RONCHI SOURCES

The Scribed Slit

Copyright – P. J. Smith

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

The Scribed Slit

A very satisfactory slit for Ronchi Testing can be made by scribing a sharpened needle across freshly painted glass. There are some tricks, but once these are mastered, it is possible to make a large batch of reliable slits in a very short time.

This is a photograph of a typical slit produced this way. The width is close to 35 um across which is a good size unless very fine gratings are in use. It is quite possible to reduce this to 20 um by carefully honing the needle on 1200 wet and dry sandpaper on a flat surface such as a piece of glass. Much finer becomes difficult. No special care was taken to produce this slit in a batch of 10. Four were rejected and done again, the others were excellent. The paint may be slightly transparent but this does not matter much.

These slits take up no more than a few mm of room, and can be less than a few mm thick including the diffuser. The minimal size allows them to be placed almost on axis. See the Close in Brite for a very simple but successful implementation of this.

In my opinion, the metal blade slit is slightly better and is more permanent but is an unnecessary luxury.

Over time, these scribed slits become damaged. This has less impact on the Ronchi test than expected. But every so often (months or years) the slits must be redone.

Some of the tricks to obtaining good slits by this method are :-

1. The paint must be applied to a polished glass surface. It must never be ruled on ground glass as this makes for a ragged slit and blunts the needle immediately. Fine grinding the other side of the glass to act as a diffuser makes for more even illumination over a wide angle. See illustration A below.

2. The needle may be carefully sharpened by stroking and rotating on extremely fine (1200) wet and dry carborundum paper stretched on a flat surface such as glass. See illustration C below. This will

produces a slit of about 20 - 30 um wide. A blunter needle may be used to make slightly wider slits.

3. Only high gloss black paint should be used. It is best applied from a spray pack in a few light passes about 20 seconds apart. Do not worry if the paint is not quite opaque. See illustration B below.

4. It is crucial the paint must be allowed to partially dry before scribing. Some trial and error is needed. The correct consistency is 'almost tacky', although you will not be testing the surface which is ready for scribing for fear of damaging it.

5. The needle is best pushed forwards, point first, at a slight angle maybe 20 degrees from vertical. The needle seems to cut a cleaner slit this way. There is more tendency for the needle to tear the slit edges if it is dragged so the point is last. It is possible to achieve perfect slit edges without any tearing, but some experiment may be needed. See illustration D. The straight edge used must be tilted up slightly so it does not touch the paint except at the extreme edge.

6. Imperfect 'slits' are easily cleaned off with solvent for a new attempt.

A simple diffuser for the light is important to obtain even illumination over a wide angle. One way to achieve this is to place a small piece of glass, lightly ground on each surface, just behind the slit. 500 carborundum works well. A scribed slit can be lightly ground on the back, but the scribing must be done on a polished surface.

Microscope slides provide an ideal source of glass but small pieces cut from window or picture frame glass may be used. The glass should be perfectly polished and not have any frosted effect, which is sometimes found on picture frame glass.

One advantage of the scribed slit is that it can be placed within 1 mm of an edge. This has been done on the rig in illustration F where the line points to a very fine but long slit running the whole length of a microscope slide. More details will be found on this under the Brute.

The end result is seen at F. These small pieces of glass are easily Blue Tacked in place, which allows for some adjustment.

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