Optical, NV and thermal imaging comparison

Optical, NV and thermal imaging comparison​

I really don't hope that the following few posts are regarded as pretentious rubbish by the good members of the SD forum
That is not their intent
Rather, since there appears to be an ever increasing interest in night vision (NV) and thermal spotting and shooting of targets after darkness that i thought some explanation of how these technologies work might be useful for both new comer and maybe some more experience users as well
I'm pretty sure not all will agree with the everything i say and I'm more than happy with that situation - disagreement usually gets to the root of a situation more quickly and often leads to a deeper understanding of a situation- which is beneficial for all involved
Anyway, here's the first part

Comparison of Optical, Night Vision and thermal devices

• In terms of image detail , glass is still king and likely to be for the foreseeable future.
• Digital sensors have yet to approach the acuity of the human eye when stationary behind a glass scope so it is expected that glass will continue to be the first choice for target and long range shooting (and, for some, daylight deer stalking)
• Many glass scopes can provide optical zoom which although it reduces field of view can and does significantly improve target detail.
• At present, and in the foreseeable future, true optical zoom is not a feature which Is likely to become available on digital scopes
• The major shortcoming of glass scopes is that they are limited to use in visible light. Although the very top end scopes can work adequately well in the lower light conditions which exist at dawn and dusk.
• However, when ambient light levels become too low, a glass scope is essentially blind (except with the use of external lighting such as a lamp)
• Given that many of our quarry species are more active at night (plus a law change in Scotland now allowing the shooting of deer using night vision and thermal scopes) both Night Vision and thermal imaging devices have become increasingly popular methods of allowing hunting to continue through the hours of darkness.
• Both night vision and thermal devices are correctly termed digital devices because they use digital sensors to convert light or heat from the natural world into electrical signals which can be made visible for viewing by the human eye, even although the original light or heat may not be visible to the human eye
• The main difference between night vision and thermal devices is the wavelength of radiation at which they are designed to operate
• Digital scopes which work on both visible light and near infra red light operate with wavelengths between 0.4-1 microns, with visible light generally regarded as covering the wavelength range from 0.4-0.8 microns
• At these wavelengths, incoming photons have sufficient energy to cause electrons to be released from the sensor and create the electrical signal which forms the basis of the image seen by the user
• Thermal scopes are designed to work on long wave infra red radiation with wavelengths centred around 10 microns because this is the wavelength of the heat emitted by our quarry species. At this wavelength the photons have 90% less energy than visible/near IR photons resulting in the need for a completely different type of sensor
• This large difference in operating wavelength results in completely different materials being used for the lens and sensors of each type of device
• A digital scope can use standard glass lenses and widely available and cost effective CMOS sensors (essentially the same as found in commonly available digital cameras) while thermal devices must use much more expensive germanium lenses and much lower resolution (and much more expensive) amorphous silicon or Vanadium oxide sensors correctly known as an “uncooled microbolometer”
• It is this major difference in the cost of lenses and sensors that mainly accounts for the large price differences between night vision and thermal devices
• Pixel size in digital and thermal devices.
• From our earlier discussion we know that image detail improves as the sensor pixels become smaller and the focal length of the objective lens increases
• To add to this we need to understand that to obtain a useful signal from a sensor, the physical size of the sensor should not be smaller than half the wavelength we want to detect.
• So, if we want to detect a wavelength of 10 microns, the pixel size should not be smaller than 5 microns and if we want to detect a wavelength of 1 micron, the pixel size should not be smaller than 0.5 micron
• This takes us to the reason why thermal is unlikely ever to produce as detailed an image as night vision (let alone glass!)
• The sensor pixels in our current generation of digital scope sensor are typically 1.55 – 3 microns (HIK Alpex 4k uses a 2 micron pixel sensor) which is already smaller than the 5 microns any thermal pixel is likely to be in the foreseeable future
• As an example, if we compare a typical thermal scope with a 12 micron sensor and 50mm lens to a HIK Alpex 4k which uses a CMOS sensor with 2 micron pixels and also has a 50mm lens, we can use our simple calculation to show that at 100m a single thermal pixel can “see” a square with sides 24mm long.
• Doing the same calculation for the Alpex 4k shows that a single Alpex pixel “sees” a square with sides 4mm long
• In other words the physical detail in the Alpex 4k image is 36 times greater than in the thermal image
• In daylight mode the Alpex image will contain both brightness and colour information which makes target identification for the human eye significantly easier
• At night, using an infra red light source, all colour information is lost and for target identification the user only has the contrast between the quarry and it’s background for identification. If some of the IR is reflected from the eyes of the quarry it will become much more visible. IR is reflected and scattered by most organic material such as grass, bushes, undergrowth etc and this can cause quarry located near or in this organic material to be very difficult or impossible to see clearly
• Although a thermal image is unlikely ever to better the physical detail of a digital image, the ability of a thermal imager to display very small differences in temperature more than makes up for its physical image shortcomings, frequently allowing quarry to be easily detected at considerable ranges when, under the same circumstances, detecting the same quarry at the same range with NV would be much more difficult or impossible
• The common refrain of “spot with thermal, shoot with NV” has its origins in this difference between the two technologies
• Which method to use - glass, NV or thermal?
• This is very, very much a user choice based on many factors but ultimately coming back to far fewer.
• IMHO the choice of method should not start until the answer to these three questions is yes
• Is it legal?
• Is it safe?
• Is it humane?
• Once legality and safety are assured and the risk of an inhumane kill minimised, then it is down to the user to decide which method best suits the circumstances. All three can be perfectly acceptable, whilst in different circumstances, none would be acceptable.

 

[Nickb]

Very informative.

 

[mealiejimmy]

Part 2
Digital scope image modes – daylight colour. NV, no IR, NV with IR

• Wavelength ranges
• Our eyes are sensitive to wavelengths in the range 0.4-0.8 microns
• We don’t suddenly start to perceive light at 0.4 microns and then suddenly stop perceiving light when it reaches 0.8 microns.
• It is gradual process and the sensitivity of our eyes to visible light follows a fairy predictable cosine curve as shown below
• 
  
• In normal daylight the human eye is most sensitive to green with a wavelength of around 0. 55 Micron
• The CMOS (Complimentary Metal Oxide Silicon) sensors we typically use for digital/NV scopes are sensitive to wavelengths in the range 0.35-1.05 micron
• The wavelengths from around 0.8-1 micron are known as “Near IR” and typically used for digital NV with 0.85 micron and 0.94 micron being, by far, the most common
• It is this ability of CMOS sensors to convert near IR photons into electrical signals that is the working principle of all digital NV
• A typical CMOS sensor response curve is shown below
• 
• 
  
• The energy of photons with wavelengths above 1.05 micron is insufficient to cause electrical signals to be produced by a CMOS sensor
• Although CMOS sensors are not as sensitive to NIR at 0.85 or 0.9 microns as they are at the centre of the visible range, much work has gone into improving their NIR performance and we now have sensors more than capable of producing high quality NV images

• 
  
• Visible photons contain both colour and brightness information (colour in the wavelength and brightness in the number of photons)
• However, when a photon hits a CMOS sensor only brightness information is transmitted – all colour information is lost (the electron doesn’t know the wavelength of the photon that hit the sensor
• To produce a colour image from a CMOS sensor requires a red, green or blue colour filter to be placed over every sensor pixel followed by some very clever mathematical algorithms to recreate a colour image
• The most common colour filtering scheme is known as a Bayer filter and it’s general schema is shown below

• 
  
• 
  
• Because our eyes are most sensitive to green light the most common filter colour is green
• With this scheme, each pixel can now produce both a colour and brightness signal.
• However to produce a colour image close to or the same as seen by the human eye takes a great deal of processing power which usually results in the loss of some or most of the signals from some pixels.
• In normal daylight conditions these signal losses due to the filtering and processing of the signals from the individual pixels are easily made good by the intensity of the daylight reaching the sensor.
• However, as ambient light levels drop as dusk and darkness approaches, these signal losses due to filtering become significant and the quality of the colour image degrades
• 
  
• Another effect that must be considered when using a digital scope in daylight mode is the unwanted effect of the sensor detecting ultra violet and NIR radiation. Both of these wavelengths will badly effect the final colour image that the sensor can produce so it is vital that, for daylight colour imaging, both ultra violet and NIR are blocked from reaching the sensor.
• This is typically done using a small glass plate placed close to but moveable from the surface of the CMOS sensor. One side of the plate is coated with a filter material that blocks ultraviolet light while the other side of the plate is coated with a filter material that blocks NIR
• With this filter in place, only visible light reaches the sensor allowing an accurate daytime colour image to be produced
• 
  
• 
  
• When the digital scope enters NV mode, the filter is automatically moved away from the sensor allowing any ultraviolet and NIR present to reach the sensor
• The clicking noise commonly heard when moving between day and NV modes is simply the small glass filter moving into and out of position in front of the sensor
• Entering NV mode also causes the following to happen:
• The image becomes black and white
• The complicated algorithms used to create the colour image are switched off and all of the signals from each pixel are used to create the image
• The combination of the UV/IR cut filter being removed and all of the signals from each sensor pixel becoming available, increases the brightness level of the image and it is very often possible to see quarry quite clearly in this mode without any use of IR illumination
• It is probable that it will become sufficiently dark that even NV mode without IR will be insufficient to produce a usable image and at this stage some IR illumination will be required
• IR illuminators
• Currently, two types of IR illuminator predominate. These are LED (light emitting diode) and VCSEL (vertical cavity surface emitting laser)
• In general, LED is better for shorter range work where it provides sufficient illumination and a “cleaner” less speckled beam, whereas vcsel IRs tend to be used for longer range work where it’s significantly higher power output allows quarry at greater ranges to be seen clearly.
• It should be noted that there are no safety concerns with any currently available LED based IR.
• Because of their significantly higher light output power, care should be exercised when using vcsel IRs and they should NEVER be viewed directly from close range

 

[mealiejimmy]

Part 3

Components of a digital scope and calculating total scope magnification

All digital scopes contain the following components

• Objective lens
• The purpose of the objective lens is to focus any radiation entering the lens onto the digital sensor located behind the lens
• The lens works by bending the light passing through the lens so that it all points towards the same point in space
• The amount that the light/heat can be bent depends on the shape of the lens and the material from which the lens is made
• Refractive index is the name given to describe how much a given lens material can bend light/heat.. Refractive index varies slightly with different mixtures of glass (and this difference is used extensively in making multiple element lenses) but is around 1.5-1.6
• The refractive index of germanium is over 4.0 and this is indeed an advantage when making germanium lenses for thermal imagers since it allows simpler manufacture of good quality low f number lenses
• The main features of the lenses used in NV/thermal systems are as follows:
• Lens material – glass for daylight/NV, germanium for thermal
• Lens focal length – this is the distance behind the lens at which the radiation passing through the lens comes to a focus. Focal length is almost always specified in mm.
• Focal length is one of the most frequently used and most important parameters of a lens. Lenses can be made from single glass or germanium elements or multiple glass or germanium elements to improve their performance
• Lens aperture – aperture describes how much light can enter the lens. Mathematically the lens aperture is the focal length divided by its entry diameter
• For example a lens with a focal length of 50mm and an entry diameter of 25mm would have an aperture of 2.0 (50/25) This aperture would normally be written as f 2.0
• Note that smaller f numbers mean larger apertures and therefore more light/heat passing through the lens and reaching the sensor
• Typical traditional cameras and many modern digital camera have lenses with apertures that will go from f1.8 – to more than f 16. These high f numbers are perfectly normal due mainly to the large number of visible photons in daylight
• In NV scopes, the apertures of objective lenses are typically larger (lower f number) so that more light can pass through the lens and reach the sensor
• Typical apertures for NV scopes are in the region f1.2 – f1.4
• Because of the nature of the sensors used in thermal scopes, lens aperture is even more critical and most thermal scopes have objective lenses with apertures of f1.0 or even f0.9 (where the diameter of the lens is greater than the focal length)
• Depth of field
• This refers to the linear distance within a field of view where all objects are in focus. In simple terms, the larger the f number, the greater the depth of field
• With the low f numbers associated with NV and thermal scopes, it is to be expected that frequent re-focussing for targets at relatively closely spaced ranges would be required

• Sensor
• In many ways the sensor is the heart of a digital/ thermal scope since it is here that photons of light/heat from the outside world are converted into the electrical signals which (eventually) form the image we see when we look at the scope display
• Daylight/NV scopes use CMOS (Complimentary Metal Oxide Silicon) sensors which can directly convert the energy of incoming photons releasing electrons which form the basis for the image seen at the end of the process. These incoming photons have wavelengths in the range 0.4-1 micron
• Thermal scopes work at much longer wavelengths (around 10 microns) and at that wavelength the photons have insufficient energy to release electrons from a CMOS sensor
• Photon energy is directly proportional to frequency (and inversely proportional to wavelength) meaning that a visible/NV photon at 1 micron has around 10 times more energy than a thermal photon at 10 microns
• This large discrepancy between the energy of visible/NV photons and thermal photons has led to the development of an entirely different type of sensor capable of detecting thermal photons
• This type of sensor (used in thermal spotters and scopes) is correctly known as an “uncooled microbolometer” and is based on sensing pixels which display large changes in electrical resistance for very small changes in temperature.
• A small electric current passes through each pixel and as it’s temperature changes with the amount of heat it receives, a varying voltage is developed across the ends of the pixel and it is this voltage which (after much signal processing) forms the basis for the image the user sees on the display
• Cooling the sensor significantly improves it performance, but the weight, size and power requirements of a cooling pump to get the sensor down to -200 degrees centigrade is completely impractical for a hand held or rife mounted device
• Cooled microbolometers are widely used in military and industrial applications where the limitations of weight, space and power do not apply.
• 
  
• Signal processing electronics
• Perhaps the best way to think of the signal processing electronics is simply to consider it a “black box where very clever stuff happens”
• Signals from the sensor enter at one end of the black box, we connect batteries and buttons to allows us some control and signals come out of the other end which drive the display and allow us to see what the sensor sees
• 
  
• Display
• Like the sensor, the display consists of pixels with the main difference being that these pixels convert electrical signals into light which we can see
• The materials used, the physical size of the display and the number of pixels in the display has improved and increased over the years, resulting in current displays producing much better quality images than earlier models.
• However, even, these larger displays are too small for the unaided human eye to see the level of detail that can be displayed and some sort of magnifying lens assembly is required
• 
  
• 
  
• Ocular lens assembly
• This is the lens assembly placed between the display and the human eye to allow the display to appear large enough for the unaided human eye to view it comfortably.
• All ocular lens assemblies are adjustable (dioptre adjustment) to allow the vast majority of human eyes to be able to see the display in focus.
• In fact, setting the dioptre for your eyes so that the display is in focus is the first thing that should be done when using the scope
• Once the display is in focus, the target can be brought into focus using the focus adjustment of the objective lens
• Note that when fully focussed (ocular and objective) the image plane (the display) is the same for both images so there is no parallax error.
• Eye relief – it should be noted that most digital/thermal scopes have much less eye relief than typical glass scopes: 45-50mm is not unusual compared to 90-100mm for many glass scopes
• Users need to be aware of this to prevent possible injury on rifles with significant recoil
• 
  
• Calculating total digital/thermal scope magnification

The four values needed to calculate total magnification are;

• Focal length of the objective lens
• Diagonal size of the sensor
• Diagonal size of the display
• Focal length of the ocular lens
• (all values in mm)

The total magnification is given by (Focal length of objective/diagonal size of sensor)* (diagonal size of display/focal length of ocular)

In almost all cases the focal length of the objective lens will be known and the diagonal size of the sensor easily calculated, The diagonal size of the display can usually be determined, but the focal length of the ocular lens is very rarely given.

Using the manufacturers specification for total magnification is often needed to calculate the focal length of the ocular lens

Examination of the formula and plugging in known values leads to some simple conclusions

• Increasing the focal length of the objective lens increases total magnification
• Reducing the size of the sensor increases total magnification
• The ratio of objective focal length to sensor size has the largest effect on the total magnification
• The limitations of the human eye mean that there is a limited range of values that the ratio of the display size to ocular lens focal length can take. Measurements of the total magnification of many digital scopes show display size to ocular focal length ratios in the range of 0.6-0.8 are usual. As display sizes become larger, this ratio tends to increase slightly
• Example: a thermal scope with a 12 micron 640x512 sensor and a 50mm lens has a diagonal sensor size of 9.835mm, so the objective lens to sensor size ratio is 50/9.835 = 5.08
• Given a typical display size to ocular lens ratio of around 0.7 the total magnification is 5.08*0.7 = x3.556 which is very close to the manufacturer specified value of x3.6
• Note that the actual dioptre setting of the ocular lens can have significant effect on the total magnification although this effect may not be noticeable due to the variations in visual acuity between users
• 
  
• Digital Zoom
• Digital zoom is the method by which a digital scope makes an object appear larger.
• Referring back to the calculation for total magnification it is easily seen that making the effective size of the sensor smaller will achieve this goal.
• Making the effective size of the sensor smaller is achieved by ignoring or switching off sensor pixels and creating an image from the remaining central working pixels which is spread over the full size of the display
• For example a 12 micron sensor with 640x512 pixels will use all of those pixels to create an image when set to it’s base magnification
• When set to x2 digital zoom, the image will only be produced by the central 320x 256 pixels, but that image will be spread over the full size of the display
• It can be seen that when x2 digital zoom is used, the image is created from only 25% of the original sensor pixels with 75% of the potential information discarded.
• As digital zoom is increased, fewer and fewer sensor pixels are used to create the image seen on the display
• Using fewer sensor pixels to create the image and continuing to spread that image over the full display leads to a significant reduction in image sharpness, commonly known as “pixellation”
• The cost and complexity of making a digital scope with optical zoom that will retain zero at all magnifications against the essentially zero cost option of digital zoom means that it is very unlikely that digital scopes with true optical zoom (like a glass scope) will become available any time soon, if ever.
• Having said that, the current range of NocPix Ace thermal scopes do have a very rudimentary optical zoom fitted to the ocular end of the scope
• This is not a true optical zoom in that the limit on image detail set by the size of the sensor pixel and focal length of the objective lens still exists, but using a large display with many pixels and then using a relatively low power zoom lens (x1-x3) to make the image on the display larger can make the target appear larger without increasing pixellation
• At present the jury is still out on the usefulness of this feature – some like it, others ignore it
• This magnifying effect may be likened to watching TV from a much closer distance than normal

 

[mealiejimmy]

Part 4 (final part)

Simple maths for Night Vision and thermal imaging

Night vision and thermal devices all use sensors to convert light or heat to electrical signals which can then be processed to create an image visible to the human eye

• Every sensor comprises individual sensing elements called PIXELS (short for Picture Element)
• The smallest thing any night vision or thermal device can “see” is a single pixel
• This means there is a physical limit to the level of detail that any sensor can produce
• At any given range, a single sensor pixel can “see” an object with a size given by this following simple formula
• Minimum object size= (pixel size /objective lens focal length) * distance to object
• Example: A thermal scope with a 12 micron sensor, 50mm lens and a distance to the object of 100m
• Answer = (12/50) *100 = 24mm
• So, any thermal scope with a 12 micron sensor and 50mm lens cannot “see” anything smaller than a square with 24mm sides at a distance of 100m.
• Currently the standard size for thermal sensor pixels is 12 microns, so the minimum size of object that can be seen with a single pixel depends purely on the focal length of the objective lens
• Here are some examples of the minimum object size visible for thermal imagers with different objective focal lengths
• 6mm = 200mm
• 10mm = 120mm
• 15mm = 80mm
• 19mm = 63mm
• 25mm = 48mm
• 35 mm = 34mm
• 50mm = 24mm
• 60mm = 20mm

The smaller the answer, the more detailed the image

6.Making the answer smaller requires either or both smaller sensor pixels and/or longer focal length lenses

The industry standard for pixel size has been 12 microns for 3 or 4 years now (having reduced from 25 microns, then 17 microns down the current 12 micron level in the previous 8-10 years)

Smaller pixels will eventually arrive but don’t expect anything anytime soon, and when it does arrive it won’t be cheap and almost certainly won’t have the levels of temperature detail we currently enjoy

Bigger lenses are easier and we are starting to see lenses bigger than the 50mm that we have become accustomed to. There are several thermal scopes with 60mm lenses now available and even a couple of scopes with 640 sensors and 75 or 100mm lenses

Given that a thermal scope has to be mounted on a rifle and often carried around, there are limits to the physical size and weight of “big lens” thermal scopes

8.Note that the number of pixels the sensor contains did not come into calculation. The size and number of pixels in the sensor sets the overall size of the sensor and this value is used to determine field of view and, to a large extent, overall magnification

More sensor pixels provides a wider field of view (and less total magnification) but having more “pixels on target” does allow a clearer image of a target to be seen

9. The electrical output from a single sensor pixel eventually becomes a dot or dots on the display which indicates how much heat has been detected. In white hot mode, more heat is displayed as a whiter dot (or dots). In black hot mode this is simply inverted with more heat showing a darker dot. Typically there are 256 discreet shades of grey between pure black and pure white. When colour mode is used, rarely are more than 8 or 16 colours available.

The ability of the human eye to distinguish fine tonal detail and the lack of colours available, generally means that an image can more clearly be seen using white hot or black hot modes rather than any of the colour modes. Having said that, if you like a particular colour mode and find it useful, then use it

10. Detection range should be the same for all thermal imagers with the same size sensor pixels and the same focal length lenses. In practice, because manufacturers calculate external factors differently (target size, probability of detection, etc) detection ranges for thermals with the same size pixels and same size lenses are often different

Buying one thermal rather than another based purely on detection range is discouraged

11.Thus far, this discussion has been mainly concerned with physical image detail, but thermal imagers are essentially devices that display differences in temperature, so some discussion related to this aspect of their performance is needed

Along with pixel size and the number of pixels in each sensor, manufacturers specify temperature performance and this is normally written as NETD

NETD stands for Noise Equivalent Temperature Difference and is a measure of how close in temperature two objects can be and still be seen as two separate objects

NETD is quoted in mK (MilliKelvin) which is a very, very small temperature difference. A one degree centigrade difference in temperature is 1000mK, so a 15mK NETD means the thermal imager can ”see” differences in temperature of 15/1000 degree centigrade

By comparison, the smallest temperature differences we can feel with any part of our bodies under absolutely ideal conditions is no better than 70mK

NETD values have become lower as thermal imaging technology has improved, dropping from 50-80mk a few years ago to as low as 15mK today

Lower NETD values allow smaller differences in temperature to be seen

This improvement is most visible in poor thermal conditions

The level of moisture in the atmosphere between the target and the thermal imager has the greatest overall effect on the temperature performance of a thermal imager. More moisture between the target and thermal imager results in more absorption and scattering of the heat before it reaches the thermal imager

Thermal imagers give their best images in low humidity conditions

When humidity levels are typically less than 90%, there will be little visible difference in the temperature detail of an image from thermals with quite a wide range of NETD values

Between 90-94% thermal performance will degrade, with live quarry remaining visible, but background detail becoming less and less visible.

In my personal experience when the humidity reaches 95% it’s time to go home

Most of the live quarry we look at with thermal imagers is considerably warmer than their surroundings so detecting animals against their background is not a particularly difficult task even for a thermal with a high NETD value

Seeing the background clearly and putting the quarry into the context of that background with a view to a potential safe and humane shot – particularly in poor thermal conditions is where a thermal with a low NETD value can prove useful

 

[cjm1066]

Thanks for sharing these articles - very informative and interesting.

 

[Ajax]

Great write up, absolutely awesome

 

[rem284]

You are truly an amazing asset to many on here and other forums. Thank you

 

[TringSaint]

Bruce,
I know you are the authority on the Zulus range of NV scopes, so I was keen to get your thoughts on the thematec units and whether the entry price unit was any good or did you need to really look at the mid or top price unit if you wanted to get a scope that really worked for both thermal and NV?
Thx
Steve

 

[Freeforester]

Great sticky stuff there Bruce, thanks for ‘enlightening us.

Now, does anyone know where I can get a bit of luminous paint for my front bead?

 

[Polar Lynx]

Very impressive. Now we all want to know what B. sees in his crystal bowl i.e. what will come. Solution of fusion images f.e. Other materials apart from germanium etc.