NV Tecnologías

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GENERAL SETUP OF DIGITAL DEVICES

In general a digital night vision device consists of an objective lens, light-sensitive sensor, blocks of electronic image processing and control, display and eye piece.

Power supply of digital night vision devices is performed with the help of replaceable power elements (batteries), rechargeable batteries of the same size or integrated rechargeable batteries. Devices can be equipped with a socket for obtaining power from external sources (e.g. car power grid, compact external rechargeable batteries).

For work in low light conditions digital night vision devices are often equipped with integrated Infrared illuminators based on laser diode or LED. For increased comfort of use digital night vision devices can include remote control system with major functions – in this case user can control the device with the help of remote control (RC).

Digital devices can be equipped with rails for mounting them on weapons. 

As in any optic observation device objective lens is meant for projecting image on the surface of the sensor which in its turn transforms reflected from the observed object light into an electric signal. 

As a light-sensitive element digital night vision devices utilize CCD or CMOS sensors.

CMOS SENSOR	CCD  SENSOR

Usually electronic processing block consists of one or several boards (depending on the configuration of device), on which they put specialized circuits, that perform processing of signal, obtained from the sensor  and further transmission of signal to the display where image of observed object is shaped. Boards contain major controls of device, they also contain power supply circuits of the whole unit and of separate circuit elements. 

Owing to the fact that digital night vision devices utilize micro-displays, in order to observe the image it is necessary to use an eye piece that operates as magnifying glass and allows viewing the magnified image.

The most often used displays in digital night vision devices are LCD displays of transmissive type (display is lighted up from the back side) or OLED displays (when electric current is applied its substance starts to emit light).  

Application of OLED displays bears a series of advantages: ability to use the device in lower temperatures, higher brightness and contrast of image, a simpler and reliable construction (backlight sources as in LCDs are not present). Apart from LCD and OLED displays digital devices can use micro-displays manufactured according to LCOS (Liquid Crystal on Silicone) technology – a type of reflective displays. 

In contrast to night vision devices based on image intensifier tubes (one can call them analogue), digital night vision devices permit to implement a larger quantity of user adjustments and functions. E.g., brightness adjustment, image contrast, image color selection, additional information in the field of view (current time, battery charge indication, icons of activated modes etc.), additional digital zoom, “Picture in Picture” function (permits to show in a separate small window additional image of the whole object or its separate part including magnified image), temporary display deactivation  (for energy saving purposes and masking of observer at the expense of absence of illumination from the operating display).  

For saving image of observed objects digital night vision devices may incorporate video recorders that allow making photos and videos.

In digital devices it is easy to implement such functions as wireless connection (e.g. Wi-Fi) data transmission (photo video) to external receivers; integration of laser rangefinders (data from the range finder can be introduced into the field of view), GPS-sensors (possibility to establish coordinates of observed object). 

To the advantages of digital night vision devices it is also possible to attribute the ability to work in the daylight conditions without fear of flashes of light or intensive light sources which may damage night vision device based on image intensifier tube. 

Reticle in digital riflescopes as a rule is also digital that means that the image of the reticle during the video signal processing is overlaid on the image in the screen and moved electronically which allows avoiding the necessity of mechanical parts for making ballistic corrections. These mechanical parts are often used in analogue night vision and daylight riflescopes and demand a high precision in manufacturing and assembling process. 

Additionally it allows avoiding such typical to optic or night vision riflescopes effect as parallax due to the fact that the observed image and image of reticle are located in the same plane – in the plane of display.

Relation

The larger is the focal distance of objective, size of display; the larger is magnification.
The larger is the focal distance of eye piece, size of sensor; the smaller is magnification.

1x	2x
3x	4x

RESOLUTION

Resolution defines ability of device to show separately two points or lines located close to one another. In technical references of device this parameter can be indicated as “resolution”, “resolution limit”, “maximal resolution”.  In principle it has the same meaning. Usually resolution is indicated in lines per millimeter (lpm) but it can also be indicated in angular units (second or minutes).   

The bigger is the value of resolution in lines per millimeter and the smaller is the value in angular units the higher is resolution. The higher is resolution; the clearer image observer can see.  

For night vision devices it is advisable to have resolution not less than 25 lpm – such resolution allows distinguishing the figure of a man from an animal or other object similar in size at 100m distance. 

For measuring resolution of night vision devices a special equipment - collimator is used. Collimator allows creating an imitation of image of special test-object –illuminated line test chart that is located at a certain distance (usually at 100m).

Line test chart

After looking at the image of test-object through the device a conclusion is made about resolution of night vision equipment – the smaller lines of test chart can be seen clearly separated from one another the higher is resolution. 

Normal resolution	Low resolution

Resolution is defined by the parameters of optic elements of device, sensor, display, quality of electronic circuitry solutions implemented in the device and also by the algorithms of signal processing. 

Overall resolution of the unit depends on the parameters of objective lens. Other things being equal the bigger is the diameter of the objective lens, the bigger its magnification and light-gathering power; the more small details it will be possible to see. 

Resolution depends on the resolution of objective lens and eyepiece. Objective lens creates the image of an object on the plane of the sensor and in case when resolution of the objective is insufficient further improvement of device’s resolution is impossible. In the same way low quality eyepiece is able to deteriorate the clearest image created on the display by other components of the device.

Large influence on the unit’s resolution is exerted by sensor characteristics. First of all it’s the sensor’s resolution – quantity of pixels (usually indicated as a product of pixels in a line and in a column) and their size.   

Relation:

The larger is quantity of pixels and the smaller is their size; the higher is resolution.
This statement is fair if sensors have identical physical size. Sensor with bigger pixel density on a unit of surface has a higher resolution.

Unlike black and white sensors resolution of color sensors in general will be smaller by 30-40%, which is caused by a different pixel structure – one pixel of color sensor consists of combination of three subpixels, each of them registers the light only of a certain part of spectrum (respectively - red, blue, green). It is achieved at the expense of application of colored light filters, letting through the light of only one color.   Thus when monochrome light hits the pixel of color camera the signal will be registered only by one subpixel only, at the same time in black and white sensor the signal will be registered by each pixel on which the light will get. This is one of the reasons why the application of color sensors in night vision devices is limited and often not rational.

Unit’s resolution also depends on the parameters of display which shows the image. Same as in the case of sensor the defining role here belongs to display’s resolution (amount of pixels) and pixel’s size. Density of pixels in the display is described by such parameter as PPI ("pixels per inch") – this characteristic defines the number of pixels that are located in one square inch of display’s area.

In the case of direct image transfer (without scaling) from sensor to display, resolutions of both parts would be the same. In this case the decrease of devices resolution is eliminated (resolution decrease happens when resolution of display is lower than resolution of sensor) and there is no need to apply expensive display (display’s resolution is higher than resolution of sensor) When sensor produces signal in standard analogue TV format (e.g. PAL(625 lines in a frame) or NTSC(525 lines in a frame)) the use of sensor with higher resolution than resolution of TV-signal format becomes unreasonable.   

Digital night vision devices can use different algorithms of useful signal processing which can influence the overall resolution of device. First of all one can speak of “digital zooming” when formed by sensor image is processed digitally and transferred to the display with a certain magnification.  In this case the overall resolution of device is decreased. Similar effect can be observed in digital cameras during “digital zooming”.  

Resolution of device is also influenced by “Binning” (algorithm that increases sensitivity of device by means of summing of signals from several neighboring pixels which results in proportional decrease of resolution). 

Along with indicated earlier factors it is necessary to mention several other factors which can lower device’s resolution. These are noises of different kinds that distort useful signal and in the end worsen image quality. It is possible to distinguish the following kinds of noise: 

Photon noise.  Is the result of discrete nature of light. Photons of light fall on the photosensitive sensor’s surface not simultaneously and not uniformly in space. 

Dark current noise (“snowfall noise”). If device’s objective lens will be covered by lightproof lid, it will be possible to see “dark” frames on the display.  The main reason of this noise is thermoionic emission of electrons (spontaneous emission of electrons in the result of heating of sensors substance). The lower is the temperature the lower is dark current signal that is smaller noise.

Transfer noise. During the transfer of charge inside the sensor a certain part of electrons that make up useful signal is lost. They are grabbed by defects and impurities that are present in the sensor’s crystal.

Readout noise. When signal accumulated in pixel is taken out of sensor, turned into a voltage and amplified each element acquires additional noise called readout noise.

For reducing noises digital devices apply different software algorithms of image processing which are often called noise reduction algorithms.

Apart from noise, electric interferences resulting from mistakes in configuration of device internal parts (position of electronic boards, connection wires and cables inside the device) or from mistakes in board layouts (position of conducting ways, presence and quality of shielding layers) can significantly lower resolution.  Interferences can be caused by mistakes in device’s electric circuit: wrong choice of elements for creating different filters and for power supply of electric circuits.   That is why electric board designing, signal processing software coding, creating electric board layouts are important and difficult challenges for designing of digital night vision devices. 

Image resolution of digital night vision device depends on conditions of observation. The higher is the illumination level of observed object the clearer image we will see in device. Thus a conclusion can be made that maximal resolution of digital night vision device will be achieved in almost daylight conditions of observation or with the help of powerful IR illuminator.

SENSITIVITY

In order to characterize sensitivity of digital video cameras they often use the parameter of minimal illuminance level on the observed object when device is still able to produce image. 
This definition is the most suitable for digital devices operating in the visible range of spectrum. For visible range a unit of sensitivity measurement is a light unit – “lux”. 

Since digital devices are supposed to operate during the night, when infrared spectrum prevails, in order to characterize their sensitivity it is more correct to use energy units that describe light flux.

Thus the parameter of sensitivity of digital night vision devices can be described as the minimal value of power of infrared light that enters digital night vision device when device is still able to produce the image with resolution providing recognition of observed object (conforms to resolution of 25 lpm).
Apart from that there is also parameter of spectral sensitivity – minimal power on the given wavelength of infrared spectral range. When spectral sensitivity is indicated they also indicate the wavelength of light at which this value was achieved.
 
In contrast to illuminance in visible range of spectrum, illuminance in infrared range is impossible to be measured in lux. In this case it is reasonable to use a universal unit of measurement – watt.

Sensitivity of device depends on the following parameters:

• Light-gathering power and quality of objective lens 
• Sensor parameters – physical size, its type and sensitivity
• Display parameters – brightness and contrast, resolution
• Algorithms of signal processing
• Quality of electric circuitry solutions implemented in the device 

For obtaining high sensitivity in digital night vision device it is necessary to collect all photons of light entering objective lens and transfer them without losses onto the light sensitive surface of sensor. An important role in the process of transfer belongs to objective lens and such its parameters as light-gathering power, amount of lenses in optic scheme, quality of antireflection coatings on the lenses, presence of blackening on the lens’s butts (in order to avoid dispersion of light inside the objective). 

Relations:

The higher is the light-gathering power of objective lens (increases when entrance pupil is increased and focal distance is decreased); the higher is overall sensitivity of device.

The more lenses are used in the objective; the smaller is light-gathering power and respectively the sensitivity of device.

The higher are optical transmission coefficients of lenses constituting objective; the higher is sensitivity.

Sensor is the main receiver and converter of light into electric signal.  It is the sensor that to a great extent defines sensitivity of device. Sensitivity of sensor depends on the pixels’ size and their density on the sensor. Other things being equal the bigger is the size of pixel; the higher is sensitivity of sensor. The smaller is the correlation of total area of sensor to total area of all pixels; the higher is the overall sensitivity of the sensor.  

Recently many manufacturers of cheap night vision devices started using cheap sensors for photo cameras (often color sensors). These sensors have good sensitivity in visible (daylight) range of spectrum, but very low in infrared range.   Technical description of such devices lacks any information about sensitivity but proudly states large amount of megapixels.   It is easy to make a conclusion that despite high sensor resolution in the night such device is not able to produce quality image without a powerful illumination source because its sensor has low sensitivity in infrared range of spectrum. 

Second common mistake is when sensitivity of digital night vision devices is indicated in photometric units (lux), here values of sensitivity can reach ten thousandth fraction of lux, which is significantly higher than analogue night vision devices based on image intensifier tubes generation 2+ or higher can provide.  Such supernatural sensitivity can be explained in a simple way.  As a rule for sensitivity measurement a luxmeter is used, its spectral characteristic coincides with spectral characteristic of human eye (see diagram). Same as human eye luxmeter is able to perceive (measure) illumination only in visible range of spectrum from 380 nm to 780 nm. It means that when measuring illumination in the night with the help of luxmeter obtained values of illumination will be close to zero because in the night visible range illumination is practically absent. But on the other hand strong infrared illumination is present (see diagram of natural illumination of night sky), and luxmeter is unable to register it, whereas night vision devices can easily register this illumination. As an example the diagram shows spectral sensitivity graphs of SONY CCD sensor and generation 2+ image tube intensifiers.

Spectral sensitivity is used as a parameter which characterizes the ability of a night vision device to work confidently in the night. As a rule it is indicated on one or several wavelengths of spectral range. For understanding “quality” of digital night vision devices the most optimal way is to have information about spectral sensitivity at such wavelengths as e.g. 780 … 810 nm (average value of infrared illumination of stellar sky; in this range sensors have average sensitivity) and 910 …940 nm (high value of infrared illumination of stellar sky; invisible infrared range, sensors are still able to be sensitive).

Comparing values of spectral sensitivity of several digital devices it is possible to make certain conclusions about their ability to “see” in the night. Here one should remember that sensitivity of digital device is defined not only by the sensitivity of its sensor but also depends on such parameters and characteristics of device as resolution of objective lens and eye piece, display resolution, light-gathering power of objective lens, sensor quality (absence of noises), quality of circuitry solutions (absence of interferences), applied algorithms of signal processing.

Contemporary digital night vision devices use two main types of sensors – CCD and CMOS. Major difference between these two types lies in the electronic organization of signal readout from pixels. In CCD (Charge Coupled Device) signals from every pixel are transferred sequentially to sensor’s electronics and then the amplification of overall signal takes place. In CMOS (Complementary Metal-Oxide Semiconductor) signals from all pixels are read simultaneously and amplified by means of amplifiers individually for each pixel. For this reason (necessity of having a part of sensor used by a large quantity of individual amplifiers) the density of pixels in CMOS sensors is lower than in CCD sensors and respectively their sensitivity is also lower.  In the last few years new technologies appear for CMOS sensors manufacturing (such as EXMOR SONY, BSI (Toshiba, Omnivision)) their essence is in increasing the density of pixels on the sensor’s surface which leads to increase in overall sensitivity of sensor. Parameters of such sensors have reached very close the figures of CCD sensors and the best samples surpass them in certain parameters.   

Night vision unit’s display also influences the overall sensitivity of device, first of all at the expense of its resolution and contrast/brightness values. 

It is possible to make certain conclusions about how digital night vision devices will operate if compared to night vision devices based on generation 2+ or 3 image intensifier tubes. On the sensitivity diagram it is clearly seen that CCD sensor and photocathode of image tube intensifier of 2+/3 generation have better sensitivity in infrared range 750-850 nm and worse in range higher than 900nm. 

Comparing these data with graph of spectral spread of natural night illumination it is possible to make a conclusion that in passive mode of operation (without auxiliary infrared light) advantage (higher sensitivity) in the night will be on the side of night vision devices based on image intensifier tubes of 2+/3 generation. 

Important thing here is that in the range higher than 900 nm digital night vision devices still have a certain sensitivity (when the wavelength increases, sensitivity decreases gradually) at the same time the sensitivity of night vision devices based on image intensifier tubes of 2+/3 generation decreases quickly to zero. For this reason night vision devices based on image intensifier tubes are ineffective when used with “invisible” infrared illuminators (e.g. 915 nm or 940 nm), whereas digital night vision devices are highly compatible with them. Due to the fact that analogue night vision devices (esp. gen 2+) often require additional illumination when used outside the city (e.g. for hunting) a factor of compatibility with invisible infrared illuminators becomes a substantial advantage of digital night vision devices.          

In the context of this topic sensitivity is a minimal value of power of infrared radiation. That is why the less is its numeric value in watts; the better is sensitivity. 

For comparison purposes we shall look at measured values of sensitivity for Yukon and Pulsar night vision devices (see table) at 780 nm wavelength. Digisight N750 at wavelength 780 nm will be much more sensitive than NVMT Spartan 3x42, but less sensitive than Phantom 3x50 generation 2+. At 915 nm wavelength Digisight N750 will already have an advantage over Phantom 3x50 generation 2+. 

FIELD OF VIEW

This parameter characterizes the size of space that can be observed simultaneously through the device. Usually in technical specifications field of view is indicated in degrees (field of view angle is indicated in the picture below as 2Ѡ) or in meters for some known distance (L) to observed object (linear field of view indicated in the picture below as А).

Field of view of digital night vision devices is defined by the focal distance of objective lens (fob) and by physical size of sensor (В). Usually as the sensor’s size for calculations of field of view they take width (horizontal size), in the result they get angular horizontal field of view:

2Ѡ=2*arctg((B/(2* f_o))

Knowing the vertical size of sensor (height) and diagonal size it is possible to calculate angular vertical and diagonal field of view. 

Relation:

The larger is the size of sensor or the smaller is focal distance of objective lens; the greater is field of view angle.

The greater is field of view of the device; the more convenient it is to observe objects – there is no need in constant movements of the device in order to view the necessary part of space.   

It is important to understand that field of view is inversely proportional to magnification – increasing magnification of the device leads to shrinking field of view. 
At the same time when field of view increases detection and recognition distances will decrease because first of all magnification will decrease and secondly if infrared illuminator is used for comfortable observation it will be necessary to use infrared illuminator with wide annular divergence of beam (it should approximately correspond to angular field of view)   this will in its turn lead to decrease of luminance in respect to the surface and respectively will lead to decrease of infrared illuminator’s distance.

EYE RELIEF

Eye relief is the distance from the external surface of the last lens of the eye piece to plane in which observer’s eye is located when observed image is optimal (the largest possible field of view, minimal distortions).  This parameter is very important for weapon sights, their eye relief should be at least 50 mm (optimal 80-100 mm).  Such big value of eye relief is necessary to avoid injury of observer with eye piece because of the recoil during the shot. In night vision devices eye relief as a rule equals to the length of eye shade which is necessary to mask illumination of the image intensifier tube or screen.  

DETECTION AND RECOGNITION DISTANCE.

Detection distance – maximal distance from observation device to some object (usually man) which may be detected with the help of the device. 

Recognition distance – maximal distance at which the observer can recognize the type of observed object (human, animal, building, etc.). 

These values are not constant for a specific device and depend on the following parameters:  

• Magnification;
• Resolution;
• Sensitivity of device;
• Observation conditions;
• Characteristics of observed object;
• Contrast level between object and background;
• Use of infrared illuminator;

Devices that have greater magnification (other things being equal) allow bring closer viewed objects and respectively detection and recognition distance will be greater in such devices.    

Resolution of device influences to a great extent recognition distance – high resolution of device allows the observer more confidently recognize the type of observed object thanks to clearer image of object’s details. 

Sensitivity of digital night vision device influences detection and recognition distance in the same way.   Devices with better sensitivity ensure clearer and more contrast image of observed object at large distance than devices with smaller sensitivity. 

Apart from device’s parameters detection and recognition distance is influenced significantly by observation conditions and characteristics of observed object. Observation conditions will be defined by level of natural night sky illumination and by transparency of atmosphere. If illumination level and transparency of atmosphere decrease (smoke, fog, dust particles, etc.) detection and recognition distances will also decrease.  

Object’s reflecting properties will influence detection and recognition distances in the same way; they will be defined by color and texture (glossy or opaque) of object’s surface and also by the level of contrast of object compared to the background.  For instance it is easier to detect and recognize an animal that is located on the background of snow field than on the background of forest edge, green grassland or field.  

In low light conditions detection and recognition distance can be increased with the help of infrared illuminators. Apart from increasing the overall illuminance of  object infrared illuminator’s radiation in some cases is well reflected for instance from animal’s eyes, resulting in the fact that animal can be detected at quite large distances – during observation through night vision device the eyes will be seen as bright spots. 

IR ILLUMINATORS

It is worth mentioning the application of IR illuminators together with digital night vision devices. Usually night vision devices have integrated IR illuminator. At the same time on the market there is a big quantity of IR-illuminators sold as accessories which are intended for combined use with night vision devices. 

Based on type of emission source IR illuminator can be divided into two main groups – LED illuminators and laser illuminators. 

In LED illuminators a semiconductor diode is used, it emits radiation at a certain wavelength of infrared range. On the market it is possible to meet LED illuminators with different wavelength (most often used 805 nm, 850nm, 940nm) and with different power. 

Laser illuminators are manufactured on the basis of laser semiconductor diodes. In respect to LED illuminators laser illuminators have significant advantages. 

Fisrt of all their radiation is coherent meaning that all photons of light in a beam have the same energy, direction and wavelength. Due to this fact a beam of light has a high energy density in a narrow spectral range which preserves even at long distances.   LEDs have dispersed radiation which is characterized by a wide spectral range and large energy losses at the distance from radiation source.  It means that having equal power laser illuminator is able to illuminate observed object located at longer distances than LED illuminator; in other words “operation distance” of laser illuminator is greater than of LED illuminator.  

Secondly energy consumption in laser illuminators is significantly lower than in LED illuminators with the same power. 

Main parameters of illuminators are radiation power and annular divergence of beam. 

Radiation power is the main defining factor for distance of IR illuminator. It depends on the type of used source, optic scheme and quality of lenses and anti-reflecting coatings manufacturing. In the majority of produced LED illuminators maximal radiation power ranges from 30 to 100 mW (low values of power in integrated IR illuminators, higher values in detachable illuminators produced as accessories). 

In laser illuminators maximal power can fluctuate from 10 to 50 mW having approximately the same energy consumption as LED illuminators. 

If several illuminators are used simultaneously (e.g. integrated and detachable external illuminator) overall illuminance of spot will be summed up, but only in case when the observed object is at the distance not exceeding maximal distance of operation of each of these two illuminators (meaning that each of the illuminators is able to light up observed object at this distance). If the distance to an object will exceed maximal operation distance of one of the illuminators than observed object will be illuminated only by one illuminator, which is more powerful meaning that at this distance there will be no summing up of illuminance spots. 

To disadvantages of laser illuminators one can attribute certain danger which they produce for human eye during direct viewing in cases when radiation exceeds 1-st class of laser safety. For this reason in the majority of countries only illuminators of the 1-st class of laser safety (totally safe) are allowed on the civil market. This fact to a great extent deters wide spreading of laser illuminators. 

Correctly designed 1-st class laser illuminators surpass ordinary LED illuminators in operation effectiveness because having relatively equal values of operation distance they have smaller size and consume less energy.  

Annular divergence of a beam of IR illuminator must be close to angular field of view of night vision device in order to illuminate total area that can be seen through the device. The larger is annular divergence; the lower is illuminance on the area and respectively illuminance distance is smaller. In practice IR illuminators have uneven spread of energy (illuminance) on area of the spot.  As a rule central area of the spot has a larger energy than area closer to the edges. In fact it means that if annular divergence is increased the user will notice to a great extent decrease of illuminance in the area close to edges of the spot, at the same time central area will be illuminated more intensively. 

When selecting IR illuminator for night vision device it is necessary to take into account spectral range in which night vision device operates. Maximal effectiveness (illumination distance) will be in illuminator with wavelength at which night vision device has the highest sensitivity.  For instance, when IR illuminator with 808 nm wavelength is used, digital devices will have better vision than with 940 nm wavelength meaning that sensitivity of sensor at 808 nm is higher.

It is worth mentioning one more advantage of digital night vision devices when they are used in combination with IR illuminators.  Obviously if compared to image intensifier tubes, sensors of digital devices have lower overall sensitivity, but significantly higher spectral sensitivity in range 900 nm and higher.  Radiation in this range is already invisible for human and animal eyes.  All this allows successful application of invisible range IR illuminators for additional illumination of observed objects in combination with digital night vision devices.  At the same time this invisible IR illuminator will be practically useless in combination with analog night vision device.   This is especially true when digital night vision devices are used for hunting: hunter can confidently use “invisible” illuminator for additional illumination – animal does not see it and does not get scared.  

Influence of power of IR illuminators and their type on recognition distance depending on device’s sensitivity is shown in table 1 (for laser illuminators) and 2 (for LED illuminators). Data is given for the following conditions: moonless night, cloudy sky, transparent atmosphere (no fog, no haze). As observation object a man-sized human figure was used in camouflage clothes located on the background of the forest edge. Annular divergence of a beam of IR illuminators is 5-7 degrees.

Table 1

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