RONCHI  SOURCES 

Pinholes

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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.

Point Sources

The Point source requires no alignment and may be used with any variant of the Ronchi test. 

This sounds ideal. 

Despite this, a good point source is much harder to make easily than other types and it is more difficult to align the test to the much weaker returning rays.

For some special applications, some of the smaller point source arrangements fulfil a special need.

Below are some types and ideas if you wish to use a point source.

Despite the apparent simplicity of a Pinhole set-up I strongly recommend, especially for beginners, you regard it as the least desirable option for Ronchi testing and use in preference a slit, which is more forgiving.

LED or Bulb with Pinhole

One favourite is a LED with the front lens sanded off.  The frosted remaining plastic helps with diffusion and a pinhole can be placed very close to the small emitter in the LED.  It is cool, requires little energy, and is small enough to be placed near axis without obstructing the eye.  It is simple.  Despite this, perfectly even illumination is difficult to achieve. 

Small incandescent bulbs also work and if a LED is unavailable will be quite adequate. 

Using another lens to focus the diverging rays from the light source towards the pinhole ensures more even illumination.  A microscope objective is great for this and is often used in professional equipment.

Another point source is obtained by shining a light beam onto a small spherical reflector such as a polished ball bearing.

 

LEDs of 5 and 10 mm diameter are readily available.  5 mm is usually sufficient and is easier to mount close to the axis.  It is probably easier to align the larger one with the pinhole.

It is better to grind off the collimating portion of the LED so the small chip source can be placed closer to the pinhole.  Leave this a ground finish so a separate diffuser is not needed.

The Viewing Grating is of course placed close alongside the source.  No alignment of Grating is required but the return beam is often harder to locate compared to a slit or grating source..

No examples of test rigs using pinholes are given but it would be easy to convert the Basic Rig or Deluxe Rig.  In a similar way, a lash up using modelling clay to hold pinhole assembly and grating is easy.

 

Remote light source

It may be convenient to locate the luminous device in a remote position and channel the light to a pinhole, slit, or grating source.  Advantages are :-

1.      Keeps heat remote from the optical path.

2.      Obstructs light less.

 

This may be implemented by :-

(a)    A beam of light from, for example, a slide projector.  This can be placed to one side and the beam aimed at a small mirror or prism or spherical reflector such as a polished ball bearing.  A more modern variation is to use a laser beam.

 

(b)   A glass rod or sheet that constrains the light by total internal reflection, leading it to a small pinhole or slit.  A rod of glass may be truncated to increase the throughput. An example of a device using a microscope slide to channel light is given under The Brute although it actually uses a slit.

 

(c)    An optic fibre whose end may act as a pinhole.  The end should be well finished for even illumination.  A variation is to form a small ball lens on its end by fusion.  This gives an even, wide cone of light. It is difficult to couple a light source to the optic fibre without much loss of light. One makeshift way is to drill a tiny hole in the front of a LED,  insert the fibre, and bond in place with some transparent glue. By the time the light has traversed the length of the fibre, any uneven illumination has usually been smoothed out.   The end should be polished flat or a minute bead formed on the end to provide an evenly illuminated cone of light.  This is harder than it sounds but once done the result is a fine point source.  A laser is not needed but is very convenient and probably easier and cheaper to obtain than a good narrow collimated beam of white light these days.

 

The Laser Diode

 

The modern Laser Diode without a collimating lens provides a possible ‘pinhole’ source.  See Laser Diode.

Some Examples

 

                        

In both of these examples a Laser beam shines on a small steel ball.  The ball should be polished to ensure an even spread of light emanating from the virtual pinhole.  The laser beam can simply shine through the centre of the glass in the primary since it need not be free of aberrations until after it is reflected from the steel ball.

 

 

 

Here two different features are shown.  The laser beam can be off axis and the testing telescope in this case may be less than full diameter as long as we assume a rotationally symmetric test surface.

 

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