Wide field-of-view digital night vision head-mounted display

Michael P. Browne*a, Bobby D. Footeb

aSA Photonics, 915-D Terminal Way, San Carlos, CA 94070; bVision Systems International, 641 River Oaks Parkway, San Jose, CA 95134

ABSTRACT

SA Photonics and Vision Systems International (VSI) are developing an innovative wide field of view digital night vision head mounted display (HMD). This HMD has an 80 degree field of view and has been designed to minimize weight, peripheral obscuration and forward projection. Digital night vision sensors enable electronic image enhancement and VSI’s Zero A/C Integration enables the HMD to be integrated with legacy aircraft and provide symbology overlay and recording without the need for an expensive drive electronics box.

Keywords: wide field of view, digital night vision, head mounted display

1. INTRODUCTION Night vision has been a key enabling technology for the past 30 years that has allowed US pilots to “own the night”. In many engagements, our dominance of the nighttime environment was the decisive factor in victory. One of the big disadvantages of night vision systems is that they have not provided pilots with good peripheral vision, since most have a total field of view (TFOV) of only 40 degrees. A large survey of USAF pilots found that the most often requested improvement to night vision goggles was to have a larger field of view.1

SA Photonics is working with Vision Systems International to develop a wide field of view digital night vision module for integration into VSI’s Targo™, a low cost, low integration HMD solution for light attack, airlift and trainer aircraft.

Wide field of view night vision goggles, such as InSight’s PNVG and Kollsman’s QuadEye system were designed to address this need. In the 15 years since the PNVG was designed, tremendous advances have been made both in microdisplay technology and high-resolution digital night vision sensors, which enable a new class of wide field of view digital night vision systems.

2. SYSTEM DESCRIPTION Targo™ is an innovative modular day-night HMD system and it consists of a helmet shell with separate day and night modules, shown in Figure 1. The day module is monocular, and uses off-the visor projection with a 20° field of view. It has a monochrome green display that is presented in the right eye only. The night module is a wide field of view digital night vision system designed by SA Photonics. Unlike other HMD’s, Targo™ does not require any bay installed electronics units. Targo™ was designed to integrate easily into legacy aircraft with minimal cost and no requirement for additional cockpit volume.

*m.browne@saphotonics.com; phone 1-408-348-4426; saphotonics.com

2.1 Targo™ components

Figure 2 shows the Targo™ system block diagram. Targo™ is different from most legacy HMD systems as it uses helmet-mounted miniaturized electronics, and does not require large dedicated electronics boxes in the aircraft. The Targo™ system consists of a helmet system with day and night modules, a tracker reference unit (TRU) and the aircraft interface unit (ACIU).

MCMC

QDCQDC

TRUTRU

Power

Legend

HMD system LRU

Avionics system

AC

IUA

CIU

Data Video up

DVRDVR

Video down

A/C VideoA/C Video

TRU Service connector

Operating Console

Operating Console

Lower HVI

Upper HVI

Helmet electronics

BMCDay module

Discrete, Analog, Serial

MCMC

QDCQDC

TRUTRU

Power

Legend

HMD system LRU

Avionics system

AC

IUA

CIU

AC

IUA

CIU

Data Video up

DVRDVR

Video down

A/C VideoA/C Video

TRU Service connector

Operating Console

Operating Console

Lower HVI

Upper HVI

Helmet electronics

BMCDay module

Night module

Discrete, Analog, Serial

Figure 2: Targo block diagram.

Figure 1: Targo™ helmet with day module (left) and 80° night module (right).

The helmet system consists of a helmet with a display module (day or night) and a backpack electronics unit, which includes the upper section of the helmet vehicle interface (HVI) harness and is mounted on the back of the helmet. The display modules interface to the electronics system by way of a blind mate connector (BMC). The day display module includes the image source and relay optics, the tracker sensor and debriefing camera. The night module includes the tracker sensor and the wide field of view digital night vision module designed by SA Photonics. The Tracker Reference Unit (TRU) is shown in Figure 3. This aircraft mounted miniature unit serves as a reference to the TARGO™ Day/Night Configuration tracker. The TRU is 2.0 by 2.0 by 6.7 cm and is located in the forward section of the aircraft near the HUD. Integration of additional LRU’s into any airframe is difficult and costly. VSI has recognized this issue and has created an Aircraft Interface Unit (ACIU), shown in Figure 4. The ACIU can be integrated into a platform with minimum impact, requiring no cooling or special rack. The ACIU regulates the HMD’s power from aircraft power (per MIL-STD-704) and provides an aircraft interface layer for the HMD. This interface can include Mission Computer communication (Ethernet & 1553B, ARINC based on configuration), video communication (HMD’s debriefing video, aircraft video for HMD display) and other interfaces. The ACIU is 3.5 cm by 3.5 cm by 15 cm and can be located anywhere in the crewstation, but is usually located under the control panel. 2.2 Targo™ features

A top-level listing of the Targo™ system features is given in Table 1.

Figure 3: Tracker Reference Unit.

Figure 4: Aircraft Interface Integration Unit.

Table 1: Targo™ features.

Feature Characteristic Field of View Day- 20° diagonal

Night - 80° horizontal x 30° vertical Power 17 watts Weight HMD – 4.5 pounds day or night configurations

AC installed equipment < 5 pounds A/C Interface 1553 and/or Ethernet, wireless capable Brightness Meets 1.2 contrast in 10,000 ft-L with dark visor

(Targo™ has dual visor configuration) Safety Configuration tested to 600 KEAS

Low voltage QDC tested to ACES seat configuration To maximize the benefit from a digital night vision system, a high speed video processing system is needed. Because we wanted to be able to retrofit this system easily into existing airframes, we did not want a system that had a large electronics box. As such, we needed a state of the art video processing system that could be mounted on the helmet. VSI, Rockwell Collins Display Systems (RCDS) and the U. S. Government are funding the development of on-helmet processing technology key to producing a high resolution digital vision system. In addition to tiling the two sensors into a single display, the video processor will also have the capability to perform the following:

• Digital distortion correction • Contrast and edge enhancement • Video compression and recording • Symbology overlay • Scintillation (speckle) reduction • Sensor fusion

None of these operations are currently possible with traditional night vision sensors. We feel that the “digital advantage” will bring a significant increase in utility to operators above that provided by conventional night vision goggles. The current direct view NVGs offer zero latency of the real world when viewed by the pilot. It is key that our system also provide a near zero latency system to achieve the performance experienced by the pilot with NVG’s. The on-helmet video processor is being designed to have low latency as well as low power and weight. This system avoids the typical frame delay and lag seen by most electronic imaging systems by processing the video real time in a parallel fashion. 2.3 Digital night module description

The 80 degree high resolution night vision system (HRNVS) module developed by SA Photonics is shown in Figure 5. This design was driven by our goals to 1) improve resolution, 2) reduce weight, 3) improve center of gravity, 4) reduce forward projection and 5) reduce peripheral obscurations compared to existing conventional wide field of view night vision systems, while providing the advantages of a digital night vision system.

Figure 5: SA Photonics’ 80 degree horizontal field of view digital night vision system.

HRNVS utilizes two solid-state digital night vision sensors per eye and couples them to a single high-resolution microdisplay. By not tiling or seaming the optical system, the user will have a panoramic view of the night vision imagery without the visual distractions caused by using multiple displays or eyepiece optics per eye. The objective specification for the HRNVS digital night module is shown in Table 2.

Table 2: Objective specification for the HRNVS digital night module.

Specification Value Horizontal Field of View 80 degrees Vertical Field of View 40 degrees Binocular Overlap 30 degrees On-axis Exit Pupil Diameter 15 mm Off-axis Exit Pupil Diameter 12 mm Eye Relief 30 mm System Resolution >1.3 cy/mr Forward Projection <2.5” Halo Diameter 0.0 mm Distortion (compensated) <1.0% Center of Gravity Ejection Safe Total Headborne Weight (display plus O2 mask, hose, receivers, visor, etc.)

<4.5#

2.4 Digital night module components

The digital night module consists of the following:

• Digital night vision sensors • Custom objective lens with multiple filter options • On-helmet video processor and electronics • Helmet interface kit • Adjustment system • Digital microdisplay • Eyepiece optics

Each of these are discussed more fully below. The digital night vision sensors are EBAPS (electron bombarded active pixel sensors) sensors from Intevac. These sensors have a GaAs photocathode, just like standard Gen III night vision goggles. Behind the photocathode is a silicon CMOS sensor, which provides the digital output. The objective lens for the HRNVS has been custom designed for the EBAPS sensor by SA Photonics. This objective lens has very good performance over the entire field of view and has been designed with a wide chromatic bandwidth so it can operate well with NVIS filtering or unfiltered. The electrical system block diagram and data path is shown in Figure 6. The electrical system consists of the 4 sensors which send imagery to the on-helmet video processor discussed above. This video processor then outputs imagery to the left and right displays. In addition, symbology can be injected from the ACIU and imagery can be compressed and recorded for after action review.

The power distribution system consists of a single input voltage from aircraft power which is then regulated to supply the sensors, displays and video processor. The helmet interface kit allows the HRNVS digital night module to interface directly with VSI’s Targo™ helmet system. In addition, the night module can be adapted to attach directly to an HGU 55 or 56/P helmet via an interface kit that uses a standard night vision goggle mounting plate attachment. The adjustment system has been designed to address the widest possible range of users, based on anthropometric studies done by both the USAF and the Army. The adjustment range for the fit system is shown in Table 3. These adjustments have been designed to address the 1st percentile female through 99th percentile male population.

Table 3: HRNVS adjustment range.

Adjustment Range (mm) Interpupillary Distance (IPD) 55-75 Vertical 35 mm Fore/Aft 36 mm

We have selected eMagin’s OLED display as the microdisplay source for HRNVS. This display has a small size and requires no front or backlight, which reduces the size and weight of the eyepiece, as well as simplifies the packaging and electronics requirements. The eyepiece optics are a patented design which provides high resolution and a wide field of view in a small package with minimal obscuration of the outside world or cockpit.

Figure 6: HRNVS electrical system block diagram.

2.5 Digital night module features

There are a number of advantages in having a digital high resolution, wide field of view night vision system like the HRNVS. These include:

• Lower cost. Because they can be made on a standard semiconductor production line, the cost of the digital night vision devices in large volume production can be much less than the cost of standard night vision tubes.

• HRNVS will weigh less, have a reduced forward projection and have a much better CG than the ANVIS or PNVG systems. The greatly reduced forward projection and weight of HRNVS will increase operator safety, decrease fatigue and allows the display to be stable even when pulling high-Gs.

• Increased capability. Digital night vision devices allow for image processing, which current night vision tubes cannot do. Advances in image processing and algorithms can dramatically reduce “speckle” or scintillation in a night vision sensor. As such, the user will be able to see more at night, giving them more of an advantage over the adversary.

• Zero-halo. The EBAPS sensors have a proprietary halo reduction structure that reduces halo around bright light sources to near-zero.

• Information overlay. With a digital night vision system, we can easily overlay aircraft symbology such as heading, airspeed, and targeting information, exactly like that presented on a head-mounted display or head up display.

• Ability to stow each eyepiece independently, with minimal swept volume. We designed the HRNVS module to have independently stowable eyepieces. We have also designed a novel stowage mechanism for HRNVS, as shown in Figure 7, which we believe will be easier to use than ANVIS or PNVG. Instead of flipping upwards like ANVIS or the PNVG system, making a too-high, too-forward CG even worse, the combiners on HRNVS simply pivot out of the way, hardly changing CG at all. In the stowed position the HRNVS does not block the operator’s peripheral vision at all, what they see is just determined by the extent of the helmet. Since the stowed postion is flat against the side of the helmet, and because we minimized the forward projection of HRNVS, the total swept volume, as shown in Figure 8, is much less than for PNVG or ANVIS. This means that pilots will have more freedom to turn their heads in the cockpit (especially for a “check 6” manoeuver) without worrying about impacting the canopy in either the deployed or stowed mode.

Figure 7: HRNVS with stowed eyepieces.

3. CONCLUSIONS The VSI and SA Photonics team has been successful in the design of a modular day/night helmet mounted display system that provides 24 hour functionality for pilots in light attack, airlift and trainer aircraft. This system can be easily retrofit into legacy aircraft, without the need for expensive aircraft modifications. The day module provides aiming capability along with a “HUD on the head”, while the night mode provides for immersive, wide field of view night vision imagery with significant advantages provided by a digital sensor system. The improved CG and the reduced weight, forward projection and peripheral obscuration of the night module compared to other night vision imaging systems will provide the pilot increased usability, comfort and safety.

ACKNOWLEDGEMENTS

SA Photonics gratefully acknowledges the financial support of the USAF, Army and VSI under SBIR Phase II Fast Track contract number FA8650-09-C-6018.

REFERENCES

[1] Donohue-Perry, M. M., Hettinger, L. J., Riegler, J. T., and Davis, S. A., “Night vision goggle (NVG) users' concerns survey site report”, Dover AFB DE (Report No. AL/CF-TR-1993-0075) Wright-Patterson AFB, OH, (1993).

Figure 8: HRNVS swept volume is less than PNVG or ANVIS.