Rafe wrote:
>It's generally
>when either the bulb or its "ballast" is near
>the end of its life.
Thanks for adding that note, Rafe. I didn't *think* I was going nuts, or at
least not just yet, and I'm seeing that effect in an 18-month-old HP
scanner. Banding on the edges, just where you'd expect it with a bad
ballast.
Best regards--LRA
>From: Raphael Bustin <rafeb@channel1.com>
>Reply-To: filmscanners@halftone.co.uk
>To: filmscanners@halftone.co.uk
>Subject: Re: filmscanners: LED Illumination for Film Scanners
>Date: Thu, 21 Jun 2001 17:39:31 -0400 (EDT)
>>
>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.
>
>
>
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