ebaps : next generation, low power, digital night …...ebaps®: next generation, low power, digital...

Presented at the OPTRO 2005 International Symposium May 10, 2005, Paris, France.

1

EBAPS®: Next Generation, Low Power, Digital Night Vision1

Verle W. Aebi, Kenneth A. Costello, Philip W. Arcuni, Patrick Genis, and Stephen J. GustafsonIntevac Corporation

Santa Clara, CA USA

1. Abstract

Intevac has developed a new Low Light Level Camera sensor technology for application to a variety of lowlight level imaging applications. The new sensor is an Electron Bombarded Active Pixel Sensor (EBAPS).EBAPS technology is based on use of a GaAs photocathode derived from Generation-III image intensifiertechnology in proximity focus with a high resolution, backside thinned, CMOS Active Pixel Sensor (APS)imager anode. The electrons emitted by the photocathode are directly injected in the electron bombardedmode into the CMOS APS anode. In this approach low noise gain is achieved in the CMOS anode viaconversion of the high energy photoelectron (1 – 2 KeV resulting from the high voltage bias applied betweenthe photocathode and CMOS anode) to electron-hole pairs in the anode via the Electron BombardedSemiconductor (EBS) gain process. The electrons are collected in the APS pixel and subsequently read out.The EBS gain process is inherently low noise with an excess noise factor (Kf) of less than 1.1. This issubstantially less than a microchannel plate based Generation-III image intensifier (MCP, Kf of 1.8) or theavalanche gain process in an Electron Multiplying CCD (EMCCD, Kf of 1.4). The low noise EBS gainprocess eliminates the need for an MCP and enables higher SNR at the lowest light levels. This offers thepossibility of higher performance for an EBAPS based camera relative to a standard Image Intensified camerabased on Gen-III tube technology using an MCP for gain or EMCCD based cameras.

This low noise gain advantage has been combined with modern semiconductor packaging and manufacturingapproaches to enable a small integrated EBAPS module which can be mass produced at low cost in anautomated ultra high vacuum production packaging system. This new sensor manufacturing approach allowshigh volume, cost sensitive, markets to be addressed. It also enables a variety of sensor formats to be easilyaddressed as it allows combination of standard CMOS APS imaging chips with a GaAs photocathode in theEBAPS configuration. This allows customization of the EBAPS for a given camera application.

EBAPS technology will be described with its application in a first generation EBAPS sensor and low lightlevel camera (NightVista) developed for commercial security camera applications. The NightVista camerahas a 1/2 inch optical format and a VGA (640 x 480) array with a 12µm pixel. The camera incorporates agated high voltage power supply for automatic gain control. It also incorporates 2 point non-uniformitycorrection (NUC), bad pixel replacement, and histogram equalization image processing functions. TheEBAPS sensor, high voltage power supply and camera electronics combined weight is 45 grams (not includingcamera housing). This is approximately 60% of the weight of a Generation-III image intensifier module asused in a standard night vision goggle. The EBAPS sensor and electronics are also ideally suited to headmounted system packaging and enable system designs with minimum forward projection relative to currentlyfielded night vision goggles.

Results will also be presented for a next generation EBAPS camera based on the ISIE6 (Intevac SiliconImaging Engine), SXGA (1280 x 1024 array, 6.7 µm pixel), EBAPS sensor with a 2/3 inch optical format.The ISIE6 EBAPS has lower readout noise than the NightVista EBAPS sensor for improved low light levelperformance and supports a 27.5 fps readout rate. Finally performance modeling will presented on a larger 1inch optical format, SXGA, ISIE10 EBAPS sensor under development for an EBAPS camera targeted forfuture high performance head mounted night vision applications.

1 It should be noted that the U.S. Government makes no official commitment nor obligation to provide any additional

information or an agreement of sale on any of the systems/capabilities described in this paper.


2

1. Introduction

Low light level cameras have a number of significant, dual use applications. These include traditionalmilitary head mounted night vision and commercial applications including surveillance, medical, andscientific applications. Modern night vision systems are rapidly transforming from the presently used directview systems to camera based systems as evidenced in a number of US military programs such as Future ForceWarrior or Digital Enhanced Night Vision Goggle (DENVG). These systems are driven by advances in videodisplay and processing. Video based systems allow image processing including fusion with other imagery suchas from a FLIR sensor in addition to image transmission for remote display and image recording or localdisplay of imagery from a weapons mounted sensor or imagery from a remote sensor. Surveillanceapplications are predominately video based where camera cost, size and performance are often critical.Scientific applications require cameras with good photon sensitivity over a large spectral range and highframe rates. These applications, and others, are driving the need for improved low light level sensors withdirect digital video output.

Two technology approaches are used today for high performance low light level video cameras. The first isbased on a Generation-III (GaAs photocathode) or Generation-II image intensifier fiber optically coupled t osilicon imager (either CCD or CMOS) to form an Image Intensified (I2) camera. The second uses anElectron Multiplying CCD (EM-CCD) as the low light level sensor. Both technologies have differenttechnology advantages and disadvantages relative to a camera based on the EBAPS sensor.

The traditional I2 camera utilizes an image intensifier originally optimized for direct view night visionmilitary system applications. In this approach the scene to be imaged is focused by the input lens onto thephotocathode faceplate assembly. The light energy liberates photoelectrons from the photocathode to forman electron image. The electron image is proximity focused onto the input of the microchannel plate(MCP) electron multiplier, which intensifies the electron image by secondary multiplication whilemaintaining the geometric integrity of the image. The intensified electron image is proximity focused ontoa phosphor screen, which converts the electron image back to a visible image. A fiber optic then transfersthis visual image to a standard CCD or CMOS image sensor, which converts the light image into electrons t oform a video signal. In these existing I2 cameras, there are four interfaces at which the image is sampled,and each interface degrades the performance of the camera. In addition the camera is composed of a highcost image intensifier which is composed of a number of custom, high cost, parts (MCPs, fiber optic, andvacuum assembly, in addition to the photocathode). Finally size and weight are not optimized relative to amodern focal plane based camera.

Recently EM-CCD devices have been developed and commercialized for low light level imagingapplications.1, 2 In these devices a low noise avalanche gain process is employed in the charge domain in amodified readout register of the CCD. This gain mechanism increases the signal sufficiently to effectivelymitigate the noise effects of the on-chip output amplifier. This enables high sensitivity, low light level,operation at video readout rates. The EM-CCD requires high clock voltages (on the order of 20 to 35 volts)on some of the register gates to achieve charge multiplication gain. Dark current reduction in the CCD bycooling is also required to achieve good low light level sensitivity.

The cost, size, weight, and performance disadvantages of the I2 and EM-CCD low light level cameras hasbeen addressed by development of Electron Bombarded (EB) silicon imager technology.3, 4 In thistechnology photoelectrons from a photocathode are accelerated to and imaged in a silicon imager anode(CCD or CMOS imager) directly (Figure 1.1). Gain is achieved by electron multiplication resulting when thehigh velocity electron beam dissipates its energy in the silicon of the imager chip to produce electron-holepairs by the electron-bombarded semiconductor gain process. The EB gain is high enough to mitigate thenoise effects of the on-chip amplifier and other camera electronics as is the case with the EM-CCD.

Noise is generated in all the elements in the multiplier chain of the conventional I2 camera, particularly inthe MCP where the electron multiplication statistics result in an excess noise factor on the order of 1.8 for a


3

modern Generation-III image intensifier and 1.4 for a Generation-II image intensifier. EM-CCD camerashave an excess noise factor determined by the electron multiplication statistics of the avalanche gain processin silicon. This is on the order of 1.4 for a well designed electron multiplication gain register.1 In contrast,very little noise is generated by the Electron Bombarded gain process in an EB silicon imager, where theelectron multiplication occurs by electron-hole pair formation as the accelerated electron beam travelsthrough the silicon. The EB gain process is essentially deterministic with a resulting excess noise factor near1.035, substantially less than for EM-CCD or I2 cameras. The EB silicon imager eliminates the MCP,phosphor screen, and fiber optics, and does not require focal plane cooling to reduce dark current due to thelow emitted dark current from the photocathode at ambient temperature and as a result both improved imagequality and increased sensitivity can be obtained in a smaller sized camera relative to an I2 or EM-CCDcamera.6

The initial approach selected for the EB silicon imager was based on a backside thinned CCD mounted behindthe photocathode in a standard 18mm diameter image intensifier tube package, but with the phosphor outputscreen replaced with the packaged CCD imager. The EBCCD effort demonstrated the viability of theintegrated EB imager concept and reduced camera size and weight relative to CCD based I2 cameras throughthe elimination of the fiberoptic output and its associated optical bonding issues, but the costs were notsufficiently reduced to meet high volume military and commercial market requirements. Further size, weight,and power reductions were also required to meet the needs of head mounted night vision applications.

Large format CCD camera electronics consume several watts of power due to the CCD clocking requirementsmaking them impractical for battery operated applications and require external electronics for a completecamera. The size of the external camera electronics presents an obstacle to applications that would benefitfrom miniaturization of the camera. EM-CCD cameras consume significant additional power due to the needto cool the focal plane, required to reduce dark current noise. Finally CCDs require specialized semiconductorprocessing lines that are not compatible with mainstream CMOS semiconductor fabrication technology.This substantially complicates application of the silicon foundry model so successfully used in the CMOSindustry to the fabrication of specialized CCDs for Low Light Level cameras further increasing the cost ofEM-CCD and EBCCD cameras.

These disadvantages have been addressed by moving to new CMOS imagers or Active Pixel Sensors (APS) asa replacement for the CCD and re-designing the vacuum package to take full advantage of packagingadvances in the semiconductor industry, resulting in the Electron Bombarded Active Pixel Sensor (EBAPS®)concept. This approach has addressed the performance, size, power, and cost disadvantages of present LowLight Level cameras.

FIGURE 1.1 EB Silicon Imager

PHOTOCATHODE

SILICON IMAGER

VIDEO OUT

PHOTOELECTRONS

CONTROL SIGNALS

VACUUMENVELOPE

IMAGE PHOTONS

2. EBAPS® Design

The recent development of high performance CMOS imagers enables the EBAPS sensor and allows it t oaddress some of the key deficiencies in previous low light level cameras. This includes substantial reductionin electronics size and weight due to the ability to integrate much of the camera electronics on-chip with


4

CMOS technology. Power is substantially reduced in a CMOS imager based camera with an order ofmagnitude reduction possible relative to EM-CCD and EBCCD cameras. Overall camera size is reducedrelative to an I2 camera by application of industry standard semiconductor sensor packaging approaches thatreduce sensor size significantly relative to a military image intensifier tube.

Ultimate performance of an EBAPS sensor will be determined to a large extent by the CMOS imagerarchitecture and design. First, it is essential that the CMOS imaging area have 100% fill factor (no deadarea). Any reduction in active area will result in lost photoelectrons. This is equivalent to a reduction inphotocathode quantum efficiency or sensitivity. At the lowest light levels (starlight or overcast starlight),low light level camera performance is dictated by photon statistics. It is essential for the imager to detectthe maximum number of photons for adequate low light level resolution and performance. Second, theCMOS imager architecture must maximize integration of the image photons with close to 100% duty cycle.This requirement when combined with high fill factor enables the collected signal to be maximized for goodlow light level performance.

Essentially 100% fill factor can be achieved in a CMOS imager with a properly designed backside illuminateddesign. A backside thinned format enables 100% fill factor for an arbitrary pixel size for back illuminatedoperation. A standard CMOS imager cannot be used in a frontside illuminated electron bombarded modesince the metal and dielectric stack (typically 4 to 5 microns thick for a modern sub-micron CMOS process)will block the electrons from reaching the silicon at moderate acceleration voltages (2 kV typical). Inaddition fill factor would be restricted to the optical fill factor of the CMOS imager which is typically lessthan 50%. In a backside thinned CMOS imager the chip is flip-chip bonded onto a carrier substrate and thesilicon substrate is removed by mechanical and chemical thinning. The free silicon surface is then passivatedto reduce carrier recombination at the surface. A properly designed pixel will allow a majority of thegenerated charge to be collected by the photodiode in the pixel regardless of photoelectron impact positionin the pixel. This enables high single photoelectron signal-to-noise (SNR) to be obtained with 2 kV electronenergy EB gains on the order of 200. SNRs above one are achieved for a single photoelectron if the on-chipelectronics noise (pixel referenced) is less than the EB gain.

A rolling shutter approach is implemented in the CMOS image chip to enable integration of the image signalat close to a 100% duty cycle. The above approach of a properly designed backside thinned CMOS imagerwith a rolling shutter architecture enables essentially full utilization of the available signal from thephotocathode. This is essential for high performance low light level imaging where the ultimateperformance is determined by the photocathode quantum efficiency and the signal limited shot noise.

The other critical requirement for a low light level camera is high dynamic range to accommodate the intra-scene dynamic range of a nighttime scene with lighting (on the order of 105 or 106) as is often the case inurban environments. CMOS imagers with extended dynamic range capabilities are common today.7, 8 Inaddition the anti-blooming structures used in CMOS imagers are effective and do not impact fill factor. Theprogrammable extended dynamic range capability of CMOS imagers is not available in CCD sensors which areinherently linear devices. The dynamic range of I2 cameras is set by the MCP and is not programmable.This level of dynamic range will result in better intra-scene performance than that obtained with either anEM-CCD, EBCCD or an I2 camera. This will result operationally in the capability to better observe scenedetail in dark areas of scenes which contain light sources.

Low voltage operation is also important to ease power supply requirements for a gated power supply. Gatingcan be used to reduce duty cycle for exposure control in high light level conditions. Gating is controlled bythe camera automatic gain control (AGC) algorithm. Minimization of the total voltage swing enables asmaller, more power efficient, power supply. This is important for low power, miniature camera design.

Other design requirements for the EBAPS are determined by requirements for high performance, reliability,minimum size, and low cost. High performance is achieved through use of a high quantum efficiency GaAsphotocathode. GaAs has good sensitivity in the near IR (600nm – 900nm) region where a higher photonflux is available at night than in the visible region of the spectrum. Reliability considerations are driven by


5

the GaAs photocathode requirement for an ultra-high vacuum (UHV) environment for photocathodestability. A critical aspect of this program has been selection of UHV compatible materials and thedevelopment and demonstration of cleaning procedures and processing techniques that allow goodphotocathode life to be achieved while maintaining acceptable CMOS imager performance. Low cost hasbeen demonstrated through a sensor development process that has considered manufacturing cost throughoutthe product development cycle. Part count has been minimized in the EBAPS with the device consistingessentially of a packaged CMOS imager with an input window incorporating a photocathode as shown inFigure 1.1. This has been achieved by adopting a proximity focused sensor design using standardsemiconductor packaging approaches for the sensor. This has enabled the overall package size to beminimized and has also enabled adoption of automated manufacturing approaches.

Video based head mounted night vision system requirements also drive sensor design choices for an optimumCMOS chip for this application. Overall constraints are low light level performance optimization, systemsize, weight and power consumption. System angular resolution versus light level is a key systemperformance metric that should be maximized with the goal of equal or better performance relative t opresently deployed direct view night vision goggles. Ultimately angular resolution is determined by sensorpixel format and system field of view. Today the accepted field of view for a head mounted system is 40°horizontal. Pixel format is limited by presently available microdisplays suitable for head mountedapplications. Today SVGA (800 x 600 pixel) format displays are available with SXGA (1280 x 1024)displays now reaching the market. There is no near term prospect for larger format microdisplays in thenear term (next 2-3 years). The field of view requirement when combined with the SXGA formatfundamentally limits ultimate system resolution, regardless of light level. This limit is 0.92 cycles permilliradian. Today typical direct view goggles with a 40° circular field of view have limiting high light levelresolution of >1 cycle per milliradian.

At low light level (1/4 moon illumination and below) system resolution as measured by standard tasks such asmaximum range for man recognition begins to be limited by the SNR of the system, not pixel count. SystemSNR is a function of the lens f/#, lens transmission, and effective focal length.9 For a fixed system field ofview and lens f/#, system SNR for a given scene object subtense is proportional to the product of the lensclear aperture diameter and the square root of its transmission.9 Typically the lowest manufacturable f/# lensfor night vision applications is on the order of f/1.2. Lens aperture diameter will thus be directlyproportional to focal plane size with the above assumptions and low light level performance will increasewith focal plane size. The optimum CMOS format for best low light level performance is the largest sizedetermined allowed by the application and other system constraints.

Size constraints have limited head mounted image intensifier tube format to 18mm diameter tubes. Largerformat 25mm image intensifiers have not been used for head mounted applications, but have been used fordrivers viewers on vehicles or weapon sights. An 18 mm diagonal format has been chosen as the optimumCMOS focal plane size for head mounted applications as this is consistent with presently fielded headmounted night vision systems. This requirement combined with the SXGA pixel array format determines anoptimum pixel size of 10 – 11µm for head mounted night vision applications.

3. EBAPS Sensor and Camera FamilyThe requirements stated above have been used as design architecture guidelines for the EBAPS sensors andcameras developed at Intevac. Two generations of EBAPS sensors have been developed with the thirdgeneration EBAPS in development. The first generation EBAPS, NightVista, is based on a 1/2 inch imageformat, VGA (640 x 480, 12µm pixel), CMOS imager. The NightVista CMOS chip has an integrated highperformance analog signal processor comprised of a programmable gain amplifier (PGA), a high speed 10 bitA/D converter, and fixed pattern noise elimination circuits. The second generation ISIE6 camera has threekey improvements over the NightVista: first the optical format has been increased to 2/3 inch; the focalplane is based on a SXGA (1280 x 1024, 6.7µm pixel) CMOS imager; and the read noise has beensubstantially reduced. The combination of these improvements results in substantially improvedperformance at all light levels. The ISIE10 camera further optimizes low light level performance relative t o


6

the ISIE6 for head mounted night vision with an increased sensor size (1 inch optical format) obtained byenlarging the pixel size to 10.8µm with further incremental reduction in read noise and increased frame rate.The key CMOS imager chip specifications of the NightVista, ISIE6, and ISIE10 EBAPS are summarized inTable 3.1. Common characteristics of the EBAPS sensors which are determined by the photocathode aregiven in Table 3.2.

TABLE 3.1 NightVista, ISIE6, and ISIE10 key specificationsNightVista ISIE6 ISIE10

Format VGA 640 x 480 SXGA 1280 x 1024 SXGA 1280 x 1024Pixel Size 12.0µm x 12.0µm 6.7µm x 6.7µm 10.8µm x 10.8µmOptical Format 1/2" (9.8mm diagonal) 2/3” (11mm diagonal) 1” (17.7mm diagonal)Frame Rate 30 frames per second 27.5 frames per second Up to 37 frames per secondVideo Output RS-170 or interlaced

digital video10 bit Digital Output,progressive scan

10 bit Digital Output,progressive scan

TABLE 3.2 Common EBAPS Sensor CharacteristicsPhotocathode GaAs (500nm – 900nm Band)High Voltage Power Supply Gated for Dynamic Range Control24 Hour Capability Daytime imaging with High Voltage off

The proximity focused sensor design combined with semiconductor style packaging results in small formfactor for the sensor. A photograph of the three EBAPS sensors developed by Intevac is shown in Figure3.2.

FIGURE 3.2 GaAs EBAPS®: NightVista, ISIE6, and ISIE10

4. EBAPS Performance

An EBAPS based camera has some significant performance differences relative to a standard I2 camera. Inparticular since the sensor does not utilize a microchannel plate it can be operated in a day only mode withno high voltage applied to the sensor. This is a result of the longer cutoff wavelength of silicon relative t oGaAs. The GaAs photocathode acts as a long pass filter in front of the backside thinned CMOS image chip.The GaAs photocathode begins transmitting light at 750nm with close to 100% transmission forwavelengths longer than 900nm. Silicon has some sensitivity out to 1100nm wavelength. Thus the CMOSimage sensor directly detects photons in the 750nm to 1100nm wavelength band in an un-intensified modeof operation. Typical spectral response curves of the intensified night mode of operation with the GaAs

NightVista ISIE6

ISIE10


7

photocathode and high voltage applied is shown in Figure 4.1 (blue curve). The un-intensified day mode ofoperation with high voltage off with direct photon detection by the Silicon CMOS imager is also shown (redcurve). This mode of operation enables high resolution near IR imagery to be obtained in the day with noimpact on sensor operational life.

When operating in day mode the degradation in image quality resulting from the proximity focused electronoptics is no longer present and the resolution is much improved with MTF similar to a standard CMOS imagesensor. AGC in the day mode sensor is obtained through integration time control on the CMOS chip.

Figure 4.2 is an example of typical day mode imagery obtained with the NightVista camera. For comparisonpurposes Figure 4.3 is an example of night mode imagery obtained by stopping down the lens used for theimage obtained in Figure 4.2 at essentially the same time and illumination conditions. The variation inimage contrast is a result of the different spectral sensitivity bands for the two images.

FIGURE 4.1 GaAs EBAPS® Spectral Response

FIGURE 4.2 NightVista Day Mode Imagery FIGURE 4.3 NightVista Night Mode Imagery

Limiting resolution versus light level measurements have been performed in the laboratory for both theNightVista and ISIE6 cameras. In these tests an Optoliner with a calibrated 2856°K light source was usedwith limiting resolution measured using a backlit, 100% contrast, 1951 Air Force resolution target. Theresolution target is read by observing a computer monitor adjusted for optimum brightness and contrast. Theresults are shown in Figure 4.4. As expected the ISIE6 camera has substantially higher resolution at all lightlevels. This is a result of the smaller pixel size (6.7µm versus 12 µm) and the lower read noise of the ISIE6chip.


8

FIGURE 4.4 Limiting resolution vs. faceplate FIGURE 4.5 ISIE6 camera low light level imageillumination

Night time imagery has been captured with the ISIE6 camera. A night time image of two men in BattleDress Uniform (BDU) against a green grass background captured with the ISIE6 camera is shown in Figure4.5. The SXGA format and lower read noise result in substantially higher resolution and image quality thancan be obtained with the VGA format NightVista camera.

Range performance modeling has been conducted on the EBAPS camera family under development atIntevac. Modeling was performed using the NVESD II2CCD camera model using input parameters given inTable 3.1. The system parameters were held constant with a 40° horizontal FOV using a f/1.2 lens. Thetask modeled was for recognition of a man dressed in Battledress Uniform (BDU) against a green grass orfoliage background. The model results are shown in Figure 4.6. A substantial increase in recognition range isobtained for each camera generation.

FIGURE 4.6 EBAPS camera family relative recognition range performance

The NightVista and ISIE6 laboratory and field test results support the model predictions. The increase inrecognition range from NightVista to ISIE6 to ISIE10 is a result of a decrease in CMOS read noise withEBAPS generation, an increase in focal plane size allowing increased light gathering capability, and anincrease in format from VGA for NightVista to SXGA for ISIE6 and ISIE10. The relatively slow drop off in


9

range performance versus light level for all three cameras is one of the advantages of a camera based, indirectview, system versus a standard direct view night vision goggle. This is a result of separating displaybrightness from the low light level sensor function. In the camera based system using a microdisplay thedisplay brightness can be separately optimized for best eye performance, removing the falloff in eyeperformance with reduced display brightness as occurs in a direct view goggle at starlight and belowillumination levels.

5. EBAPS Product Features

The EBAPS camera family offers substantially smaller size and weight than presently available low light levelcameras. The commercial NightVista and ISIE6 cameras are shown in Figures 5.1 and 5.2. The NightVistacamera EBAPS sensor, electronics, and high voltage power supply weigh 45 grams (not including case andother mechanical mounting components). ISIE6 and ISIE10 cameras will be only slightly higher in mass dueto the increased sensor size. A standard 18mm format image intensifier tube in contrast weighs in excess of80 grams. The addition of CMOS image sensor and electronics along with a fiber optic taper for opticalcoupling of the tube output to the CMOS image sensor would increase overall weight of an I2 camera to the150 gram range. The other critical advantage of EBAPS for HMD applications is the low sensor profile.This enables a reduction in forward system projection in excess of 3 cm for a helmet mounted EBAPScamera relative to a standard goggle or I2 camera solution. This reduced forward projection improvesergonomics of a head mounted system by improving the center of gravity and reducing risk of entanglementof the system in an operational environment with branches and other obstacles.

FIGURE 5.1 NightVista camera FIGURE 5.1 ISIE6 commercial camera

Camera power has also been addressed with the FPGA based NightVista camera consuming 1.1W for theRS170 video configuration. The ISIE6 camera consumes 1.8W while processing 4X the number of pixels persecond. FPGA based camera designs for the ISIE6 and ISIE10 cameras are targeting 1.2W powerconsumption upon completion of on-going power reduction activities. An ASIC based ISIE10 camera wouldconsume <1W of power. Less than 1W of power consumption is targeted for battery operated, headmounted, applications.

The EBAPS cameras all have a number of common features that improve overall system performance.These include sophisticated AGC algorithms that control camera exposure through the miniature, gated, highvoltage power supply. The AGC algorithm allows the user to select either the entire frame or a userselectable window for exposure control. Average brightness in the window along with the percentage ofallowed saturated pixels is also selectable. Non-Uniformity Correction (NUC) to remove fixed pattern noiseis performed on a pixel-by-pixel basis using a standard two point correction algorithm. In contrast t othermal imagers the gain and offset parameters required for NUC correction in an EBAPS are stable overtime and can be stored in the camera at the factory. The cameras also perform bad pixel correction using


10

standard approaches. An important camera image processing function is the histogram equalizationalgorithm. Histogram equalization is critical to optimize display of the imagery on a standard 8 bit displayfor viewing and to obtain the performance benefits of an indirect view, camera based, night vision systemrelative to a direct view goggle. It does this by optimizing mapping of the image data to the 8 bit display.The image processing functions in the EBAPS cameras are all performed with only a few lines of latency asrequired for head mounted applications.

6. Conclusion

Planned future military night vision equipment will be video based using a head mounted low light levelcamera coupled with a microdisplay. Commercial products with significant potential are also primarilycamera based for 24-hour security monitoring applications, scientific applications, and medical applications.This program has demonstrated a family of EBAPS based cameras which meet the requirements for bothcommercial applications where cost and performance are critical and next generation video based headmounted night vision applications. Future work at Intevac will include completion of the ISIE10 EBAPSsensor and camera later this year along with camera power reduction activities. The ISIE10 EBAPS will haveoptimum performance for camera based head mounted night vision applications. Expected performance forthe ISIE10 EBAPS is comparable to presently fielded Gen-III night vision goggle systems. Other workunderway includes development of prototype EBAPS based HMD systems which will leverage the packagingadvantages of the EBAPS camera technology.

7. Acknowledgements

This work has been supported by Intevac with partial support by the US Army. I would also like t oacknowledge the continuing support of the US Army RDECOM and CERDEC and Night Vision andElectronic Sensors Directorate and the many useful discussions with them during the course of this work.

8. References

1 M. S. Robbins and B. J. Hadwen, “The Noise Performance of Electron Multiplying Charge-Coupled Devices,” IEEETrans. Electron Devices, Vol. 50, pp. 1227-1232, May, 2003.

2 J. Hynecek and T. Nishiwaki, “Excess Noise and Other Important Characteristics of Low Light Level Imaging UsingCharge Multiplying CCDs,” IEEE Trans. Electron Devices, Vol. 50, pp. 239-245, Jan., 2003.

3 V. W. Aebi, K. A. Costello, J. P. Edgecumbe, J. J. Boyle, W. L. Robbins, R. Bell, D. Burt, A. Harris, I. Palmer, andP. Pool, “Gallium Arsenide Electron Bombarded CCD Technology,” SPIE Vol. 3434, pp. 37-44, 1998.

4 M. Suyama, A. Kageyama, I. Mizuno, K. Kinoshita, M. Muramatsu, K. Yamamoto, “An electron bombardment CCDtube”, SPIE Vol. 3173, pp. 422-429, 1997.

5 R. A. LaRue, K. A. Costello, G. A. Davis, J. P. Edgecumbe, and V. W. Aebi, “Photon Counting III-V HybridPhotomultipliers Using Transmission Mode Photocathodes”, IEEE Trans. Electron Devices, Vol. 44, pp. 672 – 678,1997.

6 G. M. Williams Jr., A. L. Reinheimer, V. W. Aebi, and K. A. Costello, “Electron-bombarded back-illuminated CCDsensors for low light level imaging applications”, SPIE Vol. 2415, pp. 211-235, 1995.

7 E. R. Fossum, “CMOS Image Sensors: Electronic Camera-On-A-Chip,” IEEE Trans. Electron Devices, Vol. 44,pp. 1689-1698, 1997.

8 Pixim Inc.: datasheet D2000 Imaging System (On-line). Available:http://www.pixim.com/products/Pixim_D2000_Product_Brief.pdf

9 Edward J. Bender, “Present Image Intensifier Tube Structures,” Electro-Optical Imaging: System Performance andModeling, Lucien M. Biberman, Editor, SPIE Press, pp. 5-1 – 5-96, 2000.

ebaps : next generation, low power, digital night …...ebaps®: next generation, low power, digital...

Documents