Re: filmscanners: LED Illumination for Film Scanners

[rafeb <rafeb@channel1.com>]

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 
phenomena.

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.



rafe b.