Re: One more thing about light boxes.

[Chris Albertson <chrisalbertson90278@yahoo.com>]

Tom,

Why not use a strobe light?  Just a normal flash like
on a 35 mm camera.  These have closely controlled color
temperature (same as sunlight) and the good ones have
repeatable light output that is adjustable over four
to six powers of two.  There is no warm up period and
you trigger them by closing a contact.   You do not
have to worry about it aging.  If the strobe is to
bright you can get ND gel filters from B&H Photo for
a few bucks

Best of all the flash provides 100% shutter efficiency.
This gets around the problem of the Mk IV's slow moving
shutter.

All of the methods of making even illumination depend on mixer boxes.  The best
shape is a sphere because it lacks corners
and edges.  

Tom Droege wrote:
> 
> You can't dim a tungsten lamp.  You have to run it at the right temperature
> to get something that has the right spectrum.  Now you have something that
> ages. Since the filament wears over time, it is a big project to adjust the
> illumination.  You have to do it with neutral density filters.  These have
> problems of their own.  Even so, you can't count on the spectrum being
> constant by just adjusting the brightness.  You really have to take a
> spectrum.  The whole thing gets really complicated at the 0.001 level where
> we want to work.
> 
> I think it may give better results to use several LEDs of different colors
> and pulse them.  These can be made pretty well behaved.  I like to drive
> them with 1-2 ns 100 volt pulses and put an ampere or so through
> them.  This makes a pretty uniform pulse.  Many of the problems you have
> with getting uniform light out of an LED go away when you pulse them with
> an avalanche transistor.  There are temperature effects and the like that
> are hard to control at low voltage.  Now the brightness can be adjusted by
> how many pulses one puts in the 100 second open time.  One can easily get a
> million to one dynamic range.
> 
> I have tested PM tubes this way at Fermilab, and got very nice linearity
> curves over 6 decades.  I was really testing that my electronics was
> linear.  One does this by running the PM tube into a cascode amplifier for
> the fans of such things.
> 
> This scheme allows one to make nice linearity curves.   Even Andrew has not
> yet asked for linearity data on the CCD.  We probably need to check
> linearity as we get near full well.  We could get it this way.
> 
> As you all can see, this is a topic with far more discussion than action.
> 
> Tom Droege

-- 

--
   Chris Albertson             
   Redondo Beach, California
   home:   310-376-1029   chrisalbertson90278@yahoo.com
   cell:   310-990-7550
   office: 310-336-5189   Christopher.J.Albertson@aero.org