Copyright – P. J.
Smith – pjifl@bigpond.com.au
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.
GOTO RONCHI INDEX