Infrared Thermometer Guns - Are they useful?

[obesepanda]

I have been trying to research and understand whether handheld infrared thermometer guns use ultrasonic sensors. I am investigating the accuracy of these devices and understand that due to the inverse square law when the distance between the radiation source and the measuring device increases by 2 the intensity of the infrared radiation would decrease to 1/4.  

What I am trying to understand is whether distance is factored into the algorithms used to convert surface temperature to a 'body temperature' reading as this seems like an important factor. The only thing I have been able to find is that depending on whether you want to to measure close range or long range the device would require a different aperture known as the D/S ratio.

Any help or a point in the right direction would be very helpful and much appreciate.

Thank you

[studiot]

5 minutes ago, obesepanda said:

I have been trying to research and understand whether handheld infrared thermometer guns use ultrasonic sensors. I am investigating the accuracy of these devices and understand that due to the inverse square law when the distance between the radiation source and the measuring device increases by 2 the intensity of the infrared radiation would decrease to 1/4.  

What I am trying to understand is whether distance is factored into the algorithms used to convert surface temperature to a 'body temperature' reading as this seems like an important factor. The only thing I have been able to find is that depending on whether you want to to measure close range or long range the device would require a different aperture known as the D/S ratio.

Any help or a point in the right direction would be very helpful and much appreciate.

Thank you

Yes they do compensate for distance .

 

Read here how.

https://www.grainger.com/know-how/equipment-information/kh-370-infrared-thermometers-qt

Scroll down to the paragraph headed

Using an Infrared Thermometer: Understanding Field of View and D:S Ratio

[obesepanda]

12 minutes ago, studiot said:

Yes they do compensate for distance .

 

Read here how.

https://www.grainger.com/know-how/equipment-information/kh-370-infrared-thermometers-qt

Scroll down to the paragraph headed

Using an Infrared Thermometer: Understanding Field of View and D:S Ratio

Thank for your the reply, so the distance is being compensated by taking the average over the entire field of view? What I am trying to understand better is why the temperature reading drops by a whole degree when increasing the distance between the forehead by just 2 cm. Surely the forehead is still within the field of view, if so why can't the device calculate the distance from the forehead and produce the same reading as before? 

Thank you

[studiot]

There will be an optimal range of distances for which a given instrument is designed.

Outside of this erroneous readings may be expected.

Have you read the manufacturer's instructions for your particular instrument ?

Edited November 8, 2020 by studiot

[obesepanda]

36 minutes ago, studiot said:

There will be an optimal range of distances for which a given instrument is designed.

Outside of this erroneous readings may be expected.

Have you read the manufacturer's instructions for your particular instrument ?

Yeah, the suggested operating range is 3-5 cm. Here is an example of measurements taken at 3,5,7.5 and 10 cm 

Distance (cm)	Reading
3	36.8
5	36.6
7.5	36.5
10	36.3

But, I am struggling to understand why the known distance cannot be used compensate for the temperature decrease. 

Thanks

[StringJunky]

34 minutes ago, obesepanda said:

Yeah, the suggested operating range is 3-5 cm. Here is an example of measurements taken at 3,5,7.5 and 10 cm 

Distance (cm)	Reading
3	36.8
5	36.6
7.5	36.5
10	36.3

But, I am struggling to understand why the known distance cannot be used compensate for the temperature decrease. 

Thanks

The background in the instrument's field of view, outside/around the measured object, will increasingly influence the target  reading as you step back... it doesn't just 'see' the area occupied by the target..

Edited November 8, 2020 by StringJunky

[studiot]

So within the stated operating distance range of your thermometer  it reads 36.7^o ± 0.1^o.

Is the resolution of your instrument 0.1^o ?

Then your readings would be within calibration.

[swansont]

 

2 hours ago, obesepanda said:

Thank for your the reply, so the distance is being compensated by taking the average over the entire field of view? What I am trying to understand better is why the temperature reading drops by a whole degree when increasing the distance between the forehead by just 2 cm. Surely the forehead is still within the field of view, if so why can't the device calculate the distance from the forehead and produce the same reading as before? 

Thank you

 It’s not just that you increased the distance 2 cm, it’s that you see something with a different temperature when you do this. If the object was huge, you’d be OK at a larger distance.

There’s no calculation, as such - it’s all driven by geometry. The device collects all the photons in its field of view. If those include photons from something other than the target, the reading will be off.

[obesepanda]

Resolution

0.1 degrees Celsius (0.1 degrees Fahrenheit)

Yes that is right, the resolution is 0.1 degrees. The Model is the YK-001. 

5 minutes ago, swansont said:

 

 It’s not just that you increased the distance 2 cm, it’s that you see something with a different temperature when you do this. If the object was huge, you’d be OK at a larger distance.

There’s no calculation, as such - it’s all driven by geometry. The device collects all the photons in its field of view. If those include photons from something other than the target, the reading will be off.

Yes I understand the field of view will increase as distance increases but I didn't realise it would increase so much from a 2.5-5 cm increase.  Also, if the aim is to measure body temperature and we know that the reading will decrease with distance, can't the measured distance compensate for the decrease?

No worries all, thank you for the info. I think I understand... if the device says 3-5 cm accuracy then it means that at that temperature the 'field of view' will most likely fit within the forehead region. Anything greater will perhaps expand to include other areas of the head where the average temp of the entire area will be returned.

 

[swansont]

21 minutes ago, obesepanda said:

Resolution

0.1 degrees Celsius (0.1 degrees Fahrenheit)

Yes that is right, the resolution is 0.1 degrees. The Model is the YK-001. 

Yes I understand the field of view will increase as distance increases but I didn't realise it would increase so much from a 2.5-5 cm increase.  Also, if the aim is to measure body temperature and we know that the reading will decrease with distance, can't the measured distance compensate for the decrease?

The increase has to be taken in the context of the distance. 

A 5 cm increase starting from 5 cm doubles the linear distance of the field of view. It quadruples the area.

If the field of view is ~ 60 degrees, that’s going from a 5 cm x 5 cm patch to 10 cm x 10 cm. If you’re aiming at your forehead, you now start to include things that are not your forehead. Your hair has a different temperature and emissivity than your skin.