Re: 4x5 Light panels

[Richard Rylander <rlrylander@xxxxxxx>]

Robert Thorpe wrote:

> One thing that concerns me about any of these light panels is their
> size. Even the 4x5 models are an inch taller than a slide, minimum.
> It's even worse when you count in the frame of the device. You start
> with a relatively graceful viewer then on the back end you have this
> comparitively huge backplate. Is it just me or wouldn't many of you
> prefer a couple of bright halogen viewer bulbs and sufficient diffusing?
> It would be one thing if none of us ever built anything but I think
> just about all of built our own viewers. We are not intimidated by
> assembling a number of parts.

My sentiments, exactly.  Rather than buy a commercial light panel, then cut
it apart, I made my own Saturn viewer backlight from raw components (Cold
Cathode Fluorescent Light and inverter - available from Digi-Key
[www.digikey.com] at $10 for the bulb and $15 for the inverter).

The particular CCFL I used is manufactured by JKL (model BF5118 – 5.6mm dia
x 118mm long, 5000K color temperature).  I used tab-and-slot mahogany
construction to match the viewer which the backlight slips onto between the
existing semicircle "wings" (essentially adding no real bulk to the original
viewer).

Very uniform illumination is produced by cutting an acrylic mirror to form a
trapezoidal reflector/diffuser arrangement with a narrow apex mirror
parallel to the diffuser and in contact with the rear surface of the bulb.
The (top-to-bottom) middle of the diffuser sees just the front surface of
the bulb, while moving toward the top or bottom sees more of the reflected
rear bulb surface (kind of a modified dipole radiation pattern with more
light coming out the sides than the ends due to the bulb itself being
opaque.  The approximately compensates for the increasing distance of the
bulb from the center of the diffuser (also, the light should fall off as 1/r
instead of 1/r^2 since it is closer to a line than point source).  A piece
of matte vellum is added to the existing acrylic diffuser to increase the
light integration within this chamber.  A piece of UV-absorbing acrylic was
also added for eye safety.

The backlight weighs just over 200 grams (with four internal AA batteries –
or an AC adapter can be plugged in), added to the 300 gram viewer makes a
very hand-holdable package (just under 18 oz.).  The lamp/reflector assembly
lifts out to change batteries (which last 5-6 hrs.).  If you don't mind
spending a little time tinkering, a relatively cheap (compared to commercial
light panels) backlight can be made that matches the viewer cosmetically,
and produces bright, uniform illumination.
Richard Rylander