IR thermometer vs thermal imager

[chirhone]

In the end, if you do not want image resolution, the sum of the outputs from a number of small sensors is effectively the same as the total output from a single sensor of the same area so there's no point. If you need to detect a small power flux then it may be cheaper to have a single big sensor and a body thermometer can expect a known amount of power flux over the normal range of temperatures within the proper distance. For a thermal imaging camera you can get the advantage of using a lens which will have more light gathering power and this increase sensitivity (i.e. signal to noise ratio).

How about signal to noise ratio? In SLR cameras, the sensors are big and images are much cleaner than that of say smartphone cameras. So doesn't the same comparison hold for the ir thermometer and imager with the latter getting a bit few fluxes and can this lower the temperature?

 

[chirhone]

How about signal to noise ratio? In SLR cameras, the sensors are big and images are much cleaner than that of say smartphone cameras. So doesn't the same comparison hold for the ir thermometer and imager with the latter getting a bit few fluxes and can this lower the temperature?

Or to reverse it. The FLIR imager is $2500, the IR thermometer is only $20. So can the much expensive electronics and better signal to noise ratio of the FLIR create higher temperature than the china made generic thermopile sensor used in the ir thermometer when both aim at the wall of uniform texture or detail and same emissivity?

 

[sophiecentaur]

How about signal to noise ratio? In SLR cameras, the sensors are big and images are much cleaner than that of say smartphone cameras. So doesn't the same comparison hold for the ir thermometer and imager with the latter getting a bit few fluxes and can this lower the temperature?

Size of array, size of pixel and other factors (quantum efficiency etc.) are all relevant. In the end, thought, it's total amount of radiation that gets to the sensor (/sensors) that counts. Big sensor arrays are another matter but the image size in an entry level IR imager will be pretty small cos you pay for area in image arrays. We're not being precise enough for a definite conclusion here but, for an uncooled sensor, there won't be a vast difference in SNR, for a given (total) area and cost.

Can you explain what you mean about "lower the temperature".

 

[chirhone]

Size of array, size of pixel and other factors (quantum efficiency etc.) are all relevant. In the end, thought, it's total amount of radiation that gets to the sensor (/sensors) that counts. Big sensor arrays are another matter but the image size in an entry level IR imager will be pretty small cos you pay for area in image arrays. We're not being precise enough for a definite conclusion here but, for an uncooled sensor, there won't be a vast difference in SNR, for a given (total) area and cost.

Can you explain what you mean about "lower the temperature".

Oh. I realized fluxes can be numerous or few but it is independent of the wavelength, so i guess it won't accept the temperature if more fluxes reach your sensors. Also:

https://en.wikipedia.org/wiki/Wien's_displacement_law

"Wien's displacement law states that the black-body radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature. The shift of that peak is a direct consequence of the Planck radiation law, which describes the spectral brightness of black-body radiation as a function of wavelength at any given temperature."

 

[sophiecentaur]

Oh. I realized fluxes can be numerous or few but it is independent of the wavelength, so i guess it won't accept the temperature if more fluxes reach your sensors. Also:

https://en.wikipedia.org/wiki/Wien's_displacement_law

"Wien's displacement law states that the black-body radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature. The shift of that peak is a direct consequence of the Planck radiation law, which describes the spectral brightness of black-body radiation as a function of wavelength at any given temperature."

You are confusing me now. What is the bit about "more fluxes"? There seems to be a problem in translation??

Also, Wien's displacement law can only be used if there is spectral analysis of the incident radiation. A post higher up suggests that the resolution could be difficult to resolve the spectrum of a body at 300K with a simple device.

 

[Dr_Nate]

The peaks are so closed together. How does a thermal imager distinguish between a wavelength of say 10 microns vs 8 microns or between 1 Celsius or so?

It won't distinguish between wavelengths. It will have a window that allows a range of wavelengths through.

Unless it is detecting the intensity of the overall radiation and not the individual peaks? More intensity more radiation and the microbolometer has to be calibrated?

The Stefan-Boltzmann law tells us how much power is radiated per unit area from a surface: $\frac{P}{A}=\sigma T^4$. It is extremely sensitive to temperature. If you take in account the effect of only allowing in a range of wavelengths and the emissivity, you could estimate the temperature from the incident power.

 

[sophiecentaur]

I was thinking this problem over and did some searching. Clearly, the emissivity of a surface will affect the radiated energy and it is only measuring the details of the actual spectrum that can resolve this and yield the actual temperature of the radiator. This link is an old one but it shows how the emissivity can be compensated for by using two different IR filters to find the ratio of radiated energy over different frequency ranges. The method is knows as Two Colour Ratio Thermometry.
I get the impression, from reading around that higher temperatures are better suited to this method, which is not surprising as you can actually see the different colours of glowing surfaces as their temperature changes. Low cost medical thermometers don't seem to use such sophistication because the emissivity differences are not big (especially in-ear) and the accuracy is good enough.

 

[zoki85]

You might have gotten lucky and have similar emissivities for those two things in the wavelength range measured.

I don't think I was "lucky". As said, the industrial IR thermometer* I used was not intentended for measuring body temperature but was quite good for humans too. It even has option of adjusting emissivity to "high" (like rubber, asphalt, wood), "medium", or "low" (shiny metal surfaces). For most of instances I had experince with "medium" or "high" settings didn't gave reading that differ more than 1 °C. IIRC, with "medium" setting the result on our foreheads were in 36 °C range. Surprisingly good having on mind wide range of temperatures the instrument was designed for. But I did it only once, while ago, and don't remember of details.

*Not a cheap chinese junk

 

[chirhone]

I don't think I was "lucky". As said, the industrial IR thermometer* I used was not intentended for measuring body temperature but was quite good for humans too. It even has option of adjusting emissivity to "high" (like rubber, asphalt, wood), "medium", or "low" (shiny metal surfaces). For most of instances I had experince with "medium" or "high" settings didn't gave reading that differ more than 1 °C. IIRC, with "medium" setting the result on our foreheads were in 36 °C range. Surprisingly good having on mind wide range of temperatures the instrument was designed for. But I did it only once, while ago, and don't remember of details.

*Not a cheap chinese junk

What is really the difference in temperature between core and surface temperature of the human body? Is it about 1.5 C?

In hundreds of thousands of Ir thermometer worldwide. Did they just add about 1.5 C to the surface temperature to come up with Body mode?

Also my Imager has top emissivity of 0.95 only and not 0.99. This produces a bit higher temperature that coincides with that of the Body Mode in the ir thermometer.

Even though the ir thermometer sells for $20. I got it for $100 because of shortages due to the virus where thousands of checkpoints use it. If my Imager has the same reading. I plan to sell the ir thermometer before the virus disappears. So they just add 1.5 C to the Surface Mode to come up with Body Mode?

What you think?

 

[Drakkith]

@chirhone I've thought about it more, and I'm not sure the Body Mode on the IR thermometer is actually adding anything. Doctors and nurses already use thermometers than measure different temperatures and know when a rectal vs ear vs mouth temperature is out of the normal range.

Perhaps the Body Mode is simply setting the emissivity setting to match that of human skin, whereas the Surface Mode is using a more general emissivity setting?

 

[chirhone]

@chirhone I've thought about it more, and I'm not sure the Body Mode on the IR thermometer is actually adding anything. Doctors and nurses already use thermometers than measure different temperatures and know when a rectal vs ear vs mouth temperature is out of the normal range.

Perhaps the Body Mode is simply setting the emissivity setting to match that of human skin, whereas the Surface Mode is using a more general emissivity setting?

I read that the emissivity of the human skin is 0.98, close to a perfect blackbody.

Anyway. I just found out a while ago my Imager has an advanced menu where I can put any emissivity. So I set it to 1 and aimed at a matt wall together with the ir thermometer (in surface mode).

They showed identical result or temperature of 29.2 to 29.4 Celsius. So we can say the Ir thermometer Surface Mode has emissivity of 1.

And since the human body emissivity is 0.98 (all kinds of body?), Body Mode can't be setting it to emissivity of the human skin of 0.98. I think they arbitrarity set it to 0.95 emissivity.

Is this correct people?

Remember they use the IR thermometers in hospitals, building, etc entrances and checkpoints worldwide so we must at least have ideas how they set Body Mode. Does the internal core temperature of the human body correspond to 0.95 emissivity with respect to that of skin?

Once I have clear answer. I'll sell the ir thermometer while it's still at high price now and just use the thermal imager if setting it at 0.95 emissivity is what Body modes in those ir thermometers do.

Come August, there would be about 80,000 US deaths from conservative projections. So ir thermometers would still be hot items for a few months or so. Or if the virus would become seasonal. I guess these devices would be more demanded?

https://edition.cnn.com/2020/03/30/health/coronavirus-us-ihme-model-us/index.html

 

[chirhone]

What is the reflected temperature of the human body? I tried to google it but can't find anything. I need to enter the data in my Imager.

Also in my googling. I read more about the IR thermometer Body Mode (can you approximate how they do the mathematical algorithm mentioned in the following, is it simply changing the emissivity or something as complex like EFE?) :

https://www.npr.org/sections/health...02/how-a-no-touch-thermometer-detects-a-fever

"The thermometer tells how hot a person is by measuring the infrared energy coming off the body. "Human skin is a very good — or very efficient — emitter of infrared energy," says http://www.sanomedics.com/media-center/company-news?detail=63, chief technology officer for Sanomedics, which manufactures CareGiver, the brand of thermometer now being used in some airports.

CareGiver captures that energy coming off the body, and the device is calibrated to translate that energy reading into the temperature of an object. A sensor relies on a silicon lens to focus the infrared energy so the reading isn't "of the wall, or of your hair," says O'Hara, "We want to read the temperature of a precise spot on your forehead." That's part of what differentiates a human no-touch thermometer from an industrial one, he says — the narrow field of view. The other difference is the algorithm the machine uses in its translation.

"That's our special sauce," says https://www.linkedin.com/in/keithhoulihan, CEO of Sanomedics. CareGiver's algorithm is based on data from clinical studies of patients, some with fevers and some without. "We crunch that data and put it into a mathematical algorithm so that it converts the temperature taken from the forehead into this oral equivalent," says O'Hara. The result, he says, is a thermometer that matches an oral thermometer typically within two-tenths of a degree and ASTM International's thermometer http://noharm.org/lib/downloads/mercury/Guidance_Hg_2013.pdf to within four-tenths of a degree.".

Can you really convert the temperature taken from the forehead into this oral equivalent? Any references?

 

[Dr_Nate]

What is the reflected temperature of the human body? I tried to google it but can't find anything. I need to enter the data in my Imager.

Can you really convert the temperature taken from the forehead into this oral equivalent? Any references?

https://therm-app.zendesk.com/hc/en-us/articles/115003164472-Basic-Thermography-settings- comes from the radiation of an unwanted source that reflects off the object you are trying to measure. For example, let's say you are trying to measure the temperature of a mirror, but there is a matte black metal boiler in the image of the mirror. Because of the high reflectivity, a mirror has a low emissivity ($\varepsilon(\lambda)=1-R(\lambda)$). You won't get much thermal radiation from the mirror. But the radiation from the boiler will reflect off the mirror into your imager. This is an extreme example, but needs to be accounted for.

Given that with human skin we are dealing with a wavelength range where the emissivity is high, the reflectivity will be low, and the reflected temperature won't be much of an issue unless, maybe, you are catching a reflection of the sun off someone's skin.

Converting temperatures? They've put numbers to it so it's been measured. The collecting and analysis of data would be so straightforward that it could be a high school science fair project.

 

[chirhone]

https://therm-app.zendesk.com/hc/en-us/articles/115003164472-Basic-Thermography-settings- comes from the radiation of an unwanted source that reflects off the object you are trying to measure. For example, let's say you are trying to measure the temperature of a mirror, but there is a matte black metal boiler in the image of the mirror. Because of the high reflectivity, a mirror has a low emissivity ($\varepsilon(\lambda)=1-R(\lambda)$). You won't get much thermal radiation from the mirror. But the radiation from the boiler will reflect off the mirror into your imager. This is an extreme example, but needs to be accounted for.

Given that with human skin we are dealing with a wavelength range where the emissivity is high, the reflectivity will be low, and the reflected temperature won't be much of an issue unless, maybe, you are catching a reflection of the sun off someone's skin.

When I set my Imager to emissivity of 0.95 and aim at forehead. There is a difference of 1 Celsius when I adjusted the "Reflected Temperature" setting from 0C to 20C in the menu. Therefore knowing the reflected temperature of the human body is critical when you are taking into account 1 C difference. Do you know or can estimate what is the reflected temperature of the human body or forehead specifically (background is indoor at daytime)?

Converting temperatures? They've put numbers to it so it's been measured. The collecting and analysis of data would be so straightforward that it could be a high school science fair project.

 

[chirhone]

When I set my Imager to emissivity of 0.95 and aim at forehead. There is a difference of 1 Celsius when I adjusted the "Reflected Temperature" setting from 0C to 20C in the menu. Therefore knowing the reflected temperature of the human body is critical when you are taking into account 1 C difference. Do you know or can estimate what is the reflected temperature of the human body or forehead specifically (background is indoor at daytime)?

I think reflected temperature is 20C for the US or colder climates and maybe 26C or more for hotter climates? Maybe one can use a room thermometer to set reflected temperature values?

 

[Dr_Nate]

@chirhone I don't think you understand what I said in my previous comment, so I suggest you put more effort into trying to understand what we say.

However, I think I can make another suggestion. You can do more experimentation. I suggest you repeat this experiment of measuring your skin with different 'reflected temperatures' in order to determine the sensitivity to reflected temperatures. Using the same settings every time, measure your skin in direct sunlight, near something hot like a kettle, and while inside your freezer.

 

[chirhone]

@chirhone I don't think you understand what I said in my previous comment, so I suggest you put more effort into trying to understand what we say.

However, I think I can make another suggestion. You can do more experimentation. I suggest you repeat this experiment of measuring your skin with different 'reflected temperatures' in order to determine the sensitivity to reflected temperatures. Using the same settings every time, measure your skin in direct sunlight, near something hot like a kettle, and while inside your freezer.

I spent hours analyzing your statements and watched this video

and used the technique to measure the reflected temperature.

It showed 32 Celsius. All my life, I live in hot climate with average of 30 Celsius and never encountered any snow. If you have snow, your surrounding (like walls) indeed measured 20 or 10 Celsius?

So i should adjust the Reflected Temperature in my setting from 20C (default of cold climate) to 32 Celsius (average of my ambient surrounding surfaces).

Do you know that putting 20C and 32C in the Reflected Temperature can affect the temperature even if the emissivity is 0.95? Yes, it can change it by up to 0.6 Celsius. In forehead measuring, this is important. So I was right in my previous 2 messages. You thought only huge reflected temperature could affect areas with low emissivity. But even high emissivity like 0.95 can change the temperature by 0.6 Celsius even if the reflected temperature varies by 20C to 32C only. Do you object to this?

But I decided to keep the Ir thermometer. This is because it is more consistent as the cone is bigger so you are averaging coin size area of the forehead giving more consistent result. Using the Imager, the Spot is looking at very tiny area, and if there is some pimple or thick skin spots, the temperature can vary by 0.4 Celsius.

 

[chirhone]

@chirhone I've thought about it more, and I'm not sure the Body Mode on the IR thermometer is actually adding anything. Doctors and nurses already use thermometers than measure different temperatures and know when a rectal vs ear vs mouth temperature is out of the normal range.

Perhaps the Body Mode is simply setting the emissivity setting to match that of human skin, whereas the Surface Mode is using a more general emissivity setting?

The reason I don't think it changes the emissivity in the Body Mode is what if the ir thermometer was made in the USA and adjusted for reflected temperature of 22C and it was used in hot climate with temperature of 32C. It can change the temperature in 0.6C even if emissivity is 0.95 or higher.

Therefore it may be using other algorithms.

The world is now at war with an unknown enemy, and thousands of casualties per day. And we can only detect them via these devices. Therefore let's figure out how it works in the first place. So guys. How is the Body Mode of the IR thermometer implemented:

1. By simply adding temperature like 1.5C to the one taken in Surface Mode?
2. By changing the emissivity?
3. By very complex calculations in the cutting edge of medical physics? How?

What really is the relationship of the forehead surface temperature to body core temperature? is there an obvious relationship?

 

[Drakkith]

The world is now at war with an unknown enemy, and thousands of casualties per day. And we can only detect them via these devices.

Any properly made thermometer will be able to detect a major fever in a patient. The world's health does not depend on whether or not the body mode of an IR thermometer is reading slightly higher/lower than the actual temperature. A fever associated with a major disease is almost always well over any uncertainty or error in a thermometer, and a fever is also associated with other symptoms (sweating, chills, body aches), not to mention the other symptoms associated with COVID-19. So no, these devices are not the only way we can detect this illness. Let's not make a mountain out of a molehill with this issue.

 

[chirhone]

Any properly made thermometer will be able to detect a major fever in a patient. The world's health does not depend on whether or not the body mode of an IR thermometer is reading slightly higher/lower than the actual temperature. A fever associated with a major disease is almost always well over any uncertainty or error in a thermometer, and a fever is also associated with other symptoms (sweating, chills, body aches), not to mention the other symptoms associated with COVID-19. So no, these devices are not the only way we can detect this illness. Let's not make a mountain out of a molehill with this issue.

I mean quarantine checkpoints worldwide use solely ir thermometers in screening. They never use oral or armpit thermometers except in hospitals only.

 

[Drakkith]

I mean quarantine checkpoints worldwide use solely ir thermometers in screening. They never use oral or armpit thermometers except in hospitals only.

And those thermometers very likely work just fine.

 

[Dr_Nate]

Do you know that putting 20C and 32C in the Reflected Temperature can affect the temperature even if the emissivity is 0.95? Yes, it can change it by up to 0.6 Celsius. In forehead measuring, this is important. So I was right in my previous 2 messages. You thought only huge reflected temperature could affect areas with low emissivity. But even high emissivity like 0.95 can change the temperature by 0.6 Celsius even if the reflected temperature varies by 20C to 32C only. Do you object to this?

Using your values, I did the math and got answers close to yours. I didn't take the wavelength range into effect so they will be a little different.

The problem is that your emissivity is likely wrong. The emissivity of skin is likely above 0.98. This reduces the amount of "reflected temperature".

I think you should do the experiment I said in post #46. This will show you how sensitive it is to reflected temperature. Set the imager to one set of values and do not change the settings through the whole experiment. Measure your arm in the room 5 times. Find and record the average of these 5 temperatures. Measure the same spot 5 times with your arm in the freezer (You need to make sure your arm isn't cooling during the measurements, so do it quick and remove your arm between measurements). Find another average. Measure the same spot outside in direct sunlight 5 times. Find another average. Then you can see how different environments affect your imager.

 

[chirhone]

Using your values, I did the math and got answers close to yours. I didn't take the wavelength range into effect so they will be a little different.

The problem is that your emissivity is likely wrong. The emissivity of skin is likely above 0.98. This reduces the amount of "reflected temperature".

I think you should do the experiment I said in post #46. This will show you how sensitive it is to reflected temperature. Set the imager to one set of values and do not change the settings through the whole experiment. Measure your arm in the room 5 times. Find and record the average of these 5 temperatures. Measure the same spot 5 times with your arm in the freezer (You need to make sure your arm isn't cooling during the measurements, so do it quick and remove your arm between measurements). Find another average. Measure the same spot outside in direct sunlight 5 times. Find another average. Then you can see how different environments affect your imager.

It took me an hour to do the tests above. This is because the imager is very sensitive (it can for example image the left over heat from the floor of the feet, etc.). As soon as I put my hand in the sun, the temperature increases very fast from the heat of the sun (2 seconds later). So I have to take many shots, and since the reading varies by each spot of the hands, I have to fix it at one spot. I managed to take one good shot. The effect is either only 0.1 degree Celsius or similar.

This is in the parking with the sun in the heated part of the pavement:

This is taking it in the shade.

This is when I put my hand in the heated area of the parking.

Difference of only 0.1 degree Celsius. And 1 or 2 seconds later, the temperature begins to increase due to the heat of the sun.

The imager is set to 0.98 emissivity with reflected temperature of 20 degree Celsius.

I think putting the values in the imager firmware means it has to subtract 2% of the software "reflection temperature" of 20 degree Celsius from the raw image it's taking? I wonder what is the temperature of the sun's rays? How do you determine the temperature of the sun's rays? And if it is high or a given value. You compute it by taking say 36 Celsius (actual) minus 20 Celsius (software setting) = 16 Celsius and taking off 2% of 16 Celsius deducting from the raw image temperature? How do you compute it?

I also tried putting my hands or fingers in the refrigerator. But the temperature decreases so suddenly (in milliseconds) I couldn't distinguish the reflected temperature from absorbed temperature. The imager is too sensitive in the freezer test to produce any useful data.