

*  Actually more than a triple image but the intensity decrease of the
others makes them almost impossible to see.