All Solid-State Electrochromic Device for Helmet ...

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Mar 8, 2007 - Helmet-Mounted Displays (HMDs) do not allow the pilot to change ... deposition process enables direct application onto HMD flight visors.
All Solid-State Electrochromic Device for Helmet-Mounted Displays Hulya Demiryonta, Kenneth Shannon IIIa, Jan Isidorssona, Sharon Dixonb, and Alan Pinkusb a

Eclipse Energy Systems, Inc., 2345 Anvil Street North, St. Petersburg, FL 33710 b

The Air Force Research Laboratory, Wright-Patterson AFB, OH 45433 ABSTRACT

Helmet-Mounted Displays (HMDs) do not allow the pilot to change transmission level of a visor transitioning from high to low light levels. A variable-transmittance visor (VTV) is a possible solution. The Eclipse Variable Electrochromic Device (EclipseECD™) is well suited for these light modulation applications. The EclipseECD™ modulates light intensity by changing the transmission level under an applied electric field. The optical density may be continuously changed by varying voltage. EclipseECD™ is comprised of vacuum deposited layers of a transparent bottom electrode, an active element, and a transparent top electrode, incorporating an all, solid-state electrolyte. The solid-state electrolyte eliminates possible complications associated with gel-based technologies, the need for lamination, and any additional visor modifications. The low-temperature deposition process enables direct application onto HMD flight visors. Additionally, the coating is easily manufactured; can be trimmed, has near spectral neutrality and fails in the clear (bleached) condition. Before introducing VTV technology to the warfighter, there are numerous human factors issues that must be assessed. Considerations include optical characteristics such as transmissive range, haze, irising, internal reflections, multiple imaging, user controllability, ease of fit, and field of view. Advanced materials tailoring coupled with meeting critical criteria will help ensure successful integration of VTV technology. Keywords: helmet-mounted display, variable-transmittance visor, electrochromic device, visor, thin films

1.0 INTRODUCTION The human eye has a great range of adaption to various light levels, although high light-intensity will put strain on the eyes with time. In very bright conditions a pilot can usually soothe the strain on the eyes with a tinted sun-visor or sunglasses. With helmet-mounted displays integrated into the helmet/visor unit an extra sun-visor or sunglasses may not be practical. To adapt to various light intensities a pilot will experience during a mission, one attractive option is to vary the transmission of the visor itself, and hence adapt to the situation at hand. Furthermore, for comfort, the transmission through the visor should be continuously adjustable throughout the possible illumination levels a pilot experiences during a mission. Electrochromic Devices (EclipseECD™) have the property of being able to continuously change transmission by applying a small potential1. This is a reversible process tailorable to a range and speed for convenient adjustment of the light level to an arbitrary setting within the ECD’s range (analog as opposed to digital control), (i.e., it not only has an on/off type of maximum/minimum transmission setting, it could have a manual and /or sensor-assisted adjustment of comfort level of transmission). ECDs most prominent application to date is in anti-dazzling rear-view mirrors for automotive applications. This is a common accessory to cars today. Other automotive applications like ECD sunroofs are emerging at this time. The ECD “Smart Window” for buildings has big implications on energy-savings, though this application relies on achieving large scale deposition technologies and competitive pricing. There are also applications for ECDs in the infrared (IR), for example control of the emissive properties of surfaces. This is of particular importance in space applications.

Head- and Helmet-Mounted Displays XIII: Design and Applications, edited by Randall W. Brown, Peter L. Marasco, Thomas H. Harding, Sion A. Jennings, Proc. of SPIE Vol. 6955, 695507, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.777794 Proc. of SPIE Vol. 6955 695507-1 2008 SPIE Digital Library -- Subscriber Archive Copy

2.0 DESCRIPTION OF ELECTROCHROMISM In an electrochromic material the optical properties of the material change during intercalation or deintercalation of ion electron pairs into the host material. It is clear that a material with different states of composition may have different optical properties, (i.e., can be more or less transparent). On the other hand, the physical processes behind the changes in the optical properties in electrochromic material are not well understood for all materials. Example materials for Electrochromism are typically transition metal oxides2,3. There are cathodic and anodic coloring materials, they change from transparent to colored state upon insertion of ions-electron pairs (cathosic) and upon extraction of ion-electron pairs (anodic). The cathodic reaction can be described in chemical terms as:

MO + xe − + xI + ⇔ I x MO Transparent

(1)

Colored

Where MO is a transition metal oxide and I+ and e- are small low molecular weight ions and electrons respectively. The coloration taking place in MO upon ion/electron-insertion can be modeled with the small polaron theory5,6,7. The insertion of an ion into the MO film is accompanied by insertion of a charge-balancing electron. The electrons enter localized states below the conduction band and initiate a local polarization, thereby creating a self-induced potential well. The polaron only affects the nearest neighbors and hence is called a small polaron. The electron will change the valencestate of its host metal site from M+X to M+X-1. The coloration of the MO, or rather the absorption of light, take place when the polaron absorbs light, get excited, and jumps to an adjacent metal site. Maximum coloration is achieved with an Ion to M ratio of about 0.1-1 for different electrochromic transition metal oxides. At this ratio there are several M+X sites surrounding the M+X-1 small polaron site resulting in a high probability for the small polaron to absorb light and jump to the adjacent metal site.

3.0 THE ELECTROCHROMIC DEVICE The ECD works in a way similar to rechargeable batteries; ions are transported from one electrode to the other, and reversibly back again. The major difference between a rechargeable battery and an ECD is that the latter is transparent. There are two major groups of electrochromic materials, organic materials, where Prussian Blue is one of the most studied (Figure 1), and inorganic materials like transition metal oxides (Figure 2).

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ELECTROCHROMIC DEVICE

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Figure 1. Basic schematic of a laminated electrochromic device, indicating transport of positive ions through an polymer electrolyte under the action of an electric field1

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IONS

Light

0.6v

Low Transmission IONS

Substrate / Flexible Plastic

IONS

Light

Light

High Transmission Transparent Electrode Ion Storage Layer (IS) Electrolyte Electrochromic Layer (EC) Transparent IR Reflective Electrode Electrode Kapton Thermally conductive glue

Transparent Electrode Ion Storage Layer (IS) Electrolyte Electrochromic Layer (EC) Transparent IR Reflective Electrode Electrode Kapton Thermally conductive glue

1.6v IONS

Substrate / Flexible Plastic

Figure 2. Basic schematic of an inorganic all solid state and monolithic electrochromic device. Ion movement through an inorganic electrolyte is indicated by arows in the figure.

A transparent ECD will contain a number of layers with different properties. First a substrate, glass or plastic, with a transparent conducting layer. On top of the transparent conductor is the electrochromic layer, followed by an electrolyte and a counter electrode and a top electrode. These three layers electrochemically function in a similar to a battery. The counter electrode can be either an optically passive ion-storage layer, or an active electrochromic layer. In the latter case a cathodic and an anodic coloring material are matched to each other. Complementary layers like this will increase the coloring efficiency of the device as the cathodic layer colors upon ion insertion and the anodic layer colors upon extraction of ions. On top of these layers a final conductive layer is deposited. In a reflective ECD one of the conductive films can be exchanged to a highly reflecting metal. In Figures 1 and 2 complete devices are depicted. Transport of ions is also indicated in the figures. There are two major approaches to the electrolyte in ECDs. The first is a polymer electrolyte, where the anode and cathode are deposited on two separate substrates and subsequently laminated together with binder functioning polymer electrolyte4. Secondly, there are all-solid-state monolithic devices. This type of device all layers may be vacuum deposited single substrate. Both approaches have their advantages and disadvantages. The polymer device is relatively easy to manufacture but polymers are not completely stable to UV radiation. The all-solid-state monolithic device is more difficult to manufacture. This type of ECD is not sensitive to UV radiation. An all-solid-state EclipseECD™ ia deposited on one substrate with a good process control of composition and avoids contamination of interphases between subsequent layers. The all-solid-state monolithic devices have the advantage to be applicable on curved surfaces. Furthermore, it is possible to deposit on top of other coatings and hence the dimming effect of an EclipseECD™ is separated from, and does not interfere with the display-function of an HMD.

4.0 APPLICATIONS Eclipse Energy Systems, Inc. employs both inorganic and organic materials, as well as all-solid-state, monolithic and laminated devices. Furthermore, Eclipse Energy Systems has a long history making ECDs and a unique knowledge of electrochromic materials and systems. In addition, an extensive smorgasbord of technologies has been developed by Eclipse Energy Systems to utilize for various applications, the more obvious of which are in the visible light modulation. Eclipse also has applications for the infra-red part of the spectra, an emissivity-modulating device. The MidSTAR1 satellite launched 8th March 2007 included EclipseIR-ECD™ for thermal control.

5.0 EYE-WEAR Eclipse has two types of switchable ECDs, the sunglass-type and prescription type. The Eclipse sunglass (EclipseView™) has a transmission range from about 50% down to 10%. This range is suitable for a sport goggle in

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daylight conditions (See Figure 3). The ECD in the sports-type goggle has a very durable combination of material and has been cycled for more than one million cycles.

Figure 3. Motorcycle goggles in bleached and colored condition

This ECD is deposited on a flexible polymer substrate and cut and trimmed to fit the frame of the goggles or as a retrofit inside the goggles. Figure 4 demonstrates how the ECD can be cut to the desired shape without compromising its function.

tn

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Figure 4. EclipseView™ with a transmission range of 18% to 50% at 550 nm, transition time 5 seconds cut in half with scissors, and cycled from bleached to colored

An ECD for helmet mounted displays (HMDs) should be able to combine the day and night-visor into one single unit. This visor would require a wider transmission range than the sports visor described above, specifically, a higher transmission in the clear state. This will enable pilots to use the same visor for both night and day missions. To fulfill this goal a prescription-type visor with a transmission range from 80% to 20%, switching in a few seconds is required. This system will also be applicable for night vision goggles providing daytime functions. Eclipse has tailored such an electrochromic system to show that an ECD can meet these requirements. Figure 5 shows ECD-devices with a transmission range from 80% to 15%.

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Transmittance (%)

80

60

40

Bleached Colored

20

400

500

600

700

Wavelength (nm) Figure 5. EclipseECD™ with transmission range from 80% to 15%

In Figure 6, the speed of the device depicted in Figure 4 is shown. It is clear that the bleaching process is very quick. The transmission changes from 20% to 70% in less than 5 seconds, and continues to bleach to 80% transmission. Coloring is also fast, and within 10 seconds the transmission is reduced from 80% to 32%, continuing down to 20%. Figures 5 and 6 depict the ECD device deposited on glass. This device may be deposited on plastic substrates as well. 8 80

Current (mA)

6

60 4 40

2

I(mA)

Transmittance

T%

20

0 0

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Time (s) Figure 6. The same device as in Figure 4. Bleaching takes place in a few seconds, and coloring from 80% to 32% in less than 10 seconds, and continues to 20 % transmission

6.0 HUMAN FACTORS CONSIDERATIONS FOR VTV IMPLEMENTATION A solid-state electrochromic variable transmission visor has been described in detail. In order to ensure successful implementation of this new technology, there are a number of optical, operational, safety, and user interface design characteristics that must be taken into account during the course of development. 6.1 Optical characteristics

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6.1.1 Color Over the years, visors have been produced in a wide variety of colors ranging from the high contrast “yellow shooters visor”8 to the neutral gray (daylight or “solar”) visor and clear (night time) Lightweight Visor assembly (‘Snoopy’) utilized with the HGU-55/P helmet.9 Any tinted visor configuration should be neutral, i.e. have equal transmission for all visible wavelengths, to avoid interfering with the pilot’s perception of color-encoded cockpit instrumentation, colorcoded maps for navigation and external real-world objects. Critical red warning and yellow cautionary signals must retain their normal learned meaning and attention-eliciting attributes. Laser eye protection, gradient transmission and nuclear flash type visors are out of the scope of this discussion. 6.1.2 Transmissive range Currently, fielded flying visors (dark, neutral-tinted visors) have a specified transmission value of 15% with an allowed variance of +/- 3%.10 thus, transmission can range from 12 to 18%; averaging 15 % transmission. Clear visors range from 90% to 92% transmission being comprised primarily of un-tinted polycarbonate with an abrasion-resistant hard coat. To encompass this entire transmissive range of the two currently fielded visors, the VTV would need a range of approximately 15% transmission to about 90% transmission, (about a 1:6 ratio) as a minimum. A range of 10% to 90% should be a design goal. It should be noted that all aircraft have windscreens and canopies that attenuate light to a certain degree. Historically, the impact of the amount of light transmission of visors has been considered independently of the transmission of aircraft windscreens. This factor needs to be taken in account since the visible light transmission in currently fielded aircraft ranges from a high of about 90% in uncoated F-15 canopies to a low of about 20% when looking down toward the nose of a B-1B windscreen.11 Standard Test Method ASTM F131611 provides a test methodology for the measurement of aircraft transparency (e.g. visors and windscreens) transmissivity. 6.1.3 Haze The scatter of light and subsequent undesirable loss of scene contrast as light passes through the transparent medium is termed haze. Standard Test Method ASTM D100312 provides a test method to measure haze. The light scattered by the transparency produces a veiling luminance that can interfere with vision. Haze should be less than 4%. 6.1.4 Multiple imaging Multiple imaging can be caused by non-parallel front and back visor surfaces. When multiple imaging is present, a secondary (or even tertiary) image might be seen, which can cause misperceptions and/or distractions. For example, at night, it has been reported (for certain, thick windscreens) that the pilot was not sure which set of runway landing lights he should use due to the severity of the multiple-imaging effect. Standard Test Method ASTM F116513 provides a test method to measure the angular displacement of multiple images in transparent materials. There should be no apparent multiple imaging present in a visor. 6.1.5 Reflections Reflections can occur from the inside surface of the visor causing lower perceived contrast, masking and distractions. The ASTM F125214 is the standardized test methodology for measuring the visible light reflection coefficient. Reflections should be as low as possible and can be reduced through the use of anti-reflective coatings and treatments. 6.1.6 Non-polarizing Plastic aircraft canopy and windscreen materials can be partially polarizing due to stresses (manufactured or induced) within the materials. If the visor has a polarizing effect then it is possible that colored patterns (birefringence) could be produced, which could interfere with out-of-cockpit vision. Additionally, at high altitudes, blue sky is partially polarized, which can interact with a canopy to cause the ‘rainbow’ effect of birefringence. Having a polarizing visor could further exacerbate the effect.15 For pilots flying over water, a polarizing visor might remove surface reflections and make it difficult to judge the water’s exact location. The bottom line is that the visor must be non-polarizing.

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6.2 Compatibility with HMD designs As helmet designs evolve, the optical quality of a VTV will become a critical factor in its overall functionality. For example, in some advanced helmet-mounted displays (HMDs), the visor is an integral optical component of the entire display optical chain. HMDs, like other helmet systems, have to be able to adapt to a wide range of varying ambient illumination conditions. Using removable, flip-up/flip-down tinted visors poses a design challenge for already complex and heavy HMDs. It is highly desirable to integrate a VTV element into an HMD where it serves the multiple purposes of eye protection, display optical component, and ambient illumination controller. 6.2.1 Irising A helmet visor has a relativity large, complexly-curved surface. A VTV usually has strategically placed electrodes that distribute the activating electrical currents used to switch the device. When the current is applied, the optical density changes start first at the periphery and then move inward. If the electrodes are not properly located and/or the density transitions are very slow, irising can be clearly visible. If the transition is not smooth, irising may cause unwanted distractions for the pilot. Density transitions for a VTV should be fast, seamless and uniform across the whole visor surface. 6.2.2 Visual blockage Unlike a standard visor, a VTV has power drivers, electrodes, demarcations between active areas, and edge treatments (mounting/sealing). All of these additional design characteristics may cause undesirable restrictions of the visual field of view. These need to be minimized or eliminated so as to not impact visual performance. 6.3 Operational considerations 6.3.1 Failure mode It is paramount that in the event of power loss to the device it must fail in the clear mode. In other words if the power is removed from the device it automatically transitions to clear. 6.3.2 Ballistic eye protection A critical operational design feature that must be taken into consideration for a VTV is its ability to afford a specified level of ballistic protection from flying debris and windblast in the event of emergency egress/ejection. Standard visor designs must pass rigorous ballistics and windblast testing. VTV designs propose using a standard clear visor as the base substrate, which already conforms to current ballistic requirements. Further ballistic and windblast testing should be conducted to verify that modification of the standard clear visor with the VTV devices does not compromise the original optical characteristics and ballistic protection. 6.3.3 Switching speed The desired speed for switching between transmission states is highly application dependent. For example, in an urban combat environment, an un-mounted soldier could be moving from building to building; from very bright outdoor lighting to dark, indoor lighting. In this case, a near instantaneous VTV switching speed is essential. The darkened visor would reduce (protect) the soldier’s retinal bleaching levels thus allowing faster adaptation when entering the indoor area. However, there are other scenarios where a fast VTV switching speed is not needed or desirable. In another example, a piloting application, it may be desirable to keep the light levels to the eyes relatively constant. This might be implemented using an automated control circuit that changes the VTV transmission coefficient more gradually and perhaps proportional to external (sensed) luminance within a certain transition zone. 6.3.4 User control

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These devices are capable of continuous, variable control over transmissivity. This feature may sound ideal for certain applications, but in general, it adds an unnecessary complexity. Having a few, visually significant, discrete steps to choose from may be optimal. A discrete-step control also facilitates ease of integration with the helmet. Anecdotal evidence suggests that a simple, maximum/minimum switch is all that is really needed. Fewer steps and proper knob design would also facilitate ease of adjustment with a pilot’s gloved hand. It is possible to envision future VTV systems that are fully automated and controlled using light sensors. The design of this type of controller would need to be evaluated to determine if the set levels of the system are appropriate and do not themselves cause a disturbing/distracting effect. 6.4 Life support/maintenance considerations: fit & usage 6.4.1 Trimming Standard visors usually require custom fitting (grinding) around the oxygen mask area to insure a tight fit. The lenses of the visor are custom- trimmed to accommodate the MBU-12/P Oxygen mask contours.9 A VTV must also be customtrimmable around the nose and cheek contours without damaging the VTV’s functional integrity. If a seal is trimmed away, it must be easily and inexpensively field repairable. 6.4.2 Non-toxicity All materials used to fabricate the VTV devices and coat the visor substrate must be non-toxic and pose no health risk either to the pilot or to the environment, as a whole. This includes any required power drivers such as batteries. Batteries cannot be lithium due to cost and availability. Alkaline-type batteries with commercial off-the-shelf availability are a good choice. 6.4.3 Abrasion resistance Visors are subjected to hard use and abrasive materials (e.g., dust, sand) so a protective hard coat is required. The hard coat must be clear and spectrally neutral having no impact on the visor’s optical or electrical performance. 6.4.4 Periodic maintenance Inspections should be performed to verify that the VTV is functioning properly (i.e., transmission range, uniformity, switching speed, etc.). These tests would be conducted by Life Support and carried out utilizing fixed maintenance schedules. Minimum performance criteria would be pre-established and utilized as criteria for pass/failure of the device. Cleaning, care, and handling requirements should be conducted by Life Support personnel. The potential for refurbishment/repair would be dependent on the amount and type of damage and previous performance results. This would be handled at the Squadron Level Maintenance.

7.0 DISCUSSION AND CONCLUSION Eclipse has a very durable ECD goggle for use in sports-activities. These goggles have an approximate range from 50% to 10% transmission suitable for day-light use and have been cycled for more than one million cycles. Eclipse’s wide range ECD has a range from 92% to 1% transmission, though the switching-speed of this type of ECD is not short enough for the HMD application. The challenge for the HMD visor is the high transmission and short switching speed. To meet the requirements for HMD Eclipse has employed an electrochemical system suitable for this application, and tailored it to meet the requirements. The maximum value for the transmission is not only dictated by the active layers but also by the transparent conductor. The best transparent conductor, ITO, are deposited onto glass and heat-treated to be crystalline, and thereby achieve an excellent transmission and conductivity. Heat-treatment is not possible on plastic and hence other solutions to improve the conduction are used. This implies that plastic-substrate ITO in itself has a slightly reduced transmission compared to the best ITO deposited on glass. Eclipse has shown that ECD’s is a viable option for HMD. The range and speed is

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demonstrated, and with some more tailoring to the application should result in an HMD-ECD that significantly improves on the environment in witch a pilot is working. In order to assure a successful VTV design, advanced materials technologies must also incorporate critical human factors performance goals that are tested and accepted by the warfighter.

8.0 ACKNOWLEDGMENTS The authors gratefully acknowledge the technical expertise of Dr. H. Lee Task, Task Consulting, Tucson, AZ. 85745, in the preparation of this manuscript.

9.0 REFERENCES [1] Granqvist, C. G., [Handbook of Inorganic Electrochromic Materials], Elsevier, Amsterdam, (1995). [2] Deb, S. K., “A novel electrophotographic system,” Appl. Opt. Suppl. 3, 192-195 (1969). [3] Deb,S. K., “Optical and photoelectric properties and color centers in thin films of tungsten oxide,” Philos. Mag. 27, 801-821 (1973). [4] Azens A., Avendaño, E., Backholm, J., Berggren, L., Gustavsson, G., Karmhag, R., Niklasson, G. A., Roos, A. and Granqvist C. G., “Flexible foils with electrochromic coatings: science, technology and applications,” Materials Science and Engineering B 119, 214-223 (2005). [5] Emin, D., Physics Today 35, (1982). [6] Alexandrov, A. S. and Mott N., [Polarons and Bipolarons], World Scientific, Singapore, (1995). [7] Berggren L., PhD Thesis, Uppsala Univeristet, (2004). [8] Heckert, S. A., Hanavan, E. P., Porterfield, J. L., Self, H. C. and McKechnie, D. F., “Airborne visual reconnaissance with yellow sunglasses,” AFAMRL Technical Report 71-36, Air Force Aerospace Medical Research Laboratory, Wright-Patterson AFB, OH, (1971). [9] Gentex Corporation: http://www.gentexcorp.com. [10] Pinkus, A.P., Task, H.L. and Dixon, S.A., “Transmissivity and Night Vision Goggle Compatibility Data For Select Aircraft Transparencies,” AFRL-HE-WP-TR-2003-0015, Air Force Research Laboratory, Wright-Patterson AFB, OH, (2003). [11] ASTM F1316-90, [Standard Test Method for Measuring the Transmissivity of Transparent Parts], ASTM International, West Conshohocken, P.A., (2002). [12] ASTM D1003-00, [Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics], ASTM International, West Conshohocken, P.A., (2000). [13] ASTM F1165-98, [Standard Test Method for Measuring Angular Displacement of Multiple Images in Transparent Parts], ASTM International, West Conshohocken, P.A., (2004). [14] ASTM F1252-89, [Standard Test Method for Measuring Optical Reflectivity of Transparent Materials], ASTM International, West Conshohocken, P.A., (2002). [15] Pinkus, A. R., Task, H. L., Barbato, M. H., Hausmann, M. A. and Dixon, S. A, “Aerospace Transparency Compendium,” AFRL-HE-WP-TR-2003-0084, Air Force Research Laboratory, Wright-Patterson AFB, OH, (2003).

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