Possible light sources for Ronchi Testing.   Range from complex to simple readily available sources with comments on applicability and performance.


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.


[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.