Actually it is inversely propotional. It follows the inverse square law -
double the distance you quarter the illumination.

John

 
 
John Burke
 
(408) 515 4992

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Edward Mines
Sent: Thursday, June 14, 2007 9:00 AM
To: [log in to unmask]
Subject: [TN] black light intensity measurement

Doug-
  You ask an interesting question. I don't believe there are any standards
regarding the intensity of UV light for inspection of conformal coatings. 
   
  Before working for HumiSeal I worked for Polychrome, then one of the
bigger manufacturers of presensitized printing plates. The photosensitive
coatings on these plates are exposed with high intensity lamps in a vacuum
frame. Sometimes it was necessary to calibrate the exposure units and we had
a device we called a "puck". At least some exposure units use UV lamps. Some
lamps loose illuminating strength over a period of time.
   
  There used to be a company in Chicago (UV Process Supply?) which sold
everything related to UV curing including these "pucks". Spectronics and
other manufacturers of UV lamps may know where to get UV intensity
measurement instruments too. At HumiSeal we had a similar instrument to
meassure the strength of our UV cure unit.
   
  The intensity of UV is proportional to the distance between the light
source and the object being illuminated SQUARED  (or maybe even to a higher
power?). You couldn't look at the light sources we were using at both
employers.
   
  Now is a good time to mention that the glowing power of the fluorescent
dye in conformal coatings sometimes declines with age. We suspected that
this had to do with exposure to light. HumiSeals 1B31 & 1A33 contain a lot
of fluorescent dye; it's easy to dissolve in the solvents in those coatings.
It's not so easy to dissolve in some of the other coatings. So the glow
strength will vary from coating to coating. I think it's dependent on
coating thickness too.
   
  Don't discount that different operators will count different numbers of
defects from the same screen. When I worked at Kodak (I got around) we saw
this many times with microscopists.
   
  Don't get hung up on minute voids in conformal coatings. There are breaks
in the coating that you can't see - the backs of flat leads, bends in flat
wire (we called them "knees"). A different material should be used for
absolute coverage.
   
  Edward Mines BEChE, MEChE 

       
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