The figure below shows the physical arrangement
used to produce a light sheet from a 1kW haelogen photograph lamp.
Key features are:
- A spherical lens on the front of the case to focus either
the lamp filament or the adjustable slide (less light but more
uniform than the filament). There is a trade off between the focal
length and diameter of the lens. The larger the diameter and shorter
the focal length, the more light you will get into the sheet.
Our lenses are approx 100mm diameter with a focal length of about
- Cooling fan. The 1kW input into the compact size of the photographic
lamp means that if it is to be turned on for extended periods
(more than about five minutes), forced cooling is required to
- Linear filament 1kW photographic lamp. These lamps are much
more intense and have a much higher colour temperature than the
type used in normal flood lamps. Physically they are somewhat
more compact. Note these lamps must be mounted horizontally.
If you mount them vertically the neighbouring coils in the filament
will short, decreasing the resistance and ultimately burning out
the lamp. The lamp holders are available through, for example,
- Adjustable slide. The adjustable slide may be used to decrease
the numerical aperture of the lamp and thus increase the in-focus
depth of field of the sheet. The cost of shutting down the aperture
of the slide is, of course, a reduced light level. We typically
use a 2 to 4mm aperture.
- Baffle. The baffle is designed to both prevent excess light
leaving through the ventilation holes. Ideally a cylindrical baffle
with its axis close to the filament would be best, but at the
danger of overheating the light. In practice it may be better
to have the baffle reflect the light away from the slide
we do not want light from far from the filament otherwise the
depth of field will be reduced. In line with this, the baffle
should probably be black and perhaps slightly convex.
- Focus adjustment. It is necessary to be able to adjust the
position of the lamp and slide relative to the lens to focus the
light sheet. We have found it easiest to mount the baffle, lamp
and adjustable slide off the support and have the position of
the support adjustable relative to the lens.
- Case. Our original ones were 5 1/4" BBC disk drive cases,
but these are no longer available. The latest one was folded out
of aluminium sheet and electro-phoretic painted (black) both inside
and out. This is significantly better than the earlier versions.
- Safety. I strongly recommend fitting the lamp with a thermal
cutout. For ours, we have a 150°C thermal switch fitted to
the bottom of the case near the rear end. The original versions
also had a thermal switch to turn on the fan and keep it on until
the lamp had cooled, but we found it was a bit too slow at turning
the fan on. The mains switch is also near the rear end of the
case to prevent it getting too hot.
Figure: Sketch of lamp elevation (top) and plan
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