RONCHI 

Grating/Grating mode

V

Slit/Grating

Comparison of performance of The Grating/Grating compared to the Slit/Grating when used with different Grating materials.

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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

Some ATM’s seem afflicted with a rigid view that one must use exactly the same set-up as represented by the current fad.  Ronchi testing has successfully used a variety of set-ups over its 80 years of existence and all are sensible candidates.  The decision on what set-up to use should be made on availability of components and suitability to your needs – not by blindly following some suggested system.

The Grating/Grating mode owes its popularity to a simple set-up in a community where quality gratings are readily available.  Ironically, most seem to have forgotten that other systems also work well, are also easy to set-up, and may be more appropriate to other situations and other communities.

I hope the following photographs give a clear understanding of performance characteristics of the Grating/Grating and Slit/Grating modes of operation. 

Unfortunately the photos are not ‘standardized’ in the sense that they were taken of different mirrors, with  different grating periods, and different number of lines present.  In fact the exposures MUST be different because the slit/grating allows less light throughput.

I would like to repeat these with better camera equipment but I simply have no time left to do this.  Also, some of the mirrors now don’t exist.

In each case comparisons may be made between the same or very similar surfaces viewed with both the grating/grating and slit/grating mode.

 

The main conclusions are :-

 

·        Grating/Grating mode gives wider, more diffused lines but visible detail is similar with a good quality grating.

 

·        Slit/Grating mode gives acceptable results from mediocre gratings.  This is emphatically illustrated under Woven Gratings,

 

The conclusion is immediately obvious.  If you want to use a Grating/Grating set-up, the grating must be excellent.  But, with a lower quality grating, the Slit/Grating mode is superior.

Explanation

Ruling accuracy

In the Grating/Grating mode, the operation depends on the image of the grating being exactly mapped onto itself in a perfect Ronchi test.  If not, the Ronchigram is afflicted with considerable leakage which the Ronchigram.

Anyone who has followed Grating manufacture will know that maintaining constant line spacing is very difficult, especially over a large span.  Thus, any but the very best gratings give degraded images.

A grating pattern produced on film may look contrasty, clean, and evenly spaced.  But it is common to image one part of the grating onto another portion maybe 20 mm away.  Maintaining constant spacing over a long span is the hardest thing to achieve so some degradation often results.   The distortion of the lens producing the grating is one culprit and the linearity of a laser or inkjet printer over a span of 20 mm is just not good enough for best results.  Used with a Slit, however, the results are excellent.

 

Substrate quality

Every time light traverses the substrate it must pass two surfaces plus the thickness of the material.  Any surface defect causes some scattering of the light.

In the Grating/Grating mode 4 surfaces must be traversed which causes far more scattering so we would expect optically polished glass to be far superior to a gelatine or plastic surface.  See the Inkjet grating photographs above.  With use and exposure to the atmosphere some surfaces degrade noticeably over time.  Glass is very simply the best material.

Of course, gratings made of strands are free of this defect.

It is easy to prove that the substrate causes degradation.  Consider the following photographs.

The left hand Ronchigram results from a slit and wire grating.

When 1 and 2 pieces of blank Litho film are inserted in the light path it is easy to see the resultant degradation.

 

The conclusion is obvious.  Any grating made on inferior substrate is best used in conjunction with a Slit.

 

Slit Width

 

Another factor which should be considered when comparing performance of Grating/Grating and Slit/Grating produced Ronchigrams is the possibility of varying the width of the slit.  Special gratings have been made width different ratios of light to dark ( see  Non Linear Gratings ) but a somewhat similar effect may be achieved by varying the slit width.

 

I experimented with a quality variable spectroscope slit but its sheer size is such that it is difficult to test close to the axis.  In my opinion a variable width slit is a pleasant luxury, but it is not necessary.

 

There is more information on the effect of slit width under Diffraction.

 

Unfortunately, some variation in these photographs is due to different exposures, but the effect of varying the width of the slit is still quite obvious. 

 

100 lp/inch using different slit widths

 

 

80 lp/inch using a wire grating

 

 

150 lp/inch using a phase grating

 

If you choose to use a slit, the most practical quick solution is, in my opinion, to choose a scribed slit.  It is easy to scribe a narrower slit.  In my opinion a slit somewhat narrower than the grating spacing is best but there is little advantage is an extremely narrow one.

 

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