Gary Orlove, P.E.
ASNT NDT/PdM Level III
Infrared Training Center

When you look at lens specifications for infrared cameras, you'll see all sorts of different things. Some camera makers list the lens by focal length while others list them by Field of View (FOV) and still others may list them by magnification power. Here are some examples:

• 3X Telescope

• 20 x 15 degree FOV

• 13mm focal length

• 100 mm “equivalent focal length”

Sometimes you will see the same lens, for instance a 13mm, in two different camera lines expressed with different Fields of View (FOV)! So what's going on here? How can a 13mm lens have an FOV of 40° for one camera and 63° for another? And just what is "35mm equivalence” anyway?

This article explains the focal length specifications for infrared cameras and explains why you can't really compare focal lengths between different cameras.

What is "focal length?"
The "focal length" of a lens is the distance between the optical center of the lens and the place where it focuses its image. For film based cameras, this is the film plane, for digital and modern infrared cameras, it is the imaging sensor (for instance a microbolometer array).

An easy way to think about this is to pretend that all the rays of infrared light pass through the optical center on their way to the infrared sensor.

Simple imaging configuration illustrating focal length

What does the 35 mean in a "35mm camera?"
The 35 in "35mm camera" doesn't refer to the focal length or even to the size of the image on the film. It is the measurement of the width of the film including the sprockets. Take a look at the following figure for the dimensions.

Dimensions of 35mm film

What makes a lens wide angle or telephoto?
The relationship between the focal length of the lens and the size of the sensor array determines the field of view of the camera. If the focal length is smaller, the field of view is wider and vice versa.

For example, think about a 35mm camera. The image formed on the film by a 35mm camera is 36mm wide and 24mm high. So, for example, if the focal length of the lens is 28mm, the horizontal field of view is 65° -- a wide angle.

28mm lens field of view

If the focal length is 100mm, the horizontal field of view reduces to 20°. This produces a higher magnification, and is considered a telephoto lens. Put another way, the 100mm lens offers a 3.25x magnification over the 28mm lens. The so called “normal” lens for a 35mm camera has a focal length of 50mm and a horizontal FOV of 40°.

100mm lens field of view. Note the increased size of the arrow on the image plane compared to the 28mm lens.

What about infrared cameras?
The IR sensitive arrays on IR cameras are typically much smaller than the imaging area of 35mm film. This means that the focal length that produces a "normal" field of view is much smaller than 50mm.

Unlike 35mm film which is the same regardless of what camera you use, the size of the IR array can vary from camera to camera. This poses a problem. You cannot tell what FOV a particular focal length lens will produce without knowing the dimensions of the array. Even then you may not be able to calculate it because sometimes the active imaging area doesn’t even cover the entire array!

Because many people are familiar with focal lengths of 35mm camera lenses, many digital camera manufacturers often specify their lenses in relation to what focal length of 35mm camera lens would produce the same FOV. This is what is meant by the term “35mm equivalence”.

Consider, for example, a digital camera with a CCD array measuring 8mm by 6mm. What focal length lens would produce a horizontal FOV of 40° (the same as a 50mm lens on a 35mm film camera)? A little math tells us that an 11mm lens would produce the same FOV. So the 11mm lens on this camera has a 35mm equivalence of 50mm.

So what specification should thermographers be looking at?
For all of us in the IR business, the most important specification for a lens is the horizontal and vertical fields of view (there are others but that’s another article). Always ask your infrared camera supplier to provide you with this data in order to compare lenses from different camera models.

• In the Dark: The blind feed the blind at new Paris restaurant
  Twin Falls Times-News, ID - Sep 24, 2004
  ... Nobody can see. An infrared camera carried in by a TV crew caught many couples using the darkness to smother each other with affection. ...

• NASA Urged for Infrared Camera in Hubble
  Newsday, NY - 20 hours ago
  ... time when light first filled the universe urged NASA on Thursday to ensure a Hubble Space Telescope servicing mission includes a next generation infrared camera ...

• InSb-based camera improves train braking
  Compound Semiconductor, UK - Sep 20, 2004
  ... Working in partnership with infrared camera specialist CEDIP, and brake pad manufacturer Flertex, they aim to improve management of the heat load on brake discs ...

• Power's on, but the work has just begun
  St. Petersburg Times, FL - Sep 19, 2004
  By COLLEEN JENKINS, Times Staff Writer. Chuck Pruett aims the handheld infrared camera at his target: row after row of power lines. ...

• ICS Infrared Roof Consultants Help Government Agencies Save ...
  Emediawire (press release) - Sep 16, 2004
  ... surveys, infrared electrical surveys, roof consulting, roof evaluations, building envelope performance surveys, infrared thermography, infrared photography ...

• CenterPoint Energy Minnegasco Offers Tips to Help Minnesotans 'Get ...
  PR Newswire (press release) - Sep 15, 2004
  ... 1) a blower door test to determine air leaks; 2) a combustion safety test and CO check; and 3) an infrared inspection that uses and infrared camera to further ...

• Zoo has tall and small arrivals
  Scoop.co.nz (press release), New Zealand - Sep 14, 2004
  ... built a special off-display exhibit to encourage three-year-old female Hoi-An and 14-year-old male Hari to mate, and installed an infrared camera to track ...

• Likely First Photo of Planet Beyond the Solar System
  Space.com - Sep 11, 2004
  ... The photograph was made at ESO's Paranal Observatory in Chile with an infrared camera, which records heat rather than visible light. ...

• Robot defuses tense situations
  Raleigh News, NC - Sep 8, 2004
  ... It has eyes: four cameras with different points of view sending video to its operator. An infrared camera even sees in the dark. ...

• Experts work to monitor volcano's rumbles
  Anchorage Daily News (subscription), AK - Sep 4, 2004
  ... Waythomas visited the vent on Spurr's Crater Peak last Monday. Using an infrared camera, he said he found no unusual heating or steaming. ...

• Brazil Release: Alien vs Predator
  Gringoes.com, Brazil - Sep 1, 2004
  ... Using thermal imaging satellites, Weyland believes to have discovered the ruins of an ancient pyramid temple that predates the Egyptian and Aztec pyramids ...

• Archaeologists use new tools in search for Fort Louis
  Columbus Ledger-Enquirer, GA - Aug 29, 2004
  ... Center for Archaeological Studies, led the search using three noninvasive remote sensing technologies - earth conductivity, magnetometry and thermal imaging. ...

Here is a selection of recent new threads by IR Community members. Feel free to click the links, see how people have responded, and post your own response if you like.

• New Forum - Building Science
  We started a new forum recently for Building Science Applications. If you posted or were following a thread for a building application, you will find them moved to the new forum.

• IR Risk/Method statements
  I have been asked to write a risk assessment and method statement for insurance purposes for IR surveys. The surveys will be mainly electrical distribution systems in building services.

• Temperature of molten iron.
  We are trying to use our camera to determine the eth feasibility of molten iron. The temperature of the iron is approximately 2650 deg. when we set the emissivity at .29 our camera will not read high enough. We have a flame filter which will read up to 2732 deg. None of our other measuring devices states that it is that high so i was just wondering if anybody has any suggestions.

• Buss bar severity criteria questions
  Generally speaking, what severity criteria should apply in a heavily loaded buss bar? I'm looking at one rated for 3000A, but currently with 2000A fuses installed. With emissivity set conservatively at 0.70, I get a rise over the ambient temp inside the cabinet on the order of 46°F, and greater than 60°F compared to ambient room temp. The B phase is slightly warmer than the others, but not by more than a few degrees.
  This is in a creamery that operates 24/7, with virtually never a scheduled shutdown -- this being the main plant switchgear, it's highly critical, but a forced shutdown would be extraordinarily expensive for them.
  I did not have the means to measure such high loads, but I am given to understand that it was running at about 70 to 80% of peak at the time.

• Reporter Software
  I use a PM 575, and Reporter Software. No matter which palette I use when I save an image, it always reverts back to Iron when downloaded onto the PC. I have tried to make a Report, change the palette to grey, copy the image, and save it in IMG format using grey: it still saves it as Iron, although once into a Report and saved, it saves correctly. I desperately need to save some images in Grey, for onward transmission to another colleague, any solutions please.

• Flash Thermography
  I was wondering where I could get more information on the flash thermography kit that is shown in the IR for nondestructive testing found in the link section of this site. Specifically where I could get a flash heat source like that for the FLIR P-60 camera and what the cost factors might be.

And of course if YOU have a question or want to start a discussion on a topic, we would love to hear from you. Just post a new thread on a message board.

Brainteaser of the Month  [top]

Here is this month's brainteaser. Readers who email us the correct identification and explanation of the object(s) in the thermograms are entered into a drawing to win a prize from ITC. Please put "Brainteaser" as the subject of the message.

Click here to email your guess

Do you have an interesting image that you think would stump other thermographers? If so please email me your image (preferably in native .img, .jpg, .tif, .tgw, or .tmw format) with an accompanying visible photo and explanation. If your image is used, you receive a gift as well.

This a thermogram of a transformer (115kv high side) where a bad "tap changer" compartment was detected. This is usually indicative of bad internal contacts elevating the temperature of the internal oil in the tap changer compartment as compared to that of the main body of the transformer. Quite a serious find.

Congratulations to our winner, Dan Roark from the Knoxville Utilities Board in Tennessee; and a thank you to Peter Bruder of National Grid in Massachusetts for the thermogram. Both Dan and Peter receive a special low emissivity traveling coffee mug.

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