Why non-contact laser infrared thermometer is inaccurate?

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               Why reading of laser infrared thermometer is inaccurate?
 

Basic Principles

 

The most basic design of a infrared thermometer consists of a lens to focus the infrared thermal radiation on to a detector, which converts the radiant power to an electrical signal that can be displayed in units of temperature after being compensated for ambient temperature.

 

So we need to work two things before we start to measure.

 

1, the D:S 


The distance-to-spot ratio (D:S) is the ratio of the distance to the object and the diameter of the temperature measurement area. i.e. if your object surface is a DIA 2CM spot, the distance from the surface should be less than 24cm.

 

If it is a curved surface, so the distance should be a bit closer.  i.e. a hot water pipe.


2, the emissivity

 

As we can know, most of object surface are low reflective. The more reflective a material is, the lower its emissivity. Highly polished silver has an emissivity of about 0.02. 

Most entry level infrared meters are designed with fixed emissivity 0.95. 
So if we use such a thermometer to measure a shine surfcae the reading must be not accurate. i.e. a chrome exhaust manifold.

A professional grade infrared thermometer must have an adjustable emissivity to suit all surface.


How to avoid the problem and get accurate reading?
 

1, D:S
 

Make sure the D:S is correct.


2, the emissivity


For a professional grade infrared thermometer just set to the correct emissivity.

For a fixed emissivity thermometer,  just attach masking tape to the object surface,  then you can start to read the temperature.

 

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