Why do stronger red lasers not work in a Crookes radiometer?

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In summary: Since the effect is actually due to forces at the edges of the vanes, does it make a difference if the laser is shining on a spot in the middle of the...Summary:The Crookes tube has vanes that turn rapidly in sunlight. Lesser lights such a 5mW green laser her also work. Also a 3 and a 40mW violet laser work well. What don't work are a 1mW red diode and 10mW red HeNe laser.The device pictured (see link) is turning away from the dark side so thermal transpiration dominates.Data is a little hazy, ill-defined terms, and there is poor control of variables.
  • #1
Simon Bridge
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Off a g+ post:
https://plus.google.com/+PeterTerren/posts/Weo9gg8F5d9

Peter Terrin said:
The Crookes tube has vanes that turn rapidly in sunlight. Lesser lights such a 5mW green laser her also works. Also a 3 and a 40mW violet laser work well. What don't work are a 1mW red diode and 10mW red HeNe laser.
So why doesn't the stronger red laser work?
(I haven't measured the power on calibrated equipment). This doesn't make sense from a heat driven action point of view. Any ideas from the physics guys.

The device pictured (see link) is turning away from the dark side so thermal transpiration dominates.

Data is a little hazy, ill-defined terms, and there is poor control of variables.
But I'd put it down to the the laser spectra and the absorption spectra of the surfaces.

Also reference:
How does a light mill work by Philip Gibbs July 1996 (Usernet Physics FAQ)

Note: Terrin is one of those happy individuals who loves to "play" with science equipment for entertainment but does not automatically assume some groundbreaking discovery whenever something happens against his expectations.
 
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  • #2
The laser hits a single side, so radiation pressure and thermal expansion work in the same direction.

The absorption spectrum of the surface is certainly a possible explanation, the absorption spectrum of the glass could be interesting, too.
Alternatively: can we be sure the 10mW-laser is really emitting 10mW?

I can block the green from the green laser with an IR transmission filter and it still works to turn the vanes. No visible green passes at all.
That is another interesting observation.
 
  • #3
The laser hits a single side, so radiation pressure and thermal expansion work in the same direction.
... nice observation: can suggest pointing the laser at the other side, see if radiation presure dominates - however:

I can block the green from the green laser with an IR transmission filter and it still works to turn the vanes. No visible green passes at all.
The green laser must be emitting an IR side-band.
Looks like radiation pressure may be irrelevant.

Alternatively: can we be sure the 10mW-laser is really emitting 10mW?
No.
Even if that is what the manufacturer wrote on the tube, and it is still accurate, it may be the power consumption rather than the output... or it may be the output in the specified bands rather than the total output or anything.

the absorption spectrum of the glass could be interesting, too.
Good point - had not considered the glass. I bet lots of people have that blind-spot.
I've heard that a lot of glass absorbs infrared well ... on the face of it, that would seem a bad kind of glass to make a radiometer out of. OTOH: the glass would heat up and emit it's own infrared (etc) spectrum...

... that sounds like it could just work as an explanation: the radiation pressure makes little difference and the thermal pressure needs IR, which it gets from the glass not the laser ... the different sources heat the glass differently?

... what else am I missing?
 
  • #4
Direct heating is much more efficient and should work with visible light (as the surface is black), and apparently it absorbs at least some IR as well.
Heating via the glass tube is possible, but the glass will emit in all directions, only a small fraction hits the radiometer.
 
  • #5
Gentlemen:

I have never encountered the following explanation for the light mill's rotation, which suggests to me that it must be flawed. Wouldn't the air on the heated side of each vane rise and cooler air fill in beneath it? Some of the cooler air will come from the cooler side, wrapping around the lower edges and displacing the vane.

Comments?
 
  • #6
Stan80 said:
Wouldn't the air on the heated side of each vane rise and cooler air fill in beneath it? Some of the cooler air will come from the cooler side, wrapping around the lower edges and displacing the vane.

probably not relevant since there is a pretty reasonable vacuum in the radiometer
 
  • #7
Stan80 said:
I have never encountered the following explanation for the light mill's rotation, which suggests to me that it must be flawed. Wouldn't the air on the heated side of each vane rise and cooler air fill in beneath it? Some of the cooler air will come from the cooler side, wrapping around the lower edges and displacing the vane.
You are describing a situation that convection in the rarified gas in the bulb has a horizontal component near the vanes.
It is mentioned in the link in post #1.
 
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  • #8
I always believed the long mean-free-path theory. I wonder how that got into my head:
6wv4Euu.jpg


FFVEWhH.jpg


Since the effect is actually due to forces at the edges of the vanes, does it make a difference if the laser is shining on a spot in the middle of the vane?
 
  • #9
Keith_McClary said:
Since the effect is actually due to forces at the edges of the vanes
Where did you get that from? The force is everywhere at the vanes. Well, at all places where they are heated.
 
  • #10
mfb said:
Where did you get that from? The force is everywhere at the vanes. Well, at all places where they are heated.
That's how I interpret the explanation in How does a light mill work by Philip Gibbs.
To explain the radiometer, therefore, one must focus attention not on the faces of the vanes, but on their edges. The faster molecules from the warmer side strike the edges obliquely and impart a higher force than the colder molecules. Again, these are the same thermomolecular forces responsible for Reynolds' thermal transpiration. The effect is also known as thermal creep, since it causes gases to creep along a surface that has a temperature gradient. The net movement of the vane due to the tangential forces around the edges is away from the warmer gas and towards the cooler gas, with the gas passing around the edge in the opposite direction. The behaviour is just as if there were a greater force on the blackened side of the vane (which as Maxwell showed is not the case); but the explanation must be in terms of what happens not at the faces of the vanes, but near their edges.
Is the force proportional to the area or to the length of the edges?
 
  • #11
Keith_McClary said:
Is the force proportional to the area or to the length of the edges?
It is more complicated, no simple proportionality.
 
  • #12
I've decided that I still don't understand how these things work.
I just watched a video where someone made the vanes turn by cooling the globe.
No light was necessary.

By an odd coincidence, I just picked up a radiometer last Friday, and just finished performing some similar tests.

The first thing I did, was set it up next to my stovetop heating element, about 15 cm away.
When the element reached about 260°C, the radiometer was spinning faster than I've ever seen it spin in sunlight.
My infrared thermometer indicted that the internal components were somewhere around 100°C when I finished.

I then set up a contraption, such that I could warm the radiometer with just air.
I placed my hands next to the tile base to funnel hot air onto the bulb.
It turned slowly in the standard direction.
2016.12.02.crookes.radiometer.heated.png
Finally, I took a plastic bottle filled with ice and placed it on top of the radiometer.
As in the video, the vanes turned backwards.

2016.12.02.crookes.radiometer.cooled.png

The bottle has a concave base.

Like a lot of people, I've heard various explanations of how these things work.
But I'm now back to not understanding it at all.
 
  • #13
Simon Bridge said:
Off a g+ post:
https://plus.google.com/+PeterTerren/posts/Weo9gg8F5d9



The device pictured (see link) is turning away from the dark side so thermal transpiration dominates.

Data is a little hazy, ill-defined terms, and there is poor control of variables.
But I'd put it down to the the laser spectra and the absorption spectra of the surfaces.

Also reference:
How does a light mill work by Philip Gibbs July 1996 (Usernet Physics FAQ)

Note: Terrin is one of those happy individuals who loves to "play" with science equipment for entertainment but does not automatically assume some groundbreaking discovery whenever something happens against his expectations.

Thanks, Simon, for resurrecting the original post by Peter Terren. I had just decided yesterday to get a radiometer and begin experimenting with lasers when I came across your post. The OP had lots of good info about laser interactions. If I am not mistaken, he came to the conclusion that his problem was resolved when he tested the violet laser and realized that the measured lower output wattage was erroneous based upon the non linear absorption response of the cell at the shorter wavelengths.

My other thought about the "cause" of the effect is that the molecular flow about the vanes reminds me of that which happens concerning "winglets" at the tips of aircraft wings. A vortex develops at the edges which is attenuated by the winlets. Maybe there is something similar going on here with the thermal flow.
 
  • #14
OmCheeto said:
My infrared thermometer indicted that the internal components were somewhere around 100°C when I finished.
Internal components, or the glass envelope?
I wonder from which part of the contraption the infrared thermometer reads a temperature.

Question: How is the thing affixed within the bulb so as to minimize friction.
Is it a wire with the vanes attached that rotates?
Does the wire have a glass bearing top and bottom?
Or is the wire fixed and the vanes rotate around the wire.

I have never seen one AFAICR ( as far as I can recall ).
Thinking of making my own some day ( like a lot of things never seems to happen ).
 
  • #15
mfb said:
It is more complicated, no simple proportionality.
Area force, Einstein Edge force, edge shear (opposing rotation), and the thermal transpiration ( that some seem to propose as doing it all ).

It should be taken into consideration that a vacuum is required for motion with a heat or light source, but not a strong vacuum - in the viscinity of 1 Pa.
Varying the vacuum will, I believe, result in one effect being more dominant than the others.
 
  • #16
How is the thing affixed ...
Diagram
 
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  • #17
256bits said:
Internal components, or the glass envelope?
I wonder from which part of the contraption the infrared thermometer reads a temperature.
I'm now guessing, I don't know for sure.
But I'm leaning now to the opinion that I'm measuring the surface temperature of the globe.

The problem yesterday was that I was using 3 4 different temperature probes:
1. my fingers (conductive thermal sensor: this doesn't feel "hot")
2. my eyes (that heating element is definitely hot)
3. my infrared thermometer (something is hot)
4. my skin (radiant thermal sensor: confirms that that heating element is really hot)​

I think I previously assumed I was measuring the internal temperature, as the surface didn't feel that hot: 100°C.
But it's possible, because the glass bulb is so incredibly thin, that even though it's very hot, it doesn't have enough thermal energy to burn me, and by the time I sense how hot the globe is, my fingers have absorbed all that energy.
But I've just stuck the radiometer in the freezer, and the globe registers a consistent 16.2°C, while the freezer contents are at -11°C.

...
I have never seen one AFAICR ( as far as I can recall ).
Thinking of making my own some day ( like a lot of things never seems to happen ).
I got mine at a "going out of business sale". :oldcry:

256bits said:
Area force, Einstein Edge force, edge shear (opposing rotation), and the thermal transpiration ( that some seem to propose as doing it all ).
I have no idea what those things are.

It should be taken into consideration that a vacuum is required for motion with a heat or light source, but not a strong vacuum - in the viscinity of 1 Pa.
Varying the vacuum will, I believe, result in one effect being more dominant than the others.

So I wasn't the only person to analyze the data from the video. :oldwink:

2016.12.03.crookes.rad.p.vs.hz.graph.png


Science! :partytime:

ps. I redid the experiment this morning, and my previous comments from yesterday appear to be somewhat wrong.
Today's observations:
The device started rotating when the heating element was around 260°F, and did not really start flying until the heating element was at full output.

pps. My infrared thermometer has a maximum limit of 400°C, so I tried to collect some spectral data. I have no idea if it will be of any relevance to this problem, so I'll just keep the photos to myself.
 
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  • #18
OmCheeto said:
So I wasn't the only person to analyze the data from the video. :oldwink:

View attachment 109861

Science! :partytime:

ps. I redid the experiment this morning, and my previous comments from yesterday appear to be somewhat wrong.
Today's observations:
The device started rotating when the heating element was around 260°F, and did not really start flying until the heating element was at full output.

pps. My infrared thermometer has a maximum limit of 400°C, so I tried to collect some spectral data. I have no idea if it will be of any relevance to this problem, so I'll just keep the photos to myself.

OMCheeto; the Video in your first post provided some excellent info...and the raw data chart was nice.
Where did you get the data chart above?

But I've just stuck the radiometer in the freezer, and the globe registers a consistent 16.2°C, while the freezer contents are at -11°C
How long did you keep it in the freezer before measuring ? It has to reach equilibrium Temp.
And of course, the temperature , even if accurately measured, must be measured with constant ambient light in order to have any meaningful significance.
That is one thing I wish the fellow in your first video would have measured INSIDE the vacuum chamber (with some sort of guage) when he did the pressure measuements. -- TEMPERATURE.

Furthermore; the thing that really makes things rough is we don't know the pressure (in the dark) of these little Crookes gizmos, so we haven't a clue where on the data chart they are fallen.

My other problem is how the heck did the guy in the video determine the avg. molecular collision distance at optimum pressure was 2 cm if he didn't know internal temperature ??

At first I thought : well, we could assume ideal gas law and use Pressure = nRT/V, and since V and n are constant we could assume Temp. (if he knew it) changes linearly with Pressure, (and figure kinetic speed from that); but not so since this is NOT a closed system; energy is pouring in from both the glass being exposed to ambient outside temperature and from the photon radiation itself .
So how did he come up with a figure of avg collision distance? Anyone?
------------------------
 
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  • #19
Creator said:
OMCheeto; the Video in your first post provided some excellent info...and the raw data chart was nice.
Where did you get the data chart above?
It came from the graph in the video. I digitized it and changed everything to standard SI units.
How long did you keep it in the freezer before measuring ?
Not long at all. I was only trying to determine how it measures temperature.
Does it see through glass? I think the answer in this case is no.
I don't really know much about types of glass, nor how it blocks or allows transmission of infrared radiation.

It has to reach equilibrium Temp.
And of course, the temperature , even if accurately measured, must be measured with constant ambient light in order to have any meaningful significance.
That is one thing I wish the fellow in your first video would have measured INSIDE the vacuum chamber (with some sort of guage) when he did the pressure measuements. -- TEMPERATURE.

Furthermore; the thing that really makes things rough is we don't know the pressure (in the dark) of these little Crookes gizmos, so we haven't a clue where on the data chart they are fallen.

My other problem is how the heck did the guy in the video determine the avg. molecular collision distance at optimum pressure was 2 cm if he didn't know internal temperature ??

At first I thought : well, we could assume ideal gas law and use Pressure = nRT/V, and since V and n are constant we could assume Temp. (if he knew it) changes linearly with Pressure, (and figure kinetic speed from that); but not so since this is NOT a closed system; energy is pouring in from both the glass being exposed to ambient outside temperature and from the photon radiation itself .
So how did he come up with a figure of avg collision distance? Anyone?
------------------------
I'm curious about those things also.
Perhaps, in the morning, I'll do more research.
 
  • #20
OmCheeto said:
. I was only trying to determine how it measures temperature.
Does it see through glass? I think the answer in this case is no.
I don't really know much about types of glass, nor how it blocks or allows transmission of infrared radiation.

The transmission of IR depeds upon the type of glass; here's great little tutorial showing the transmission of various materials at various IR wavelengths.:
http://www.edmundoptics.com/resourc...e-correct-material-for-infrared-applications/

I'll address some more later as time permits.
 
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  • #21
Creator said:
The transmission of IR depeds upon the type of glass; here's great little tutorial showing the transmission of various materials at various IR wavelengths.:
http://www.edmundoptics.com/resourc...e-correct-material-for-infrared-applications/
That was one of three things I did last night, before retiring.
1. How many types of glass are there,
according to wiki, there are 49​
___and what kind are used in light bulbs?
according to one website, the answer is "Soda Lime"​
___What are "soda lime glass" IR absorption characteristics?
according to this website, "soda lime glass" is transparent between um, argh! Darned log scales! I'm guessing 200-2100 nm.​
2. What happens when you put the radiometer in the freezer? Does it go backward?
yes​
___Does it continue to go backwards?
no​
3. What happens when you take the radiometer out of the freezer? Does it rotate in the expected direction? And for how long?
see question #2​

I'll address some more later as time permits.
Me too.

ps. I just did some more experimenting this morning. My kitchen is warmer than it's been since summer. :smile:

Collected data was a bit too all over the place to make any claims, other than; "I did not burn my house down".
:partytime:
 
  • #22
Creator said:
My other problem is how the heck did the guy in the video determine the avg. molecular collision distance at optimum pressure was 2 cm if he didn't know internal temperature ??
I think he said "1 cm".

I found the answer to this at Hyperphysics.com
They have a nifty calculator, which requires only 3 variables:
Temp: 300K (room temp)
Molecular size: 365 picometers (from wiki)
pressure: 0.927 pascal (= 0.7 millitorr, from the video)​

lambda = 0.7e-2 meters = 0.7 cm
Close enough.

I went through the thread this afternoon, to see if I wasn't missing something.
I also read the wiki entry on the device.
The wiki entry is fairly decent, IMHO, as they observed just about everything that I did. Mainly, my question why it runs backwards in the freezer; "The wheel turns backwards because the net exchange of heat between the black sides and the environment initially cools the black sides faster than the white sides. Upon reaching equilibrium, typically after a minute or two, reverse rotation ceases."
This simply requires that the black side have a higher emissivity than the white side.

I was never able to determine the temperature of my stove's heating element, other than it is somewhat above 660°C, as I was able to melt a small piece of aluminum foil.
 
  • #23
OmCheeto said:
I think he said "1 cm".

I found the answer to this at Hyperphysics.com
They have a nifty calculator, which requires only 3 variables:
Temp: 300K (room temp)
Molecular size: 365 picometers (from wiki)
pressure: 0.927 pascal (= 0.7 millitorr, from the video)​

lambda = 0.7e-2 meters = 0.7 cm
Close enough.

.
Yes, he did say 1 cm. - My mistake...
BUT, The point I was making is that you CANNOT use the ambient temperature OUTSIDE the glass, especially in an OPEN system; at least not in his experimental set up because:
1. I don't think its possible for the INSIDE temp. to reach thermal equilibrium that fast, and especially since there so high a vacuum and there is a continuous flow of photon energy coming INTO the system , and thus it is no longer a closed system. Thus the Temperature used in that "nifty calculator" is unreliable . The calculator is based upon a closed system in thermal equilibrium..
2. And according to the Ideal Gas Law, each time you change the pressure inside the glass enclosure, the TEMPERATURE changes also !. So saying that the inside has the same ambient Temperature as the outside is faulty...Even if he waited 5 minutes after EACH change of pressure thermal equilibrium still would NOT be established because the photon radiation is pouring into the system the whole time.

The only way to get a true Temp. reading is to put a Temp. gage INSIDE the glass enclosure and take a Temp. reading EACH time you take a Pressure measurement.
 
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  • #24
Creator said:
Yes, he did say 1 cm. - My mistake...
BUT, The point I was making is that you CANNOT use the ambient temperature OUTSIDE the glass, especially in an OPEN system; at least not in his experimental set up because:
1. I don't think its possible for the INSIDE temp. to reach thermal equilibrium that fast, and especially since there so high a vacuum and there is a continuous flow of energy coming INTO the system , and thus it is no longer a closed system. Thus the Temperature is unreliable .

I'm not following your train of thought so far, so..., let's continue.

2. And according to the Ideal Gas Law, each time you change the pressure inside the glass enclosure, the TEMPERATURE changes !. So saying the inside has the ambient Temperature of the outside is faulty... IMHO

I've never seen anyone prove this to me yet.
Though, it's possible they did, and I didn't understand.
What part of the Ideal Gas Law states that when you change the pressure, the temperature has to change?
I asked this question 8 years ago, and no one, to my memory, responded.

pfoogle pfoogle pfoogle

Correction. 9 years ago.

A younger Om said:
Dec 20, 2007
...The combined gas law doesn't really tell me much as indicated by this calculator: http://www.1728.com/combined.htm
It just tells you that you have to know the value of 5 variables to get the 6th.
Which is totally useless. I can even use it to prove you wrong: Cut the volume in half and double the pressure and there is no temperature change.

Sooner or later I will determine what my question should be.
Anyways, I'm late for work. I'll work on the solution later.

Though, Stew said he'd already answered the question, and may have just been disgusted with my apparent impermeability to new knowledge.

ps. We appear to be drifting off topic.
 
  • #25
OmCheeto said:
I've never seen anyone prove this to me yet.
Though, it's possible they did, and I didn't understand.

.
OK; Sorry, I made another mis-statement (but only in point #2). withregards to the experimental apparatus in the video my statement that "increase in pressure changes temp." is not necessarily true because the number of moles of gas, n, is not constant. However, it would be applicable to the Crookes Radiometers IF the pressure could increase by changing the volume since n IS constant.
However, that still doesn't negate the MAIN point I was driving home, namely, the fact that the internal temperature calculation (and thus the avg. collision distance) is inaccurate because it is an open system , and all these calculators are based upon Ideal Gas Law for a CLOSED system.

The huge amount of photon radiation from outside the system skews the entire calculation and makes it intractable unless you know the exact amount of radiation energy that is being absorbed into the interior of the glass enclosure.
Again the fact that in the video set-up there is continuous radiation that is heating up the paddlewheel adds another variable to the Ideal gas equation that is unaccounted for.

As I said before there needs to be INTERNAL Temp. sensors and due to the accumulation of radiation energy the temperature measurements need to be taken EACH time a pressure measurement is taken in order to calculate mean collision distances.

In any event this is getting off topic; the real question should be why does the pressure have an optimum value for maximum rotation.
 
  • #26
Creator said:
OK; Sorry, I made another mis-statement (but only in point #2). withregards to the experimental apparatus in the video my statement that "increase in pressure changes temp." is not necessarily true because the number of moles of gas, n, is not constant. However, it would be applicable to the Crookes Radiometers IF the pressure could increase by changing the volume since n IS constant.
However, that still doesn't negate the MAIN point I was driving home, namely, the fact that the internal temperature calculation (and thus the avg. collision distance) is inaccurate because it is an open system , and all these calculators are based upon Ideal Gas Law for a CLOSED system.
I don't think the "ave collision distance" has anything to do with how the radiometer works. I think it's purely coincidence that it's the same length as the edge length of the vanes.
The huge amount of photon radiation from outside the system skews the entire calculation and makes it intractable unless you know the exact amount of radiation energy that is being absorbed into the interior of the glass enclosure.
Again the fact that in the video set-up there is continuous radiation that is heating up the paddlewheel adds another variable to the Ideal gas equation that is unaccounted for.
A huge amount is not required. As has been noted, placing your hands on the radiometer will make the vanes spin.
I just did that, and beings my hands were not scrupulously clean, I then cleansed the bulb with alcohol, which lowered the temperature of the bulb by 2°C, which caused the vanes to rotate backwards.

As I said before there needs to be INTERNAL Temp. sensors and due to the accumulation of radiation energy the temperature measurements need to be taken EACH time a pressure measurement is taken in order to calculate mean collision distances.
Given that there are roughly 225 trillion air molecules per cm3 inside the radiometer, my guess is, once again, that the mean collision distance is irrelevant.

In any event this is getting off topic; the real question should be why does the pressure have an optimum value for maximum rotation.
My guess is, that a higher numbers of molecules will make it more "soupy", increasing drag. And a lower number of molecules reduces the available carriers of motive force. So there should obviously be a "sweet spot". As to why it is, what it is, is probably beyond my ability to comprehend.
 
  • #27
Simon Bridge said:
You are describing a situation that convection in the rarified gas in the bulb has a horizontal component near the vanes.
It is mentioned in the link in post #1.

Simon:

I am describing vertical components. Air on the heated side rises. Some of the cooler air replacing it is air from the cooler side of the vane curling under the bottom edges and displacing the vane. Wide rectangular vanes set horizontally should cause faster rotation than the same vanes set vertically.
 

1. What is a radiometer?

A radiometer is a device used to measure the intensity of electromagnetic radiation, such as light or heat.

2. How does a radiometer work?

A radiometer contains a set of vanes, typically made of a lightweight material such as mica, suspended in a partial vacuum. When exposed to light or heat, the vanes rotate due to the pressure difference between the black and white sides. This rotation can be used to measure the intensity of the radiation.

3. What is the purpose of using a radiometer?

A radiometer is used to measure the intensity of radiation in various applications, such as in meteorology, astronomy, and medical imaging. It can also be used as a teaching tool to demonstrate the principles of light and heat energy.

4. What are the limitations of a radiometer?

Radiometers are only able to measure the intensity of radiation and cannot provide information about the wavelength or frequency. They are also sensitive to external factors such as air currents, which can affect the accuracy of the measurements.

5. How can we improve the accuracy of radiometer measurements?

To improve the accuracy of radiometer measurements, it is important to minimize external factors such as air currents and use calibration procedures to ensure the instrument is properly calibrated. Using a higher quality radiometer and taking multiple measurements can also help improve accuracy.

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