Choosing the Right Thermal Camera Lens

Understanding Thermal Camera Lenses

So you've decided to add a thermal camera to your inspections arsenal. Congratulations. You’ve seen lots of example images. You've poured over details such as pixel resolution and thermal sensitivity. You’ve weighed the added benefits of features such as WiFi and manual focus.


 
You're about to make your selection, when all of sudden you notice a specification called “Field of View.” This is a specification measured in degrees. Unlike degrees of Fahrenheit or Celsius, which most people understand, degrees of angle, when it comes to vision, are less common.
 
How does a narrow field of view differ from a wide? Is there much of a difference between 15° and 25°? 10° doesn't sound like much. When do you need one or the other? Is it that important to have a thermal imaging camera that can take more than one lens?
 
For building and industrial use, interchangeable thermal camera lenses run from narrow fields of view from 6° to 15°, “standard” 25° view and wide fields of view from 45° to 90°.
 
Most fixed-lens thermal cameras have a wide angle lens, typically with around 40~45° fields of view.
 
For specialty uses, like research and development, you will also see macro lenses that can see extremely sharp detail, down to 25 microns (a strand of human hair, by comparison, is about 75 microns).

Do I need a thermal camera with more than one lens?

The answer is yes. Or no. Or maybe. It all depends on your most typical uses for your thermal camera. Are you using it for a single purpose or for many different applications? Are you using it for items mostly from a distance or within close proximity? Are the areas you going to be measuring in hazardous or extremely hot?
 
You may not need more than one lens to start. But having a camera that has the ability to use a variety of lenses may make sense. As you gain experience with your thermal imaging camera, you may find more applications to use it on.
 
Something to keep in mind, if you think you will want more than one lens, check to see what it would take to add it later. Thermal lenses are made of crystal, not glass. Therefore, there are sometimes very small differences in the physical properties. This can cause calibration issues between the lens and the camera that need to be compensated for, in order to maintain the highest level of accuracy. Some thermal cameras and lenses have a chip inside them to communicate the offset information. This allows the lens to be purchased at any time and just attached to the camera.
 
Other brands require that the lens and the camera be calibrated together as a system. Once the lens is calibrated to a specific camera, it cannot be guaranteed to be accurate with any other cameras (even the same make/model). If this is the case with the thermal imaging camera brand you are selecting, it might make sense to invest in the extra lens(es) right away. This way you will not have to send the camera back nor pay for calibration. Typically, if you buy a camera and more than one lens, the manufacturer will calibrate the extra lens(es) at no additional cost.
 

Which Thermal Camera Lens is Right for Me?

Let's say you want to monitor substation towers from a distance. Here is what they could look like with a standard lens on a camera with 160 x 120 resolution. In this case, standard field of view is 17.5° x 13°.
 


These shots are taken from about 150 yards away. Lots of sky, right?

However, when you put a 2X telephoto lens, the image suddenly looks a lot closer, doesn't it?



Telephoto in this case is 8.75° x 6.5°.
 
This gives more of your target in your image.

The camera above had a resolution of 160 x 120. It isn't bad. But what if you need better clarity? Also, what if you need to get closer?

Here is how a higher-resolution thermal camera would look. These images are from a camera with 320x240 resolution and standard 24° x 17° field of view.



The other lenses are 2x and 4x. The 2X is 12° x 9°. The 4X is 6.0° x 4.5°.

What about getting in more of the image?
Thermal cameras are all about seeing the "big picture." This is true for buildings, breakers and boards. If you want to see more of something, and you don't want to be too far away, you need to expand your field of view as wide as possible.

Here is a breaker panel taken from about 5 feet away, first using a standard lens. Again, in this case, standard is 17.5°x13°.


And this is the same panel, taken with a 35°x26° wide angle lens.
 

You do not get the detail you may want, but you can quickly see the entire panel. By scanning at a wide angle, you can identify areas of interest that you may want to come back and do a deeper investigation on.

Here is a 320x240 resolution camera looking at a building. The first two pictures show one side versus the other. The third picture shows the entire building. Being able to see more gives you a lot more information at a glance.




 

Let's get small

If you want to get close-up and see details of very small objects, you will want to look into a macro lens. How small can you see? Down to 25 microns, with the right lens. Macro lenses for thermal cameras are typically used for research and development applications. This will help give you a very clear understanding of the effects of heat on your specific device.

 

It is also excellent for designing electronic circuits and components. You can see the transfer of heat as components, circuits and chips get powered up.

If you would like more information, here are some great guides:
Choosing the Right Thermal Camera Lens
Telephoto Thermal Camera Lens Application Notes
Thermal Camera Macro Lens Application Notes

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