Temp diff between Honeywell thermostat & IR thermometer

I had a Honeywell CM67 thermostat which gave a temperature reading about 3 deg C lower than when measured with my IR Thermometer (ie by pointing it at the thermostat case and display as well as the wall around it).
I've just replaced the thermostat because of a (different) fault with a new Honeywell CM907 (the new version of the CM67) and I'm getting the same thing. I know that I can change the calibration / temperature offset, but I wondering why it's happening at all. Is it something to do with the way I'm using the IR thermometer?
Thanks,
Michael
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My understanding is that the Honeywell is measuring air temperature. The IR thermometer is measuring thermal radiation (light) radiated from a surface, and this will be affected by the material, its colour, texture etcetera, that is, emissivity. http://www.openxtra.co.uk/articles/emissivity_intro.php
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snipped-for-privacy@hotmail.com wrote in

Probably - there's a health warning on the IR about reflectivity.
So be careful when looking for actual temp.
Having said that, my IR thermometer is my best friend when looking at drops, offsets, etc, a bit of black tape will help for genuine accuracy, and I generally rely most on how I feel for ambience - my room thermostat is easy to poke up a degree or two just for that particular time of day phase if I'm a bit cool.
PS
If your cat sleeps with it's nose tucked into it's tail, it's too cool; get your heating adjusted or I'll bell the RSPCA :)
miie
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I thought it might come down to something like this; so last night I did a check & put a bit of black insulating tape and a bit of (shiny) sellotope on the thermostat case. I left them for a while & then measured the temperature of the insulating tape, sellotape, the thermostat case and the wall. All of these were consistent.
However the theremostat was still 3 deg C lower.
M
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