Possible light sources for Ronchi Testing. Range from complex to simple readily available sources with
comments on applicability and performance.
GOTO RONCHI INDEX
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 source always
consists of
A
luminous device.
A
method to ensure even illumination
A
spatial filter.
The
Luminous Device
The light used need not be
monochromatic unless dispersive materials (e.g. a refractor objective) are
under test. If the Ronchi test is visual,
low powered lights are adequate. For
taking photographs you may need more power. The light source should waste
little light, yet illuminate the spatial filter evenly.
Possible sources of
illumination are :-
(a) A gas laser producing a collimated
beam.
(b) A diode laser which has an
incredibly small point source that may be used directly as a point source.
(c) An incandescent bulb. Modern variants include flashlight bulbs,
quartz iodide bulbs, and small grain sized bulbs. Some penlight bulbs came with
a built in ‘collimating’ lens.
(d) A bright LED. These may replace incandescent bulbs in the
future and may have the advantage of being closer to monochromatic which is
important in some refractive tests.
They are small, convenient, and produce little excess heat
(e) A candle or acetylene flame. While
of historical interest, these do work quite well and give a well- diffused even
coverage.
For normal Ronchi testing, don’t bother with lasers
Lasers may be used if
available but they are not necessary and have problems. Most noticeable is the inherent speckle
which I find obtrusive and almost impossible to be rid of. Unless you have a laser set up as a
precision source already, do not bother with one. There is some peace of mind in not using a laser, which will be
dangerous to inexperienced users. For
reflective tests there is no requirement the light source be monochromatic[1],
and it is actually better if not coherent. An interesting variation is to use a
small Laser diode as a pinhole source, with the voltage supply turned down so
it is not lasing. This makes it safer
and eliminates speckle associated with laser sources. The geometry is such that
the emitting element is a small wafer edge on so this is equivalent to a very
small pinhole. It sounds almost ideal
but I far prefer a common garden variety slit.
When I tried turning down the voltage, brightness decreased but there
was still speckle indicating the diode was still lasing.
A point source is ideal for
illuminating a pinhole, a line source a slit and a collimated beam a multi-slit
spatial filter.
When using a slit or
multi-slit source, the linear dimensions of the spatial filter increase to
cover a sizeable area. The illumination
should cover this area evenly to reap the advantages of these types of sources
such as greatly increased light throughput and averaging out of source and
grating imperfections.
A
Remote source of light may also be used
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.
(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 source.
Even
illumination – the DIFFUSER
The Ronchi test is not as
sensitive to uneven illumination as the Foucault test (especially the Zonal
Foucault Test), but it does have some impact.
Certainly, an insufficiently wide beam will not illuminate all of the
surface under test.
Some method must ensure
even illumination over the angular span of the test beam.
The simplest, although not
necessarily the most efficient, is a simple diffuser made of frosted glass.
Some have used a few layers
of frosted sticky tape in this role but we can do much better. A few layers of tracing paper also work.
For a simple way to make a
very satisfactory diffusing screen suitable for most of the test rigs
described, see Diffuser.
When light passes through a
fine optic fibre (see Pinholes) or is
reflected from white paper, it is often
sufficiently conditioned.
Note that, when using
a Laser Diode direct pinhole source, NO diffuser must be used.
Spatial
Filters
The spatial filter used in
Ronchi testing depends on the test variation.
It may be either :-
1.
A
pinhole. This gives results with every type of Ronchi
test. For a variety of reasons this is
the least recommended source, especially for a beginner. A quality pinhole source giving even
illumination is much harder to make than a slit. Of course, if a precision pinhole source is available, there is
no reason not to use it. But be warned
that the Grating quality to work well from a pinhole must be excellent. There are some variants of the Ronchi test,
which ONLY work with a pinhole such as the Mobsby and Circular grating, and 2
dimensional tests. A pinhole source is sometimes conveniently supplied by
(a) direct emission from a laser diode
without any collimating lens.
(b) being piped in via a single fibre
optic strand, whose end acts as the pinhole.
Sometimes a small bead is formed by fusing a portion of the end. This ensures even illumination over a wide
angle.
(c) a collimated beam of light
reflecting from a small bead or polished ball bearing. The beam is typically provided by a laser
but a focused beam of light works quite well.
Even sunlight on a small steel ball works quite well.
For more details see Pinhole
sources
2.
A
slit. This is an excellent choice, especially when
the grating is inferior. It averages
out the grating defects to such an extent that quite poor gratings give
excellent results. It also allows woven
gratings to be used, completely hiding the horizontal lines, giving excellent
results.
For
more details see Slit/Grating model
3.
A
multi-slit
which is usually implemented by
(a) passing the light through a part of
the same grating used as the occulter[2]
or
(b) placing the grating at prime focus
and projecting the emerging light from the optics under test to a distant
screen, or by viewing the image by looking into the exit pupil of the test
optics from a distance. [3]
For more details see Grating/Grating mode.
4.
A
2 dimensional
grid which is usually placed at the prime focus and used as in case 3b above as
in the Lower variation.
All have special
application, advantages, and disadvantages.
GOTO RONCHI
INDEX
[1] I find a near monochromatic source such as a red LED more distinctive in the presence of ambient room illumination which is very convenient. Note that many LED’s such as yellow are actually a mix of distinct colours. Red and good blue LED’s are usually close to monochromatic which is handy when testing refractive surfaces.
[2] This is possible in some situations such as testing a concave surface at its center of curvature. In most refractive situations it is impossible unless a nulling plane mirror is used so the test is in autocollimation mode.
[3] This is often called the ‘Lower’ test. It has been especially useful when figuring searchlight mirrors and Schmidt corrector plates.