How reliable the laser guns are for checking surface temperature?


Thread Starter

Tribhuwan Joshi

Normally I have observed that, when we want to check the temp of any surface by laser gun it shows absert values. How reliable are laser guns? Name any other more accurate device for this purpose (Cheaper of course).
I would suspect you are referring to infrared (IR) thermometers using laser
pointers - laser guns are good only for dispatching 50's era bug eyed
monsters. That being the case, then, depending upon the manufacturer, they
tend to be reliable and accurate devices when used within their performance

I'm guessing not all of the surfaces you have attempted to measure give
nonsense values, but those that did tended to be specular reflective
surfaces of one sort or another (polished chromed rolls, and the like),
and/or surfaces far away from the thermometer. If measuring reflective
surfaces you'll need to use correction factors for emissivity (either by
multiplying the displayed temperature by the appropriate correction factor,
or, in more capable instruments, by changing the emissivity factor from 1.00
to the required value). An alternative, if possible in your application, is
painting a portion of the target flat black, and measuring the temperature

Take care when measuring targets far away from the IR thermometer. All such
instruments have a 'field of view', and, the farther away you are, the wider
the sensed area. The sensed temperature is the average of the temperatures
at all points in the field of view, so, if some of the area is at 200°C, and
other portions of the area are at 100°C the sensed temperature will fall
somewhere between the two.

The following links point to a sampling of various manufacturer and reseller
web pages that discuss IR thermometry in more detail.

We've used the Land Insrument "Cyclops" for years and it takes a beating and provides repeatable readings.

Molten salt is always hotter down below than the debris floating on the top.

No non-contact works well on freshly extruded aluminum.

Rocky Jones Space Ranger never used a laser gun, he used a ray gun to blast his alien enemies.

Bruce Durdle

Radiation pyrometers measure the energy issued by a surface as radiation. Any surface will have an emissivity less than 1, and you need to factor this in to the equation - there is usually an adjustment you can make somewhere on the instrument for this. What you can't factor out is the effect of reflected background radiation.

A surface with an emissivity of 0.65 will have a reflectivity of (1 - 0.65) = 0.35, and will reflect 35% of the energy it receives from its environment as well as radiate energy based on its own temperature. If you are measuring metal temperatures in a furnace, with flame temperatures well above the metal surface temperature, you can expect indicated temperatures to be up to 100 deg C above the actual surface temperature.

Other possible effects are those due to absorption of radiation in the atmosphere between the surface and the sensor - both water vapour and CO2 can be potent absorbers at some wavelengths.

Michael Griffin

So far as I know, unless you are measuring a black body radiator, you always have to correct for emissivity. Plastics are typically around 0.95 (but it depends upon the type of plastic, filler, etc.). Most metal objects are painted or coated, so it is the emissivity of the paint or coating that matters in these cases. You can get emissivitiy tables for most common materials.

If the surface is covered with dust or dirt, it is the temperature of the dust or dirt that you are measuring. If the surface is a thermal insulator, then the temperature can change drastically within a distance of only a few millimetres. All of these means that you have to know what you are doing when using infra-red measuring equipment. You will probably never get readings from an infra-red measurement and a thermocouple measurement to agree with each other.

One exception to the emissivity correction comes with a special sensor made by one company. This sensor inherently corrects for emissivity. However the sensor must always be placed directly against the object being measured, so it isn't practical for remote sensing (but it is good for quick measurements of things like servo motor case temperatures).
I find IR themperature measurement most useful for comparisons. Examples would be finding hot spots on a plate, is a motor running hotter than last time I checked it, and is there a temperature difference on either side of a trap would be my typical uses.