RONCHI

Phase Gratings

Production and use of Phase gratings for Ronchi Testing.

Copyright – P. J. Smith

But permission is given to distribute this material in unaltered form as long as it is not sold for profit.

A phase grating retards instead of blocks selected rays.

This has the following potential advantages :-

· Because all of the light is transmitted, the image is usually brighter.

· Many of the phase related tests (e.g as developed by Zernike, Burch and others) are more sensitive. It would seem this is also true of phase gratings used in Ronchi testing.

Methods that have been used to create selective path difference in phase tests are

1. Selective etching of a glass surface via a pattern of resist. Raleigh did something similar.

2. Deposition of thin layers in a selected pattern. Zernike originally used thin squashed specs or whiskers of rosin. A more modern technique is to deposit layers through a mask as is done in electronic chip manufacture.

3. Chemically altering a portion of the gelatine layer in film. This is routinely done when recording holograms on film. Special films have been developed but any film works to some extent. In essence, the silver image is bleached away leaving some gelatine transparent but slightly altered optically.

At face value these techniques are outside an ATM’s reach but there are some ways around the problem.

Unfortunately, the available phase gratings may have little advantage over other readily available alternatives. The results are, however, interesting, and in some cases actually superior.

The first phase grating I ever used was a transparency printout by an inkjet printer. On detailed microscopic examination of the inkjet image it was obvious it would never work very well in normal Ronchi testing. But it does in fact work far better than expected. The Ronchi image also has an Ethereal quality peculiar to phase gratings. Ronchi himself produced some of the first gratings of this type.

There is extra material including microphotographs of inkjet gratings which you should examine under Inkjet and Laser produced gratings.

The next step in this interesting saga occurred when I accidently turned an Inkjet grating 90° and was confronted by an excellent Ronchi image. What was going on ??

It turned out that some types of transparent materials for Inkjet printers was manufactured by a process that left many small striations on the surface. This was acting as a phase grating.

At one time I had seen in Applied Optics a reference to using a Phase grating made by using a standard piece of cellulose sticky tape for Ronchi testing. Apparently the manufacturing process left small closely spaced striations either on the surface or the glue layer which selectively retards light. In retrospect, I can remember this product well. Newer tape seems to be improved so it is smooth, thus it is useless for this application.

I had dismissed the tape idea at the time as a useless curio. In retrospect, it was probably quite a workable process. Anyway, it confirmed what was taking place.

When the Inkjet transparency material was viewed by oblique reflection the striations were easy to see. Under a microscope the situation changes drastically and they may become almost invisible.

Special illumination shows the phase variations from surface striations.

Normal microscope illumination simple passes through the grating without showing the phase strips.

The micrograph on the right makes the phase strips easily visible by a combination of

· slit source illuminating the substage

· defocusing the microscope

If the same technique is used on an Inkjet grating image it becomes obvious that the layer of ink, even if it is largely transparent, acts as a phase retarder.

Special illumination shows phase retardation from Inkjet ink.

It would seem to me that an Inkjet printed grating is a combination of a phase and an amplitude grating depending on the opacity of the ink. The Ronchigrams from it have the Ethereal quality typical of phase gratings.

Below are Ronchigrams using blank Inkjet transparency material aligned with various width slit sources. The period of this material is close to 150 lp/inch.

Blank commercial Inkjet transparency Phase grating material with Slit.

It is obvious that this technique has much to offer if satisfactory gratings are available.

Unfortunately, the material used to make these photographs seems to no longer be available. All of the newer transparency material intended for Inkjet transparencies have a stipple finish instead of the necessary surface striations. I do have some of the original material left over but it is easily damaged by handling and deteriorates quickly when exposed to humidity and dust. This would not matter if it were cheap and common as was the original material.

Maybe some other common type of film will become available to take its place. I keep looking at all types of films, tapes and membranes and have found no replacement.

Bleached Photographic Film Gratings.

Routine photographic methods can also be used to make Phase gratings.

The Ronchigram below is of an almost perfect sphere taken with a grating made from Kodalith film, processed in D9 developer, and subsequently bleached with Farmers reducer. The grating image on the film, being transparent, is invisible to the naked eye.

Bleached Film Grating is about 200 lp/inch

While there would seem to be no compelling reason to make gratings of this type, the above do show that it is quite feasible. Of course, amplitude gratings made in a similar way but left unbleached also work well. The main advantage of the Phase grating is greater light throughput which may or may not be important.

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