Re: filmscanners: LED Illumination for Film Scanners

[Raphael Bustin <rafeb@channel1.com>]

On Thu, 21 Jun 2001, Stan McQueen wrote:

> 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.


Stan, I was thinking of more pathological behavior.

A healthy bulb, with a healthy, regulated power 
supply, is not really my concern here.  I understand 
that household flourescents are driven by 60 Hz, 
(in the USA) but I also know that the bulbs inside some 
film scanners are driven at much higher frequencies, 
and those frequencies are not well controlled.

But I have seen, on household flourescent lamps, 
situations where the plasma seems to be moving 
about and varying in intensity.  It's generally 
when either the bulb or its "ballast" is near 
the end of its life.

And I've seen banding effects in my older 
scanners (both the Microtek and the Polaroid) 
which could only be explained by time-variant 
spatial non-uniformity of the lamp's brightness.

A moderate spatial non-uniformity would be 
acceptable, IF it were time-invariant, at least 
during the course of one scan.


rafe b.