I used to use one of the early (very high $) IR cameras and when taking pictures of equipment, different materials/metals would show up as different colors, which meant different temperatures even though they really were the same temperature.

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On Mar 10, 2011, at 23:47, David Beierl <dbeierl@ATTGLOBAL.NET> wrote:

> At 11:20 PM 3/10/2011, Anthony Egeln wrote:
>> the wire. Still pegging max high with a steady flashing light, but
>> the thermal laser gun reveals a normal temp system.
> 
> IR temp guns are not the be-all and end-all of temperature
> measurement.  In order to obtain an accurate reading you have to take
> into account the field of view of the instrument, of course - the
> fancier ones will have a laser grid that directly shows the area
> measured, and the device being measured must fill this area.  If it
> does not the reading will be low.
> 
> In addition, however, is the factor of emissivity, the amount of
> infrared that a given substance will emit at a given wavelength for a
> given temperature.  On expensive instruments you set the emissivity
> on the instrument, but cheap ones ordinarily assume an emissivity of
> 95% of what a theoretical black-body radiator would emit.  This is a
> good average choice, but it fails on some materials we find
> interesting.  Aluminum is one of them.  Its emissivity under various
> conditions ranges from 0.02 to about 0.4.  IR instruments assuming
> .95 emissivity will therefor read somewhere between moderately and
> ridiculously low.
> 
> Polished metals in general have low emissivity and will read rather
> low on our cheap IR thermometers.  I just checked a stainless-steel
> pot of boiling water - the shiny s/s lid of the pot measured about
> 105-108F unless there was a layer of condensed steam on it, in which
> case it went up to 125F or so.  Rolling a kitchen thermometer across
> it demonstrated that the temperature was in fact above 200F.  On
> substituting a sheet of aluminum foil for the pot lid I obtained a
> reading of 82F.
> 
> Our aluminum engines aren't polished, and they typically are coated
> with oily dirt and such that has a higher emissivity than the metal
> itself, and that can be a saving grace; but their readings cannot be
> accepted uncritically.
> 
> ONE WAY TO GET A MUCH MORE ACCURATE READING WOULD BE TO COVER THE
> MEASUREMENT AREA WITH CLOSELY ADHERED MASKING TAPE.
> 
> Yours,
> David