6652d137   
   From: BretCahill@peoplepc.com   
      
   > It depends on the design of the particular CCD used in the camera.   
   > There are several  common types of infrared and/or low light  commercial   
   > ccd cameras.   
   >   
   > 1 Common B&W ccds  which have no infrared cut filter have peak   
   > sensitivity in the red through blue range (.4-.5 micron)  but are still   
   > moderately sensitive to  an infrared illuminator  from about .7  to 1.0   
   > micron. These are most common for cheap  short range surveillance   
   > cameras with 850nm LEDs.   
   >   
   > 2. common 3 color cameras.   The red channel of these typically has a   
   > "tail" into the infrared similar to common b&w ccds.   These are usually   
   > called day/night color cameras.  With an illuminator they display a B&W   
   > image with infrared illumination. (the red color information is shut off)   
   >   
   > 3.  Enhanced infrared cameras.  These have their highest sensitivity in   
   > the red-and infrared.  The Sony ExView cameras are an example.  They   
   > have about 3x higher sensitivity in the IR, about like 1 and 2 in the   
   > visible.  These work well to a few hundred yard range with a telephoto   
   > lenses and partially focused diode IR laser.  ExView CCDs cost more and   
   > usually have more defects than "plain" ccds.   
   >   
   > All of the above need a long pass blocking filter to ONLY be sensitive   
   > in the visible or a long wave pass filter to only be sensitive in the   
   > IR.  Types 1 and 2 often have short pass  filters to  give "normal"   
   > images pleasing to the human eye.   
   >   
   > 4. For much more sensitive cameras in wavelengths .4 to 1 micron CCD   
   > cameras can integrate for longer than normal video frames.   The   
   > sensitivity is roughly linear with the integration time. Integration up   
   > to about 2 seconds is practical before noise becomes severe.   Cooling   
   > the CCD will allow  increased  integration time.   At -40C hour   
   > integrations are practical.  The downside is that if objects move during   
   > the exposure the images are smeared.   
   >   
   > Lenses make a large difference too as lenses with small f ratios can   
   > gather many times more light than the roughly f/3.5 lens of the human   
   > eye.  Lenses faster then about F/1.4 start getting expensive.   
   >   
   > For outdoor use the clear night sky (no moon) is several times brighter   
   > in the 0.8 to 0.9 micron range because of infrared sky glow.  An   
   > integrating ExView CCD camera or a plain ccd with a gen III intensifier   
   > work well even without moonlight.   
   >   
   > So... what to use depends on your application and your budget.  Also   
   > whether your application allows the use of an IR illuminator.   
      
   It might be more cost effective to just buy a lot of cheap IR LEDs and   
   use a conventional camera, first testing for an IR cut off filter with   
   a single cheap IR LED.   
      
      
   Bret Cahill   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)