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CliveWard

Choosing a thermal rifle scope

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Hi All,

There's been a bit of talk of thermal rifle scopes here recently, so I thought this info might be useful for anyone considering a purchase.

 

You'll have to excuse the commercial bias but it's taken from our blogger site but you can stick any manufacturers numbers in you fancy and hopefully get a better understanding of how it all works together.

Anyway, deep breath, eyes down. icon_e_wink.gif

A thermal rifle scope is a considered purchase and it helps to understand the technology and how that relates to the performance of the device in the real world. Sometimes a high specification number such as resolution may look advantageous and be promoted as the 'next big thing', but can actually make no difference or even be a hinderance depending on your application.

Lets run through the major components in the optical path and try to understand them better and how they affect image quality.

Objective lens:
The objective lens presents the image to the FPA (Focal Plane Array) or more commonly termed, Thermal Core. Lenses come in varying focal lengths to magnify the image that is presented to the FPA. The longer the focal length of this lens, the more magnified the image the FPA receives. Another key factor in the specification of the objective lens is it's aperture or F number. A lens with an F1 aperture will have considerably more thermal sensitivity than a lens with an F1.4 aperture. Thermal sensitivity is important because it enables the device to resolve small differences in temperature which is important in cold damp or foggy conditions.

Thermal Core (FPA):
The thermal core converts the heat radiation from the objective lens into a digital image signal. There are two types of core, shuttered that require a mechanical shutter to cover the input of the FPA to perform a calibration and shutterless that require no calibration at all. Shuttered cores can be set for automatic calibration or manual calibration. In both instances the image seen through the scope will degrade until the calibration is carried out, where the image will 'freeze' for a short period of time. With a shutterless core, the image is constantly being corrected and never suffers from degradation or need for calibration.

Pixel pitch and resolution:
Pixel pitch is the biggest factor of how much detail a thermal rifle scope can resolve. It essentially is the size of the individual pixel in the FPA. The smaller the pixels, the smaller the details the scope can resolve. There is a simple equation to calculate this resolution for any thermal device:

Pixel Pitch (Microns) / Objective Focal Length (mm) = Pixel Field of View (MRAD)

1 MRAD = 10cm at 100m

For example the the WT1 75-3 or WT1 75-6 would be 17/75=.226 MRAD or 2.26cm at 100m. You can see we have ignored resolution. On both scopes the pixels are the same size, just that one has more of them.

So if you have a target at 100m that is made up of say 200 pixels on the FPA, in the higher resolution scope it will appear half the size as it has twice as many pixels that it needs to display in the eyepiece. The target itself won't be any more detailed.

The only benefit of a higher resolution FPA is the ability to have a low magnification, wide field of view, but still retain image quality when using digital zoom to get the target to an apparent size that makes it easier to shoot. For most shooters, this facility isn't necessary. With this wider field of view though the exposure control of the FPA is working on the entire field of view which can affect the quality of the actual target you are trying to shoot if the scene is very contrasty.
Detection / Recognition / Identification ranges:
Using the pixel pitch of the FPA and the focal length of the objective lens, an online calculator has been created using industry standard methods to allow users to compare specifications and useable ranges.

https://jscalc.io/calc/n6XzdHX2iC6dtIUt

Input the focal length, pixel pitch and target type for a range estimation. Then you can input focal lengths and pixel pitches of various devices to compare how they will perform. Recognition 70% is a reasonable criteria for matching real world observations on a fox sized target where you would be sure of taking a shot.
Eyepiece display:
This is where it all comes together. The eyepiece display is where the image is relayed to the shooters eye. If you are comparing scopes with similar focal lengths, pixel pitches and resolution, the scope that quotes the higher optical magnification will usually have a higher resolution and physically larger eyepiece display making accurate placed shots easier.

Choose the right tool for the job:
Finally it's worth mentioning choosing the right tool for the job in hand. On a long range foxing rifle a 75mm 17 micron cored scope such as the WT1 75-3 would be the 'ideal'. For a lighter weight gun for targets out to 200m then a 50mm 17 micron scope would be more suitable and offer excellent performance in a very compact package.

Ignore resolution of the FPA, unless you have a specific requirement for a very wide field of view. You won't see any more detail at range with the more expensive higher resolution scope.


Cheers





Clive

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Clive, thank you. For someone like myself with little understand of thermal scope specs that explanation is very helpful.

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Clive, thank you. For someone like myself with little understand of thermal scope specs that explanation is very helpful.

Hi MJR,

 

No problem at all. It's worth knowing.

 

We've had a 640x480 resolution 75mm thermal rifle scope available for a decent price for a year now, but pretty much to a man we advise customers not to buy it unless they have a specific need for the wide field of view on the bottom end and to buy the less expensive 384x288 because it sees exactly the same amount of detail downrange and gives a more useful magnification range for the more usual, small distant targets.

 

We have also been able to offer a 640x480 resolution rifle scope with a 50mm lens for the same amount of time. We never did because the native magnification is so low in this combination it makes it pointless for the UK market.

 

 

Cheers

 

 

 

 

 

Clive

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