Thermal Cameras - Optical vs Digital Magnification
Is it worth paying more for a bigger lens and achieve higher optical magnification? Or having only digital zoom is enough to get the job done? These two parameters play an important role in Detection, Recognition and Identification capabilities of thermal imaging devices. Let’s learn more and draw a thick borderline that will help us distinguish between optical and digital magnification.
So what’s the difference?
Despite the seeming similarity between the termsoptical and digital magnification, they are a whole lot different.
The history of optical zoom goes back centuries when people used polished concave pieces of glass, lenses, in first prototypes of micro– and telescopes. From the word “optical” one can guess that optical magnification, or zoom, uses a lens system in order to bring an object image closer. It guarantees “lossless” magnification. Every lens has a key characteristic related to optical zoom: its focal length. The bigger focal length of the lens, the greater the optical zoom.
Digital zoom is a relatively recent invention. It was first introduced in digital cameras to enhance their performance capabilities and can now be found in almost any thermal imaging device. Strictly speaking digital zoom is not “zoom” at all. It is just a simulation of optical zoom. Digital image is comprised of thousands of individual dots, pixels. When you switch from zoom X1 to, let’s say, zoom X2 or zoom X4, what your device really does is it takes the central portion of the image and electronically enlarges it. Since the total number of pixels does not change, cropping and proportionally stretching the image makes the pixels bigger thus leading to poorer image quality.
Digital zoom: do big numbers mean better performance?
Some manufacturers implement really big digital zoom factors in their thermal imagers: X8, X16 or even more. It literally cost nothing for them to offer image scaling of any multiplicity. At the same time these large numbers make a false impression that you will be able to see more details and at longer range. But given the nature of digital magnification that we just discussed, what you will actually see with high digital zoom is just a few large pixels really. So if you wondered why your thermal image did not look that great at high digital zoom, now you know.
If the detector (FPA) of your thermal imager has resolution of 384×288, then at digital zoom X4 the image you see will effectively have resolution of just 96×72 pixels. So it becomes clear why having and using digital zoom factors higher than X4 makes little or no sense.
The key to achieving best possible results in detection, recognition and identification is in choosing the optimal combination of optical and digital zoom. Most thermal imagers on the market have fixed focal length, and therefore optical zoom, but allow to vary digital magnification. So with a little aid of digital zoom you can achieve the best results in any environment.