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The specifications used in advertising for Thermal Imaging can be very misleading.
For a start, you need to understand what the "quoted" magnification actually means as this can seem very impressive.
For example, 3 X - 24 X means 3 X optical magnification only. All the other figures are digital magnification and DO NOT / WILL NOT produce the same degree of clarity as optical magnification. Usually only the first step of digital zoom is of any real benefit.
More on magnification later.
A major feature in promoting a product is to state the "DETECTION" distance.
Detection means just that, detection, and NOTHING else.
The normal standard is to "detect" a man sized target (1.7 m x 0.5 m) or a NATO target 2.3 x 2.3m at a specified distance.
e.g. Man sized target @ 3330m, NATO target @ 4500m for a specific device.
All you will be seeing is a few pixels of a "hot" spot, you will NOT be seeing it as a "man", or whatever.
From there you have "RECOGNITION", the distance, where you can recognize that it is a man (or whatever) but not who it is.
Next, comes "IDENTIFICATION", again the distance, which is often not provided, this is considerably less than recognition. Identification is regarded as only identifying that a human "target" is a "good guy" or a "bad guy" not who it is.
Experience using thermal imaging will extend your own "Recognition" and "Identification" distances once you have become accustom to how a person / animal moves, and reacts, combined with what you might expect to see in your area of operation.
The figures provided for DRI (Detection, Recognition, Identification) are derived from the "Johnson Criteria" and indicate a range of distances, and are based on two probabilities, of, 50% and 90% of accomplishing the tasks above.
These figures WILL vary CONSIDERABLY with differing atmospheric conditions and therefore can NEVER be considered as absolute.
The Johnson Criteria is purely THEORETICAL, a CALCULATION ONLY, and has no bearing on real life situations.
When comparing figures provided by various manufacturers quoting the "same" specification products, some will state an average figure between the 50% and 90% probability distances, while other manufacturers, generally those of a more specialist nature will give a more conservative figure.
This does NOT indicate a lower specification item, but a more realistic expectation of detection distances achievable under average conditions.
For example one manufacturer may state a detection distance of 1800 m while another, a conservative 1660 m for a product of the same specification.
There will NOT be any difference between the units under the same atmospheric conditions.
Smaller "targets" will obviously need to be closer for all the above categories.
Another important piece of information is the operating or refresh rate,
e.g. 7, 9, 30, 50 Hz and so on. What this means to the user is how smooth and realistic a moving image appears. Anything below 25 Hz (Standard video camera frame rate) produces a lagging image on a moving target, especially a fast moving one.
At first thought it would be fair to say that a "new picture", @ say, 9 times a second, a commonly available rate for some imported products, would or should be fine.
Sorry, no it is not, OK maybe for casual observations but not much more.
The next thing to consider is the detector pitch and FPA (Focal Plane Array) size, thermal devices were commonly 25 microns with an FPA with 384 X 288 pixels.
While this was quite reasonable for most work, it can be a little coarse and the image suffers when using "zoom" magnification settings.
An FPA with 640 X 480 of 17 um compared with one of 384 X 288 of 25 um, the optical magnification will nominally be about the same due to the 25 um FPA being about the same physical size as the 17 um unit, but the difference in resolution is noticeable.
Admittedly it may hard to justify the extra cost involved ??
Now if you have an FPA with 640 X 480 and a pitch of 17 microns and one of 384 X 288, also 17 micron, using the same focal length lens, you will have approximately 50% higher optical magnification with the smaller FPA.
This is because the larger FPA is providing a wider field of view which in turn produces a larger overall image with a lower optical magnification.
A casual look at the provided figures, it may seem to suggest the 640 X 480 FPA will contain more pixels in the "wanted target" image than the 384 X 288 FPA, but this is not so.
The specific "target" within that image, at the same distance with the same lens, either size FPA, that "specific target" will contain no more or no less pixels than the other.
The smaller FPA will exhibit a slight degradation in image "quality" due to the greater "expansion" onto the viewing OLED screen, (depending on the inbuilt software) but the higher magnification may offset any degradation (??).
When you investigate the D.R.I. figures for the difference between models with the same pixel size, and the same focal length and lens speed, you will find the D.R.I. figures are identical. There is no loss of DRI.
There is an odd manufacturer or two advertising 12 micron pitch.
Be aware that they have an FPA much smaller because of the smaller pixels, often coupled with a lower number of pixels, so when you expand the image to the OLED where you have an image large enough use, it will have no better overall image quality than a unit with 17 micron pitch!
You need to note the pitch of the detector and not confuse the pixels specified with that of the "viewing screen" specification.
Sensitivity and the lens. The "f" number of the lens governs the amount of thermal energy entering the unit just as it does with light in a camera. Consider heat and light as being the same, they are the same, other than their wave length.
The "better" manufacturers of quality thermal use f 1.0 to f 1.3 lenses against higher numbers of lesser makes.
Knowing the SYSTEM sensitivity is more important to the user than just FPA sensitivity.
System sensitivity is determined by multiplying the FPA sensitivity in milliKelvins by the the lens "f" number squared. For example, a unit with an FPA sensitivity of 50 mK using an f 1.0 lens will have an system sensitivity of 50 mK. e.g. 50 x 1.0 x 1.0 = 50, while an f 1.3 lens would be 50 x 1.3 x 1.3 = 84 mK. If you jump to say, an f 1.5 lens you will now have a system sensitivity of 112 mK. This shows the decrease in the system or overall sensitivity with the "slower" lenses.
(Formula supplied by people far more knowledgeable than me).
Don't be too alarmed about overall system sensitivity loss, (within reason), with other than low overall specification products, while it is certainly reduced by the use of "slower" lenses, the only feature that is affected is the ability to see very small differences in temperature, e.g. the shading or slight temperature variations over an animals body.
Remember, all you would encounter is only 30 - 40/1000 thousandths of one degree difference!
An "animal", for example, will still stand out well against the general surrounding countryside.
A far more important factor in overall image quality, (image detail) is governed by the built in firmware / software in the device.
With some currently produced thermal devices having a system sensitivity of around 100 mK will produce better, sharper and brighter images than a device lacking up to date "enhancements" that has a 50 mK system sensitivity!
There will be some "visible" losses of lower temperature things like trees and shrubs as they cool later into a cold night and early hours of the morning, or just plain cold nights, but the detection distance for animals will not be of great concern.
In practical terms, while the overall sensitivity is slightly lower, there is no user that will "see" the difference based on sensitivity alone.
Be aware that "lesser manufacturers" have their sensors tested at temperatures HIGHER than industry standard of 30 degrees Celsius to obtain a "better" sensitivity figure than their competitors. A HIGHLY DECEPTIVE PROCESS !
Kelvin, just to explain - One degree Kelvin is equal to one degree Celsius, they just start at different points.
E.g. 0 degrees C = 273.15 K, and 1 degree C = 274.15 K, both up by one degree, and so on, so you can see from that, in each system one (1) degree is the same.
Kelvin is the scientific system for measuring absolute zero. e.g. Minus 273.15 degrees Celsius.
1 mK is 1/1000 of a Kelvin. So with an overall system sensitivity of 60 mK for example, you can detect temperature differences of 60/1000 or 0.06 of one degree of temperature!
Have you ever wondered why some thermal items are so much cheaper than others? The lens in quality items is Germanium, NOT glass, and they are expensive. It is common to use a small diameter lens (slow speed) to save money.
Some cheap models use lenses that are not Germanium but another cheaper material that has an even lower efficiency than Germanium in passing the required infrared wave lengths for detection.
There are other factors as well, such as poorly ground lenses, also cutting down on costs, which causes the passing light / heat rays to disperse slightly and therefore not coincide exactly at an infinitely small point at the focal length, which in turn prevents them coinciding at the FOCUS point, so therefore can NEVER produce a clearly focused image regardless of how much you try!
Do NOT believe all the claims made by most suppliers, that you can see through fog, smoke, rain, etc.
Well, OK, yes you can, but with reduced performance (D.R.I).
Even atmospheric pressure alters the performance capability of thermal imaging (and all other types of NV as well).
Remember, anything, including foliage, that completely prevents heat being "seen" will prevent detection with thermal. Any obstruction to the line of sight will prevent detection with all optical equipment regardless of type.
The real benefit of thermal imaging over other forms of night vision is that you need NO light at all, visible or infrared, and at the same time it is possible to see objects that are hidden in shadows or partially obscured by foliage.
As just a point of interest, thermal will have "white hot" and "black hot" options, white hot is great for scanning for a target as the eye and brain will react to a white object quicker than a black one, but once an object of interest is located, changing to black hot tends to sharpen the image outline.
High end commercial and professional makes will change the reticle "colour" according to the white hot / black hot polarity. e.g. black reticle on a white hot target and a white reticle on a black hot one.
On the subject of black / white hot etc., colour "palettes" are available with most thermal imaging products today. These may seem attractive to some users, it is my opinion that they are more of a gimmick than a real benefit. While I do have a number of units with this option, they ALL remain on the monochrome setting, only changing the option of "black hot" or "white hot".
Something to think about. There are some "experts" in the industry who make statements about thermal, who obviously do not understand the subject.
One in particular who claims to be a major military supplier does not understand frequency / wave lengths. In his explanation of what it is all about, he makes HORRENDOUSLY incorrect statements about which way Infrared frequencies and wave lengths change with temperature change.
He has now even gone to the extent of using "bold" type to highlight his incorrect statement.
NOTE. THERMAL IMAGING FIREARM SIGHTS cannot be exported from the USA without the State Department approval due to ITAR controls.
Therefore any "AMERICAN BRAND" thermal rifle scope available commercially in Australia will NOT be made in the USA it will be a re-branded "Foreign" product.
Beware of those brands that list a page of "extras" but virtually NO actual specifications!
Site updated, SEPTEMBER 22, 2021.