Re: Pellicle mirros

[dmorton@journalist.co.uk (David Morton)]

Mark Bishop writes:
> I have a suggestion, which I've been thinking about for a long time. As
> I
> understand it, a pellicle 'mirror' does not flip up during the exposure
> but
> remains in place - hence the continuous image and very fast motordrive
> possibility. However it reduces the amount of light available to the
> film and
> finder, so the finder can be fairly dim and available-light photography
> is
> disadvantaged.
> My idea is that someone should build a camera where the mirror is in
> fact a
> prism which gains its reflectivity from a silvered layer which is made
> of some
> kind of LCD. That way, there's no need for a shutter or a mirror
> movement
> (smaller, quieter, lighter, cheaper cameras...), the delay between
> pressing the
> button and the 'shutter' opening is reduced and the photographer is
> without a
> viewfinder image for only the length of an exposure (and remember,
> persistence
> of vision means that an interruption of viewing of less than about
> 1/20sec is
> not visibly descernible). Also, unlike a conventional pellicle, there's
> no
> impact on viewfinder illumination.
> Does anyone with a more scientific background than me know if something
> like
> this is feasible?

It's been years since I worked in this field, but the problems are the
speed and the dynamic range.

You can have quick LCD light valves, and you can have low voltage/low
power ones, but there's no easy way to have a fast low power version.

The dynamic range is the killer, though: a focal plane shutter is really
jolly *jolly* good at not cutting down the light when it's open, and not
letting light leak through when it's closed. It would be quite difficult
(slight understatement) to achieve anything close to this ON/OFF ratio
with any imaginable light valve, LCD based or otherwise.

So until you can achieve an almost impossible dynamic range, you still
need to back the LCD up with another mechanical shutter (and if you're to
gain any significant benefit from the LCD, in terms of response time, then
that mechanical shutter has to be controlled by a mechanism with built-in
precognition :-} ), and so you may as well just use the mechanical shutter
and forget the LCD.

This isn't one of those inventions which hasn't happened because no-one's
thought about it, indeed it's something of a 'holy grail' for optical
designers. It hasn't happened because it's really rather difficult to do
better than whirring wheels and moving bits of cloth (or titanium). Of
course this may upset or comfort you, according to taste :-)