Measuring tempurture of a light source with a IR thermometer

[videobruce]

I was taking measurements of the temperature of some LED bulbs (while lit) and found reading varied due to distance.
Example; maximum reading were as high as 189 degrees F when taken 12-14" away, but dropped to between 160-165 degrees when taken from around 20-24" away. I figured most of the reason was the actual 'light' was the factor since I never noticed that while measuring any other surface.

This was with two different IR handheld gun style thermometers (one more expensive that the other).  Other than actually turning off the lights, is this typical when measuring any type of light source?

[Fgrir]

What is the distance-to-spot ratio for your thermometers?  You won't get a good reading if the object you are trying to measure isn't completely filling the sensitive area of the thermometer.

[videobruce]

1:8 one one and 1:12 on the other.

When I move the laser pointer around temperatures vary substantially (as it does when measuring electronic components on circuit boards. It's very specific on the exact location (with either unit).

What I didn't do was to let the bulb heat up, remove power, then measure.

[ebclr]

Worth watching

[videobruce]

The only problem with that comparison is that isn't a light source, it doesn't generate heat or light, it only reflects. 

[ebclr]

you forgot the hot water inside,

[videobruce]

I didn't know hot water emitted light??

I think you need to re-read my 1st post.

[ebclr]

Are you looking for light or heating, If you are measuring light with an infrared pyrometer, you will never have any measure related to light

[videobruce]

I'm looking to measure heat, since these are "IR" thermometers. The reason light was brought up is the objects of interest are light bulbs that appear to be skewing the measurement due to the light being emitted along with the heat, since the reading severely decreases with distance, something that doesn't happen with other object or surface I ever measured. 

[Fraser]

An IR thermometer can be a very challenging measurement tool when dealing with small targets.

In order to be accurate the target being measured needs to fill the field of view. The field of view is normally detailed in the IR thermometer manual at different distances. As the distance between the thermometer and target increases, so does the area viewed by the thermometers optics. As soon as the area viewed exceeds the target dimensions the thermal sensor becomes underilluminated and begins including the background or surface temperature surrounding the target. The reading then starts to fall as the sensor averages the whole scene temperature.

An emitter like an LED or small lamp is actually a very small target for an IR thermometer. Depending upon its size, it may be too small to fill the IR thermometers field of view. The best you can do is place the IR thermometer directly over the target in an effort to fully illuminate the sensor. Some IR thermometers have plastic optics and some just expose the sensors small window to the outside world. In the case of the bare sensor, it is often possible to place the sensor very close to the thermal target for a decent reading.

A thermal camera is a far easier  tool to use with small targets as you literally place the cross hairs over the target and the camera measures just the area within the measurement box. It is the advantage of a multi pixel array sensor versus a mono pixel IR thermometer. The pixels in a thermal camera are tiny compared to the mono pixel in an IR thermometer so illuminating a pixel or even several pixels is easy at close range on even small targets. Sadly thermal cameras cost a lot more than a cheap IR thermometer though.

I suggest you try your IR thermometer sitting over the target LED or lamp. Try buying one of the cheap IR thermometers that have the sensor window directly exposed to the outside world. Great for close-in working.

The thermal sensors are blind to visible light. The window on the sensor is opaque to light. It presents only thermal wavelengths of energy to the sensors die behind the window.

Examples of exposed sensor window IR thermometers.....


http://www.ebay.co.uk/itm/Mini-Pen-Style-Non-Contact-Infrared-IR-Digital-LCD-Thermometer-W-Battery-DT8220/321939087156?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D47230%26meid%3D5dd18f571cc84240812680ada0e62f03%26pid%3D100678%26rk%3D3%26rkt%3D15%26sd%3D252787417491&_trksid=p2481888.c100678.m3607&_trkparms=pageci%253Af2d4d51c-ac43-11e7-8fdd-74dbd180d56d%257Cparentrq%253Afcc568bc15e0a860f8ddca50fffb4a8c%257Ciid%253A1

http://www.ebay.co.uk/itm/HOLDPEAK-960C-Digital-Laser-IR-Infrared-Thermometer-With-Data-Hold-For-Cooking-A/252787417491?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D47230%26meid%3D6bc37389d41b4d18830d4785b324887e%26pid%3D100678%26rk%3D1%26rkt%3D15%26sd%3D252787417491&_trksid=p2481888.c100678.m3607&_trkparms=pageci%253A384c2f7b-ac44-11e7-961d-74dbd1806583%257Cparentrq%253Afcc7300415e0ab6bc79fbcf8fffd4002%257Ciid%253A1

I use this one as it has a long nose to reach into electronics on PCB's

http://www.ebay.co.uk/itm/Baby-Digital-Thermometer-Non-contact-Infrared-IR-Laser-Body-Surface-Temperature/222658196228?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D47230%26meid%3D6bc37389d41b4d18830d4785b324887e%26pid%3D100678%26rk%3D2%26rkt%3D15%26sd%3D252787417491&_trksid=p2481888.c100678.m3607&_trkparms=pageci%253A384c2f7b-ac44-11e7-961d-74dbd1806583%257Cparentrq%253Afcc7300415e0ab6bc79fbcf8fffd4002%257Ciid%253A1

Hope this helps

Fraser

[videobruce]

The thermal sensors are blind to visible light.

There is absolutely no interaction?

[Fraser]

No.

See the wavelength response here :

http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7FTS305-11C55%7FA%7Fpdf%7FEnglish%7FENG_DS_TS305-11C55_A.pdf%7FG-TPCO-033

Fraser

[Fraser]

Some reading for you.......

http://www.mouser.com/pdfDocs/amphenol_as_thermopile_ir.pdf

Fraser

[videobruce]

I plan on re-running the tests, heating the bulb up, then disconnecting power then take readings as I originally did (12-24" away).
Thanks for the links.

[ebclr]

Thys may help you understand the big mistakes on your setup

https://www.omega.com/temperature/z/pdf/z057-058.pdf

[billfernandez]

OP:  At (for example) 12" your 8:1 IR thermometer has a field of view that is 1.5" in diameter.  To get an accurate reading the surface that is emitting IR (due to its temperature) must fill the entire 1.5" field of view.  (In practice, the area emitting IR must be significantly larger than this 1.5" diameter area.)  If the heat-emitting object does NOT fill the entire field of view the indicated temperature will be in error.

[beuford]

Emissivity is important if the surface of the bulb where you are measuring is glossy or reflective. This would make the surface a low-emissivity one and make IR measurements difficult. Note that some LED bulbs have a plastic diffuser that is higher emissivity and will assist surface measurements.

HOWEVER, there will likely be little heat at the surface of the diffuser. The LED lights produce little IR, and the diffuser will not get too warm from the visible light alone since it is designed to pass rather than absorb these wavelengths. It's the power supply in these devices that tend to get warm as they convert between line and LED voltage.

Run the LED for a while, then remove from its socket and check the temperature of the base; you might be surprised at what you find. If you get close enough with the IR sensor you can probably pick up the temperature of the base through the diffuser.

Beuford

[K5HJ]

Visible light LEDs should not emit substantial IR so the effect of light on the sensor is quite small.

What is the diameter of the device you are measuring?
As was stated by others, you must fully fill the IR sensor. Getting as close as possible is best.  Moving farther away only makes the error worse.

If you wish to measure the die temperature of an LED, the only practical way is with a thermocouple or RTD thermometer.

A thermal camera would get you in the ballpark.

[videobruce]

If the heat-emitting object does NOT fill the entire field of view the indicated temperature will be in error.

Ok, I get the root of the problem now. The surface behind the bulb is a (mostly) white benchtop. The bulbs were just laid out on the bench (since I don't have a 2 pin socket to use). I just naturally figured it was the light output that was 'fooling' the sensor, but it's the lower percentage of area being measured that is. The benchtop is at room temperature (more or less) that drops down the reading the further the sensor moves away from it.

I don't normally measure temperature of light bulbs, I did because I was surprised these get so damn hot, too hot to actually hold the base. How I do use them is the measure components of circuit boards where I suspect heat issues with failing components.  So all of this tells me a typical IR thermometer is the wrong instrument to use. 

Not to get OT, but I stumbled upon this which I thought was kinda humorous;
http://www.nytimes.com/2007/08/07/health/07heat.html

[videobruce]

The specific bulbs I'm taking about are these;
https://www.greenelectricalsupply.com/gu24-led-bulbs.aspx

[Fraser]

Videobruce,

I suggest you buy a well made cheap mini IR thermometer like those I detailed in my post. You can recognise the type you need by the small aperture in which the sensor resides. The closer to the outside world the sensor is positioned, the better. My Actron CP7875 Mini IR Thermometer has a small metal 'cone' aperture of around 8mm diameter and 5mm depth. It needs to be placed over the target as its aperture spec is 1:1. It also has a decent update rate for 'scanning' a large area.

https://www.autoaccessoriesgarage.com/Automotive-Tools/Actron-Pocket-IR-Thermometer

Also available under the TIF brand but has a plastic sleeved aperture as opposed to an all metal one. This could be useful in avoiding shorts on PCB's whilst in use !

http://www.robinair.com/products/pocket-ir-thermometer-11

Brochure is here

http://www.robinair.com/sites/default/files/06-15.pdf

Manual is here

http://www.robinair.com/sites/default/files/TIF7201.pdf

They can cost less than $10 but can be very useful. Often more so than their larger, longer range 'big brothers'. The TIF7201 costs a little more at $20

http://www.testequipmentdepot.com/robinair/test-and-measure/thermometers/ir-thermometers/infrared-pocket-thermometer-tif7201.htm

https://www.amazon.com/Robinair-TIF7201-Pocket-Infrared-Thermometer/dp/B002Y0EUBO

Fraser

[videobruce]

Since you brought up Pyrometers which I never saw that term before, is that just another term for a IR thermometer? Looking over listings in Amazon (which is debatable on their accuracy) I saw many that use both terms in the same listing (device).

[Fraser]

An IR Thermometer is a member of the Pyrometer family.

Pyrometers are a tool that can measure temperature through non contact methods.

Early pyrometers for high temperature measurements where the target was actually glowing in the range Cherry red to Bright yellow used a filament colour comparison method. The user adjusted the brightness of a filament superimposed onto an image of the target (a telescope device was used) and when the colours of the filament and target matched, the temperature could be read off of a calibrated scale on the filament colour control.

Why did I detail the above ? Well pyrometers come in many forms depending upon age and intended purpose. IR thermometers are just one type of pyrometer. Searching for 'Pyrometer' is not the best choice when looking for an IR thermometer of the type you need. Common searches for such are 'non contact thermometer' and 'IR thermometer'.

Fraser

[Fraser]

Wiki has a decent page on Pyrometers that you might find interesting...

https://en.m.wikipedia.org/wiki/Pyrometer

Fraser

[videobruce]

This is what I meant with the previous question;

https://www.amazon.com/Infrared-Pyrometer-Thermometer-Handheld-version/dp/B00WEE39K8/ref=sr_1_15?ie=UTF8&qid=1507563656&sr=8-15&keywords=Pyrometers
https://www.amazon.com/KKmoon-UA750-50-750°C-Non-contact-Thermometer/dp/B01MSWT4X8/ref=sr_1_44?ie=UTF8&qid=1507563698&sr=8-44&keywords=Pyrometers

From that link above comes these statements;

Modern pyrometers or infrared thermometers also measure the temperature of cooler objects, down to room temperature, by detecting their infrared radiation flux.
A modern pyrometer has an optical system and a detector.

All of that sounds as both terms mean the same thing. 

Wikipedia's definition of a Infrared thermometer makes no mention of a Pyrometer.

[Fraser]

The function of a Pyrometer is the same, correct.

However the term Pyrometer when searching on the Internet will bring back results for all manner of temperature measurement devices that are not a modern Thermopile based IR Thermometer. It's a little like searching for an "internal combustion engine" when all you are really only interested in is the Diesel engine ! 

Pyrometer is an over arching category of test equipment that merely details that it is a thermal radiation measurement device.

Fraser

[videobruce]

Initially it appeared to be a different animal just adding to the confusion. 

[Fraser]

Indeed, there are many terms used. You can have 'contact pyrometers' as well !

I attach a range of Pyrometers for your interest 

As you can see, they come in many forms.

Fraser

[videobruce]

Scouring thru Amazon, I found many analog automotive gauges that came up using "Pyrometers" as the search entry.

[Fraser]

A nice article to read

http://www.explainthatstuff.com/how-pyrometers-work.html

Fraser