I have an I R thermometer and use it when setting up flow and return on radiators. Two things to bear in mind, as well as surface condition, colour also has an effect. Also it does not measure at the point of the laser light ,it requires a larger surface area for accurate results. The distance table guide for mine states
Surface diameter 25 mm – distance 300 mm
75 mm. 900 mm
125 mm 1500 mm.