RONCHI

Scanning the Surface

How scanning the grating from left to right sees past the ‘blind spot’ and creates greater sensitivity.

Copyright – P. J. Smith

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

General

Details are given in advanced interpretation on the ‘Blind Spot’.

One answer to this limitation is to use a scanning technique.

Many people today have only seen Ronchigrams in books or images from CCD cameras. They lack the experience to know that much more information is available from watching a scan that slowly sweeps the mirror from side to side. It is worth pointing out that many advanced interferometric testing systems use an equivalent technique. It usually goes under a fancy name such as phase stepping.

In the case of the Ronchi test, the information is enhanced because :-

1. The sweep passes over the central blind spot thus making it visible.

2. Small changes in pattern are more visible to the eye than static images.

3. Small changes in velocity of the test image result from a constant speed scan. Again, the eye is extremely sensitive to these variations.

This sweep should always be performed. It is especially useful when assessing surface smoothness and local dimples and pimples.

In very primitive rigs this may usually be done by simply pressing down on one side of the base of the test rig. This gives surprising control to both the sweep amount and velocity.

Most people are quite stunned at the extra detail shown by this simple technique.

It is possible to sweep an image when using a CCD or Video camera but in my opinion the image lacks a constancy of motion due to the changing frames. This is one reason that I actually prefer to view a Ronchi image by eye.

Examples

No static photographs can do justice to the detail available in a controlled scan but the following give some idea of what may be expected. Unfortunately, most of these examples are a little unusual but the general idea is well illustrated.

Both of these examples use a phase grating which accounts for the more delicate tones. Any other grating works in a similar way. The main feature to note is how detail, especially in the middle, is more obvious in some of the Ronchigrams but invisible in others. This is especially useful when the grating is drawn close to Centre of Curvature giving fewer bands. The effect is more obvious below.

A variation is to use a single wire when scanning as below.

Don’t forget that at one position a Ronchi Grating will act as a knife-edge

Of course a true knife edge can be used but any grating, especially one formed of threads does produce similar results.

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