RONCHI  RIGS

Laser Diode

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Copyright – P. J. Smith – pjifl@bigpond.com.au

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


Theoretically, any variant of the Ronchi test may use a pinhole source.

 

If the collimating lens is removed from a Laser Diode pointer the visible source is an extremely small ‘pinhole’ which is attractive for optics testing.

 

 

The un-collimated Laser Diode is less dangerous than a collimated beam but it is still a potential hazard.

 

 

Make yourself familiar with the hazards of lasers, including uncollimated laser diodes.

 

Extreme far Red Laser Diodes should not be used because the eye is less sensitive and the risk of eye damage increases because the body response mechanism is fooled.

 

 

Fortunately the Laser Diode can be detuned to act as a miniature non-lasing Red Led by reducing the voltage to the diode.  Sometimes the feedback circuit may need to be disabled for this to work.

 

 

I would not normally recommend this test set-up for normal use but, since the source is so small, and the arrangement is so simple, it can fill some special needs.  If the laser diode is used in the lasing mode a lot of speckle results which is somewhat objectionable.  It also shows up grating imperfections such as scratches and surface roughness.  The illuminating cone from the laser diode source can be uneven, especially if it is used while lasing.  This is another reason to detune it so it becomes simply an extremely small LED source.

 

 

Personably, I believe better Ronchigrams result from the use of a slit in one of the more basic rigs. 

 

 

 

The Laser Diode is simply the source and the Grating used in front of the eye in the normal way.

 

 

No diffuser is used in front of the Laser Diode.


Example

Laser Diode as Pinhole with Edmonds 100 lp/inch Grating.

This isolated photograph was taken using a Laser Diode ‘pinhole’ source.  I did not bother with other photos, because I simply was not all that interested in this arrangement.  Speckle (not really visible here) is quite objectionable.

Personally, I do prefer using a slit source which seems to me to actually show more detail.  But when one considers that a pinhole source can be used with any variant of the Ronchi test and no alignment is ever necessary, it does have something to offer.

The downside is presence of speckle (unless successfully detuned) and the fact that the grating must be of excellent quality.  It cannot be used with woven gratings to produce a line Ronchigram so precludes using this common item that, in other set-ups, can deliver Ronchigrams of equal quality.

Laser diodes can be so small that one could be mounted on the edge of a 35 mm slide mount containing a grating.


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