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Image intensification

Thermal imaging TV imaging Night vision Boresighting Image intensification Fundamentals Thermometry Literature



1 Types of image intensifier tubes

Image intensifier system IIS (night vision device NVD) are the imaging systems built using an image intensifier tube consisting of a photocathode, an anode in form of a phosphor screen, and other optional components. The tube intensifies a low-luminance image of the observed objects created on the photocatode into a brighter image created on the anode. In other words, the photocathode due to the radiation impinging onto its surface emits electrons that are focused onto a phosphor screen that emits brighter light. Spectral sensitivity of the IIS depends on design but generally IIS used for surveillance applications are sensitive to radiation from about 0.4 m m to 0.8 m m and sometimes up to 0.9 m m.

Typical image intensifier system IIS consists of an optical objective, an image intensifier tube, an ocular, a chassis, and an optional illuminator. The image intensifier tube is a heart of the image intensifier system. Therefore the term "image intensifier" is often used as synonym of the image intensifier system. Next, the image intensifier systems IIS are commonly called night vision devices (NVD) although also other infrared imaging systems like thermal imagers or LLLTV cameras enable imaging at night conditions.

The technology of image intensifier tubes has progressed steadily over the years. There are so far five generations of image intensifier tubes: 0, 1, 2, 3 and possibly 4[1].

Generation 0 refers to the technology of World War II, employing fragile, vacuum-enveloped image converters with poor sensitivity and little gain. These are single stage tubes that achieve image intensification due to acceleration by high voltage of electrons emitted by the photocathode and striking the phosphor screen. Typically S-1 photocathode (sensitivity up to 60m A/lm), electrostatic inversion and electron acceleration were used to achieve gain. Gen 0 tubes are characterised by significant distortion and necessity for active illumination (large searchlights with infrared (IR) filters). These tubes were in past used in active night vision systems cooperating with an illuminator. High power tungsten bulbs covered with an IR filter suppressing visible radiation were used as illuminators. Active character of use of first generation image intensifier systems was their significant disadvantage.

Generation 1 - First generation tubes are improved Gen 0 tubes. Typically S-10 or S-20 photocathode (photo sensitivity up to 200m A/lm), electrostatic inversion and electron acceleration are used to achieve gain. Because of higher sensitivity Gen 1 NVD were the first passive night vision systems.

Focusing is usually achieved by using an electron lens to focus electrons originating from the photocathode onto the screen (Fig. 2).

In the inverter diode tube presented in Fig. 2 an electrostatic field directs the photoelectrons and focuses an inverted image on the phosphor screen. Electron lens can be achieved by combining an electrostatical field with an axial magnetic field provided by either a solenoid or permanent magnet. An uniform magnetic field enables to achieve good resolution over the entire screen and at the same time keeps distortion low. Fibre optics windows are used in Gen 1+ tubes to minimize degradation of the image resolution towards the edge of the tube. The fibre optics enables also efficient coupling to another image tube, to an imaging detector or to photographic film.

 

a)

b)

Fig. 2. a)Diagram of a NVD built using a Gen1 tube b)photo of Gen 1 tube

First generation image intensifier tubes are characterised by good image resolution (25-30 lp/mm), a wide dynamic range (the ability to reproduce the ratio between the bright and dark parts of an image), and low noise. However, they are also characterised only by a moderate gain. Luminance gains in the single stage tubes are usually in the order of 100 to 500 (up to 1500 in Gen 1+ tubes) in situation when luminance gain in the order of 105 is necessary to achieve ability to see at overcast starlight night conditions. Due to low production costs Gen 1 NVDs still dominate in commercial market but they are rarely used for military applications.

Generation 2 image intensifier tubes represent a significant breakthrough in night vision technology. Gen 2 tube are single stage imaging tubes built using an S-25(extended red) photocathode (with sensitivity of 240-450 m A/lm) and a microchannel plate (MCP) to increase significantly luminance gain up the level about 18 000-30 000 cd/m2/lx. They are also typically equipped with automatic gain control and bright spot protection.

The microchannel plate is an array of miniature electron multipliers oriented parallel to one another; typical channel diameters are in the range 10-100 m m and have length to diameter ratios between 40 and 100. Channel axes are typically normal to, or biased at a small angle (» 8 ) to the MCP input surface. The channel matrix is usually fabricated from a lead glass, treated in such a way as to optimise the secondary emission characteristic of each microchannel and to render the channel walls semiconducting so as to allow charge replenishment from an external voltage source. Parallel electrical contact to each channel is provided by the deposition of a metallic coating on the front and rear surfaces of the MCP, which then serve as input and output electrodes, respectively. The total resistance between electrodes is in the order of 109 W . Such microchannel plates allow electron multiplication factors of 103 – 106. Spatial resolution is limited only by channel dimensions and spacing; 12 m m diameter channels with 15 m m center-to-center spacings are typical.

Fig. 3. Principle of work of a microchannel plate

                    a)        b)

 

Fig. 4. Gen 2 image intensifier tube (inverter MCT tube) a)diagram b)photo

 

                           a) b)        

Fig. 5. Gen 2+ tubes (proximity focus MCT tube) a)tube diagram b)photo

First Gen 2 tubes were manufactured similarly to the Gen 1 tubes using the inverter diode tubes (Fig. 4). Newer Gen 2 tubes are usually proximity diode tubes shown in Fig. 5. They are characterised by lower size and higher gain in comparison to typical Gen 2 tubes.

 

Gen 3 tubes are similar to the Gen 2 tubes in design. The primary difference is the material used for the photocathodes. Second generation image intensifiers use photocathodes with a multialkali coating whereas third generation image intensifiers use photocatodes with a GaAs/AlGaAs coating. This latter material is characterised by quantum efficiency in the near infrared over 10 times better in comparison to photocathodes with a multialkali coating. However, the advantage of higher sensitivity of photocathode is significantly reduced by necessity to use a protective ion barrier film to increase tube life that cause some signal losses. Therefore luminance gain of Gen 3 tubes (range 20000-90 000 cd/m2/lx) is usually no more than about 1.2-3 times higher that luminance gain of typical Gen 2 tubes (range 18000-30 000 cd/m2/lx).

Gen 4 tubes are modified Gen 3 tubes. The main difference is the protective ion barrier that was removed in Gen 4 tubes. Gated power supplies are also used to pulse on and off the Gen 4 tubes. Using these quick, controlled voltage pulses, Generation 4 tubes are able to adjust automatically to lighting conditions - less power is consumed when ample light is available, and more in darker situations. However, it must be emphasised that the existence of Gen 4 tubes is not officially confirmed by the US authorities.

All these types of intensifier tubes are also differentiated by the nominal useful diameter of the photocathode. Typical diameter values are 18, 25, and 40 mm.

There is a common view that higher generation number means better tube. It is usually true if we compare Gen 0, Gen 1 or Gen 2 tubes but do not have to be true if we compare Gen 2, Gen 3 or Gen 4. Gen 3 and Gen 4 tubes are manufactured exclusively by USA manufactures. European and Asian manufactures instead of changing technology concentrated on improving Gen 2 technology and offer different equivalents to Gen 3 tubes (Supergen, SHD-3, XD-4, XR5). These equivalents can be sometimes better than Gen 3 tubes. Gen 2 tubes are also generally more tolerant to urban night conditions.

2 Parameters of image intensifier tubes

Testing modern image intensifier tubes is not an easy task. MIL series standards propose measurement of 35 parameters to characterize fully these tubes. The parameters can be generally divided into four basic groups:

  1. Photometric parameters,

  2. Image quality parameters,

  3. Reliability parameters,

  4. Temporal parameters.

  5.  

Tab. 1. Parameters of image intensifier tubes

Photometric parameters

Image quality parameters

Reliability parameters

Temporal parameters

Electrical parameters

  1. Photocathode Luminous Sensitivity

  2. Photocathode Radiant Sensitivity

  3. Luminance Gain

  4. Saturation Level

  5. Equivalent Background Input (EBI)

  6. Operation stability

1. Limiting Resolution
(center, peripheral, high level)

2. Modulation Transfer Function (MTF)

3. Signal To Noise Ratio (S/N)

4. Blemishes (dark spots, white spots, chicken wire)

5. Output Brightness Uniformity

6. Halo

7. Useful cathode diameter

8. Image Alignment

9. Shear Distortion

10. Gross Distortion

11. Image Rotation

12. Magnification

13. Temporal magnification

14. Fixed pattern noise

15. Ion barier film defects

1. Bright Source Protection

2. Burns In (ESS)

3. Reliability test

4. Accelerated reliability test

5. Extreme temperature test

6. Extreme humidity tests

7. Shock

1. Rise Time

2. Decay Time

3. Phosphor decay time (luminance persistence)

  1. Power voltage
  2. Input current
  3. Reversed polarity
  4. EMI

 

 

Photometric parameters describe tube sensitivity to incoming light. During photometric tests tube is uniformly illuminated. No image is created on the tube photocathode. We are not interested in tube ability to create an image but in tube ability to magnify input illumination.

Image quality parameters describe tube ability to create high quality copy of the image generated at the photocathode plane. Images of some standard targets are created at the photocathode plane and quality of the image generated at the tube screen is analyzed during image quality tests.

Reliability tests describe tube resistance to harsh work conditions like extreme illumination levels, bright spots, extreme temperature, humidity, vibration etc. The tests helps us to select tubes of long life time.

Temporal parameters describe tube temporal reactions to light or voltage impulses. These parameters, especially phosphor decay time enable determination of tubes not suitable for surveillance of high speed targets.

Electrical parameters describe tube electrical reactions to light or voltage changes.

Tab. 2. Definitions of photometric parameters

No

Image quality parameters

Definition

1

Photocathode luminous sensitivity

The ratio of the photocathode current generated by the input illumination to the input luminous flux [m A/lm]

2

Photocathode radiant sensitivity

The ratio of the photocathode current generated by the input illumination at specified wavelength to the input radiant flux [m A/W]

3

Luminance gain

The ratio of the output screen luminance to the input illuminance [cd/ m2 lux]

4

Saturation level

The luminance of screen of the tube in saturated state (high input illumination) [cd/m^2]

6

Equivalent background input (EBI):

The additional input illuminance required to provide an output luminance increase equal to the mean background screen luminance due only to the tube’s dark current cd/ m2

6

Operational stability

Temporal fluctuations and drifts of average brightness of the screen of the II tube

Photocathode luminous sensitivity gives information about sensitivity of tube photocathode to polychromatic light of 2850K color temperature (the spectrum covers both visible and near infrared range). Generally higher photocathode luminous sensitivity the better the II tube. However, this rule is valid only when two tubes manufactured using the same technology are compared.

Photocathode radiant sensitivity gives information about sensitivity of the photocathode to monochromatic 880 nm infrared light. Generally higher photocathode radiant sensitivity the better performance of the II tube in near infrared range.

Photocathode luminous sensitivity is usually proportional to photocathode radiant sensitivity. Higher photocathode luminous sensitivity means typically also higher photocathode radiant sensitivity.

Both photocathode luminous sensitivity and photocathode radiant sensitivity cannot be measured when the tube is potted because there is no direct access there to photocathode. These parameters can be measured only for bare tubes not having mechanical case and electronic module. Practically this means that these parameters can be measured by manufactures of II tubes or at some repairing depots. Typical users of II tubes cannot measure these parameters without partial destroying tested tube. The parameters listed below can be easily measured by both manufacturers and typical users of II tubes.

Luminance gain gives information about how many times brightness of the image of the observed scenery on the II tube screen is higher than brightness of the original scenery. Image intensifier tubes of too low luminance gain cannot be used for observation when illuminance is low. However, we must remember that higher luminance gain means higher intensity of the noise. Increase of the luminance gain will not improve tube performance when the gain is above a certain level.

The EBI phenomenon is caused by thermal emission of the photocathode. This means that even when there is no incoming light, the photocathode emits some electrons that are later multiplied and finally generate some brightness of the screen of the tube. During observation of targets at low illuminance conditions the EBI effect creates visible haze in the image. In other words we can say that higher EBI means lower contrast of the image at low illuminance conditions.

Bad operational stability does not means that tube sensitivity is low, image is noisy or image quality is low. Bad operational stability means practically significant fluctuations of screen brightness. These fluctuations can be very disturbing for users of night vision devices. The users will be quickly tired when use NVD with tube of bad operational stability is used.

Tab. 3. Definitions of image quality parameters

No

Image quality parameters

Definition

1

Limiting Resolution (center, peripheral, high level)

Parameter defined as the smallest resolution pattern which the observer can see and distinguish between the black lines and the clear area between the black lines.

 

2

Modulation Transfer Function (MTF)

Parameter defined as an output signal modulation to input signal modulation when the signal is sinusoidal wave. It is a measure of the degradation of an image as it appears at the output screen of the tube as correlated to the input pattern which is normalized to 100 percent contrast at a spatial frequency equal to or less than 0.2 lp/mm.

 

3

Signal To Noise Ratio (S/N)

Parameter defined as ratio of the average signal at the output of an intensifier tube to the true root-mean-square (rms) value of the signal fluctuations about the average at a specified electronic measurement system bandwidth.

4

Halo

Parameter defined as a circular area of brightness evidenced on the assembly output imaging screen occurring as a result of a small bright source input and concentric with the input.

5

Useful cathode diameter

Parameter defined as diameter of part of photocathode that reacts to incoming radiation.

6

Blemishes

a)Dark spots

b)White spots

c)Chicken wire

a)The opaque or dark spots which exceed a contrast of 30 percent of their surrounding area. It is typically required that the dark spots shall not exceed the size and quantities specified in special tables.

b) The bright spots or a pattern that may flicker or appear intermittently on the image screen in one general area

c) The phenomenon defined as a predominant pattern of dead fibers which has a diameter equal to or less than about 22.5m m – 2 single fibers and whose light transmission is so degraded that with light projected through the optic, single fibers in the area of question cannot be distinguished or identified as single fibers with the use of 50 power magnification.

7

Image Alignment

Measure of displacement of the spot image that should to be created at the point of passing the optical axis through the screen.

 

8

Shear Distortion

Parameter that describes phenomenon that causes the image of a strait line to have a discrete, localized, lateral displacement (i.e., a break).

 

9

Gross Distortion

Parameters that describe phenomenon that causes the image of a straight line to curve. Gross distortion is caused by a long-range deformation or flow of fibers during fabrication.

10

Output Brightness Uniformity

The ratio of the maximum to minimum brightness variation over the useful screen area when the photocathode is uniformly illuminated.

 

11

Ion barrier film quality

Hole defects in ion barrier film

12

Fixed Pattern Noise

The phenomenon defined as discernible spatial gain variation seen on the screen of the tube. There two components: Multi-Multi Pattern Noise and Multi-Boundary Pattern Noise. MMPN is a phenomenon defined as discernible spatial gain variation between individual multi-patterns or groups of multi-patterns. MBPN is the phenomenon defined as discernible spatial gain variation between peripheral and interior channels of a multi-pattern or group of channels.

13

Image inversion

The phenomenon of angular rotation of output image in comparison to input image different than desired 180 degrees. .

14

Magnification

The radio of size of output image to size of the input image

15

Temporal magnification

The phenomenon of temporal variation of tube magnification. This means that size of output image vary with time.

Limited resolution gives information about tube ability to create recognizable image of two small targets located close each other. Higher value of limiting resolution means that the tube can generate recognizable images of smaller targets separated by smaller distance. USAF1951 standard target is typically used for measurement of the limited resolution. The task of observer is to determine the the smallest pattern that he is able to resolve. Limited resolution is measured as center resolution or peripheral resolution depending on location of input image, at low or at high illumination level.

Fig. 1. Image of USAF 1951 target during resolution test

Limited resolution is a good measure of tube ability to create image of small details. The parameter has a long history in optics and was a traditional measure of many optical systems. However limiting resolution can be poor measure of overall image quality. There are tubes of the same limiting resolution that according to human perception differ significantly in the image quality. The reason is that limiting resolution describes tube performance only in high spatial frequency range but not in middle or low frequency range that have more significant influence on perceived image quality. Because of this situation nowadays optical objectives and imaging systems are evaluated not by limiting resolution but by their MTF function. Additional advantage of MTF over limiting resolution is the fact that MTF is an objective parameter measured automatically when limiting resolution is a subjective parameter measured by humans.

Fig. 2. MTF of two tubes of the same limiting resolution

a)    b)

Fig. 3. Image generated by two tubes of the same limiting resolution but different MTF a) MTF at 30lp/mm equals to 0.4 b) MTF at 30lp/mm equals 0.25

Signal To Noise Ratio gives information how noisy is the image generated by the tube; more precisely about temporal variation of brightness of very small portions of the screen image. If we capture a sequence of images of tube screen using a video camera then temporal noise generate differences between frames. Low SNR can significantly degrade image quality, makes more difficult to recognize small details and also makes more difficult long work with such tubes.

Fig. 4. Image generated by an II tube during SNR test

Halo gives information influence of high brightness spot targets on image of neighbour targets. High values of halo means that the tube cannot create recognizable image of targets located close to high brightness sources. This parameter is particularly important if the tube is to be used at urban terrain.

Size of photocathodes are standardized: 12 mm, 18 mm, 25 mm etc. There are however some differences in size of photocathodes even offered by the same manufacturer. Useful photocathode diameter should be measured and known because it determines later important parameter of night vision devices or LLLTV cameras: the field of view. It can have also indirect influence on image quality, too.

Tube blemishes can be met in every image intensifier tube. Tubes are never flawless, and every tube will have blemishes to some degree. The basic question is if they can be treated as only cosmetic defects or as important defects that significantly degrade tube performance.

From five types of blemishes presented in Tab. 2, the dark spots are usually most common and most easily noticeable. If size and number of dark spots are too high then this defect can limit significantly tube ability to detect small target. The same is valid in case of bright spots.

Fig. 5. Photos of a few dark spots of different sizes and contrast

Chicken wire is less frequently noticeable and rarely can significantly degrade tube performance.

Image alignment effect creates situation when image of spot generated exactly at the center of the photocathode is not created exactly at the center of the tube screen but is displaced slightly from the center (up to one millimeter). It is typically measured as the distance be image of the test reticle projected on the photocathode of the assembly concentric with the optical axis and the optical axis of the assembly at screen plate. This effect has no big importance at applications when user want only to get an image of the observed targets. However, this effect is critical at military applications when the tube is a component of a sighting system.

 

Fig. 6. Displaced image of spot target due to image alignment effect (the bright spot should be in the center)

Gross distortion is usually caused by a long-range deformation or flow of fibers during fabrication. This defect can be tolerated up to high levels in most surveillance applications when image is only evaluated subjectively. However, it can be a big hindrance in applications that require precise determination of position and size of observed targets.

a)b)     

Fig. 7. a)Gross distortion target b)Image of gross distortion target distorted due to gross distortion effect (attention: the effect was significantly exaggerated)

Shear distortion is due to localized misalignment errors in the assembly of fibers or multifibers. Shear distortion in fiber bundles is sometimes referred as incoherency. It causes that images of lines have a discrete, localized displacement (Fig. 8). Shear distortion up to some level is acceptable but if occurs at some critical areas (the centre) then it can be a serious problem in boresigting applications.

Fig. 8. Influence of shear distortion effect on image of a single circle

Image inversion is a phenomenon of angular rotation of output image in comparison to input image different than 180 degrees. For single channel nigh vision devices image rotation even at the level of several angular degrees difference than desirable inversion is often acceptable and this parameter is not important. However, image inversion is a very important parameter for dual channel night vision devices when the manufacturer must build the device using two tubes of almost identical image rotation. If not then the observer can get headache and perceived image quality can drastically deteriorate.

a)b)     

Fig. 9. Image inversion phenomenon a)original input image b) output image seen on the tube screen

Ion barrier film defects are holes in ion barrier film that is to protect the photocathode against bombarding ions. From final image quality point of view ion barrier film defects can be treated as a kind of blemishes.

Fixed Pattern Noise appears as hexagonal pattern is in almost every tube, the question is only at what illumination level. As we can see in Fig. 10 there are some brighter and darker hexagonal multies and there are brightness differences between peripherial and interior channels of hexagonal multies (webbing).

Multi-Multi Fixed Pattern Noise creates brightness differences between different hexagonal multies.

Multi Boundary Pattern Noise creates brightness differences between peripherial and interior channels of hexagonal multies (webbing).

Although Fixed Pattern Noise is easily noticeable it can rarely significantly decrease image quality and degrade tube performance. It is usually only a cosmetic effect but over a certain limit it can be problem for the observer.

Fig. 10. Screen of II tube with noticeable both Multi-Multi Pattern Noise and Multi-Boundary Pattern Noise

    2.1 References

  1. Association of Nigh Vision Manufacturers, Annual meeting – report, 1999.

  2. MIL-PRF-49052G "Image intensifier assembly, 18 millimiter microchannel wafer, MX-9916/UV, 1999

  3. MIL-STD-1858, Image intensifier assemblies, performance parameters of; 1981

  4. DEP 2283A3 "Image intensifier - 18 mm microchannel wafer XX1940