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Re: filmscanners: LED Illumination for Film Scanners



At 03:36 PM 6/21/2001 -0700, you wrote:
>If I'm not mistaken, all A.C powered fluorescent bulbs "flicker" at 60/50 
>cycle (in those places that use 60 or 50 cycle AC, that is).  In fact so 
>do incandescent bulbs, for that matter, but in both cases, aspects of the 
>design reduce the visibility of this.  I think television screens also do 
>so at some frequency..., and we know that computer monitors do, and the 
>higher the frequency the less obvious, at least on a conscious level.
>
>In the case of fluorescents (and televisions and CRTs), the phosphors have 
>a long enough lag activation period to "bridge" this rather short fraction 
>of a second.  In the case of incandescents, the filament remains hot 
>enough to bridge that difference in electrical flow.

For most, perhaps all, bulbs, the phosphors do not actually have a long 
enough lag time to "bridge" the gap. Persistence of vision is the 
explanation for why it seems continuous. 50 or 60 Hz is far faster than the 
24 frames-per-second that was (still is, for all I know) used for motion 
pictures.

The thermal "inertia" of the incandescent bulb prevents it from flickering. 
That is, it can't possibly cool down enough to stop glowing as the voltage 
crosses the zero point. There just isn't enough time before the voltage 
climbs back up on the other side. (As you said.)

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




 




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