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Re: filmscanners: LED Illumination for Film Scanners

At 08:00 AM 6/21/2001 -0400, rafe b wrote:
>At 11:18 PM 6/20/01 -0400, Isaac Crawford wrote:
> >Well halogen bulbs have the same uneveness problems as regular bulbs,
> >but with the use of a mixing chamber, they can give the same results a
> >"cold" head can... I can see some potential advantages in an enlarger
> >with LEDs in this arangement, primarly concerning heat. I'm not sure
> >what sort of advantage LEDs would have in a scanner, maybe long life
> >with minimal shift (light wise)?
>The *problem* I see with cold-cathode and fluorescents is that
>they can flicker.  I'm not exactly sure why this happens.  Is
>it a power-supply problem, or is it a property of the bulb, or
>a combination of the two?
>I do know that fluorescents are based on plasma physics --
>essentially, a low-pressure mercury vapor is made to conduct
>current and emit photons as a result.  One can easily imagine
>local variations in the plasma (and very dynamic ones, at
>that!) which cause non-uniformities in the light output.
>In fact, you can do more than imagine it... most of us have
>seen it occur.  Plasmas are incredibly beautiful and complex
>There is no comparable issue with tungsten lighting or LED
>lighting that I'm aware of -- in these cases, dynamic variations
>are nearly always attributable to the power supply.  With
>tungsten lighting you have thermal time constants that slow
>down the dynamics quite a bit.

Fluorescents flicker at the AC line frequency--60 Hz in the US. This is 
because, as you say, the fluorescent light is a plasma device. The 
discharge turns on and off at the line frequency. It is not a continuous 
discharge (either in time or in wavelength). The UV from the mercury vapor 
discharge tube causes the inner coating of fluorescent material to, er, 
fluoresce. The composition of the coating determines what wavelengths (I 
would normally use "frequency" but I don't want to confuse it with "AC line 
frequency") will be emitted by the tube.

Photography by Stan McQueen: http://www.smcqueen.com


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