Re: Purple spots on pictures?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Crow Leader by way of Terry Fritz <teslalist-at-qwest-dot-net>" <tesla-at-lists.symmetric-dot-net>

Tesla list writes:
>Original poster: "Adam Britt by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><beans45601-at-sbcglobal-dot-net>
>I just got back from taking my tesla coil to school (everyone loved it, of 
>cource.) And my teacher took a bunch of pitcures of the coil in action. On 
>all of the pictures, these purple spots appear on the pictures. They are 
>digital. When he took the pictures, he left the shutter open for 1 second

This is probably noise frmo the camera itself. All digital cameras will 
make funny dots if left open for a few seconds, some more than others. The 
noise is from the CCD sensor and   circuitry to read the pixels. There are 
several ways to battle them. This is good for cameras with stuck pixels 
that are always on or brighly colored (say green or white dots that never 
go away).
- use a shorter exposure, a useless option if you want a pic with lots of 
sparks.
- cancel them in software. There are two types of software for this. I 
don't know any names off hand, but the generic one just removes what it 
thinks are random bad pixels. I'm not sure how it does this, or on what 
basis. The other type lets you "calibrate" the program by taking several 
pictures with the lens cap on (on in a dark room) to look for pixels that 
tend to be more prone to picking up colors that are not there.
- you can drop the ISO and increase the exposure time. This is not an 
option all digital cameras have, and may not work the same on all of them. 
High ISOs on digital cameras = high gain and more noise, and a lower s/n 
ratio then say a twice as long exposure at half the sensitivity.
Sort of related, here's my experience with cameras with high voltage stuff.
I use a consumer Kodak digital camera from 1998 that sometimes crashes or 
currupts images when about a foot away from equipment that sparks, and a 
Kodak professional digital camera that seems to care about nothing at all. 
The expensive camera is in an all metal body, so it seems very shielded.
Corona from my quarter crusher agitates the audio section of my Canon video 
camera, and Tesla coil has no affect in it at all, even few feet from ~3 
foot arcs.
KEN