High Resolution LED-Projector Stimulating Night Vision Devices Using Infrared Radiation

Transcription

1 High Resolution LED-Projector Stimulating Night Vision Devices Using Infrared Radiation Stephan Bissinger MOD-GmbH. Klagenfurt, Austria Johannes Kerschbaumer MOD-GmbH. Klagenfurt, Austria Hermann Fröschl MOD-GmbH. Klagenfurt, Austria ABSTRACT The development of a high resolution projector with an ordinary visible and a special invisible infrared image projection is presented. The aim is to create an imaging system which is able to produce a realistic night vision impression, both for the naked eye and modern night vision devices. The electrical and optical set up of the night vision module and its integration into a standard projector is described. INTRODUCTION The intention of this project is to develop an imaging system for day and night vision training combined in one single device. Investigated is the upgrade of a standard projector with a night vision module that is specifically adapted to this projector and shall support different generations of night vision devices with no constraints on technique and compatibility. Simulating a nocturnal environment requires low brightness output and yet sufficient image quality, which is hard to achieve with standard ultra-high-pressure (UHP) arc lamps. A special condition on the lighting is the support of a floating transition between day and night simulations, which requires a large dynamic brightness range on screen especially in the low intensity regime. Furthermore this dimming property has to be repeatable over time with a constant output quality and little effect on the display system longevity. The combination of several projectors into a high resolution system shall be practicable. Therefore the lighting and imaging has to be spatially and temporally stable with a similar appropriate contrast ratio in both the infrared and the visible output channel. Moreover the system shall be easily transportable, e.g. light weight and small size to provide an easy set up. BACKGROUND Special technologies were developed which use amplified ambient light as well as infrared radiation to gather visual information in dark surroundings. These devices produce an artificial monochromatic view of the terrain with unique temporal and spatial effects like halos and noise. An adequate interpretation of these night scenes, especially in stressful situations requires an explicit training. Usually simulations are preferable to reduce training costs and risks for human trainees and their technical equipment. Some recent night vision device training systems are built upon the principle of simulating image contents to look like a night vision device recorded image [1] and not stimulating actual night vision devices. These images, however, are displayed using standard imaging methods merely for the naked eye or need special training goggles. The described imaging system uses an advancement of this concept. It displays ordinary visible images and additionally stimulates night vision devices by generating images with a wavelength in the near infrared spectrum. The consequence is that the visible image content is superimposed with an infrared image that can only be seen through a night vision device. The principle requires an additional fourth infrared channel separated from the standard red, green and blue channel. The fourth channel effects the digital image generation, the projection optics and the video electronics. This imaging principle is already used with Laser display systems. So far the disadvantage of Lasers is their rather expensive and expansive light source and lower achievable resolutions due to their mechanical display technique. The following topics discuss the integration concerning the hardware, lighting, optic and electronic control to build up a fourth infrared channel in a standard three channel projector. IMAGING SYSTEM The display concept of front projection allows high resolution and small system size. Mainly there are two different types of projection displays available, the Digital Light Processing [2] (DLP) from Texas Instrument and the Liquid Crystal on Silicon [2] (LCoS) from Sony. The LCoS 1-6

2 system spatially splits the lamp spectrum in three major parts red, green and blue and uses a display for each of these colour intervals. The other major property is that the LCoS chip operates with linearly polarized light and therefore needs special optical parts and complex optical coatings. Furthermore their lighting system consists of UHP arc lamps, which don t meet the specification on lifetime, electronic and enhanced brightness control in respect to the present task. The DLP technology uses a field of switchable mirrors to generate the image information. The chip serially mixes pure colour images together which superimpose in the eye. Such a serial light pulse sequence is most efficiently achieved using three separated light sources of different wavelengths. This lighting system is ideal to be set up by LEDs, which can be easily pulsed and synchronized with the image sequence of the display chip. The DLP solution uses only one chip and appropriately short switching times between images seem to make it possible to introduce a fourth image channel. Furthermore a standard projector combining LED sources with a DLP chip is already commercially available. Thus a standard LED projector with a single DLP chip was chosen to be the basis for the development of the infrared module. Lighting After the selection of the display type and the lighting principle a main subject is to specify an appropriate infrared radiation source. This source has to fulfil three major requirements. First its wavelength has to be invisible to the human eye. Second it has to meet the sensitivity and minimum irradiance values required by modern night vision devices. And third it has to be optically and electronically integrable into the existing system and has to achieve a similar lifetime as the standard LEDs. All three specifications can be satisfied using the Epitex infrared LED L Wavelength This near infrared LED is operating at a peak wavelength of 750 nm with a half width of 30 nm. Strictly speaking 750 nm is not yet in the infrared spectrum but the human eye has a very low sensitivity at this wavelength, especially in scotopic vision [3]. The LED data sheet states a radiated total power of 1 W at a forward current of 0.6 A. Its angular distribution is flat with a full width at half maximum (FWHM) at 60 degrees. Sensitivity Figure 1 is a diagram showing the sensitivity of generation 2 and generation 3 night vision goggles with respect to wavelength. Figure 1: Spectral sensitivity of night vision goggles gen2 and gen3; Source: dated As one can see the sensitivity for wavelength 750 nm is close to the maximum of gen3 goggles and about 15 % for older gen2 models. The radiance of the projector must exceed the value of approximately W cm -2 sr -1 to assure to simulate the intensity of starlight and 89 % moon phase at a latitude of 45 at wavelengths between 380 nm and 900 nm [4]. This value is obtained by fitting curve 4 of Figure 2 with the function RS(λ)=A e α (380-λ) W cm -2 sr -1 nm -1, using the parameters α= nm -1 and A= W cm -2 sr -1 nm and integrating this expression from 380 nm to 900 nm. Figure 2: Spectral radiant sterance of the night sky for different phase of the moon [5] This radiance corresponds to a photopic luminance of 10 mcd m -2 and a scotopic luminance of 26 mcd m -2. The luminance φ was calculated using the equation φ=k RS(λ)V(λ)dλ, with K the equivalent value and V(λ) the 2-6

3 luminous efficiency for photopic or scotopic vision in the interval 380 nm to 770 nm. Assuming 1.2 W power output of the LED, a 100% projector efficiency and a display screen of 4 m 2, the radiation from the screen in the half space would be W cm -2 sr -1. Thus the projector needs an efficiency of <0.2 % for wavelengths around 750 nm to simulate the total power of a moon and star light night. Measurements of the prototypes total radiated power yield an average value of 20 mw on screen which corresponds to an actual efficiency of about 2%. That means the infrared source alone is able to simulate an unnaturally bright night even without any light coming from the visible LEDs. Integration The third requirement faces several subjects originating from different topics, such as optic, electric and thermal. Optics Parts First order colour lens aberrations have the biggest impact on the projection quality. The optic of the standard projector is designed to minimize the lateral colour aberration for visible wavelengths. Figure 3 shows the difference of the image length of a point source over wavelengths for non-paraxial rays for a simulated model of the projectors internal optics. higher magnification ratio, more light losses at the DLP plane and to a more unfocused boarder region at the illuminated DLP. The increase of the colour curves slope for higher wavelengths yields a similar colour aberration between 750 nm and 850 nm. From the aberration point of view every wavelength within this interval would be practicable to be used as source wavelength. The reason to choose a wavelength nearer to the visible spectrum is to reduce power losses due to reflections at coated surfaces. Further lens aberrations as spherical, astigmatism and coma add to the colour aberration but depend on system parameters like numerical aperture, angle of ray incidence and apertures. Setting the same numerical aperture for the infrared (IR) source as for the visible sources shall result in similar second order aberrations. The chosen standard projector fortunately allows a different method to combine the radiation paths that is more practical and less complex than a special colour combiner. A standard mirror of the existing optic is exchanged with a specially coated cold mirror which transmits infrared radiation and reflects visual light. The cold mirror has an average reflectivity of R > 97% for wavelengths between 425 nm to 650 nm (visible) and an average transmission of T > 93% in the interval 730 nm to 770 nm (near infrared). Starting at the DLP panel both the visible and the infrared beam share the same optic path, see schematic Figure 4. Infrared LED Cold Mirror Lenses DLP Panel Figure 3: Lateral colour curve for the standard optical system Red LED Blue LED The red image centroid at a wavelength of 625 nm and the blue image centroid at 465 nm spread about +-30 μm around the green image centroid at 528 nm. A first attempt to include the infrared path into the visible system is to exchange the colour combiner of the three visible LEDs and use a combiner with an additional dichroic plane to introduce the infrared radiation. This method is only successful if the projector optics is already suited to handle the wavelength of the additional source properly. Coupling a wavelength of 750 nm into a simulated system leads to a difference in the image length of about +60 μm, hence to a Green LED Figure 4: Sketch of the optical system At the cold mirror the IR beam is basically transmitted in a straight path. This method to integrate the infrared source doesn t change the optical properties for the visible path, except for a little loss of illumination due to a slightly worse reflective coating of the mirror. Furthermore it allows to design lenses especially for the chosen wavelength to optimise the illumination at the DLP. The 3-6

4 infrared path was simulated and a lens was designed using the software Zemax. Electric Control The infrared source needs a special electric control which guarantees a highly constant current to stabilize the radiation output especially for the Epitex LED which is generally not supposed to have a lighting quality required in projection applications. A control board was developed which basically works as a temperature regulating constant current supply up to 750 ma. Figure 5 shows a schematic diagram. Figure 5: Schematic diagram of the infrared LED control board The board has the ability to synchronize the infrared source with every visible LED thus it establishes a synchronisation with the DLP. Another essential feature is that every single LED can be dimmed in 256 brightness steps and can be disabled separately. An electric dimming to that extent is special to LED sources and gives the possibility to vary the colour intensities in each frame, which enables the processing of special image contents, see next paragraph. The board is realised with an I 2 C protocol to communicate with the internal projector electronics. Via an Ethernet port and the DCP/IP protocol a PC can directly control the LEDs driving current to adjust different brightness settings during simulation. The board also controls the cooling of the infrared source. A heat sink with a temperature probe is connected to the backside of the LED. The sink incorporates a fan which is regulated by the board and maintains an LED operating temperature under 40 C to guarantee a stable radiation and a long lifetime. Projection Modes The display system uses a 60 Hz single DLP chip technology. Single chip means single channel generation and processing of image information. A computer, the image generator, forms pictures out of a database and sends them to the projector via a standard DVI single link. Every frame consists of three pure colour image data. The projector electronics processes this signal and generates the digital image content needed by the DLP. The chip images 60 frames per second, whereas every frame consists of several sub frames. The achievable visible colour depends on the sequence of colour pulses and their intensity in the sub frames. The light sources have to be pulsed at sufficient frequencies and have to be synchronized with the DLP. The possibility to independently control the pulse intensity, width and sequence of the LED radiation is the key to introduce a forth infrared colour. Dual Channel The solution which shows the highest image quality uses two separate projectors, one for the visible content, and one for the infrared, which superimpose on the screen. The disadvantage is that the number and cost of the imaging system and the image generation doubles. The same quality would be achievable using a 120 Hz projector which displays the visual and the infrared content both at 60 Hz. Apparently this method sacrifices half the achievable brightness and also requires two image generators for the visible and the invisible content. Multiplexing/Colour Mixing The ability to separately turn on and off each light source generally enables an arbitrary combination of available wavelengths within a DLP frame. The common sequence red, green and blue can be intermixed arbitrarily with the infrared wavelength. Multiplexing means that every subsequent frame the visual LEDs are switched off while the infrared LED is switched on and the respective DLP image content is exchanged by the infrared content. This method does not work properly because the frequency of bright images alternating with infrared images (which appear as visible black images) is reduced to 30 Hz. This causes the eye to perceive flickering of the displayed scene [5]. Colour mixing exchanges only one arbitrary colour with the infrared within each frame. Although images will change their colour from frame to frame they do not show flickering and the difference in colour impression should not be noticeable in night scenes. This mode has not been tested yet because an advanced intervention into the video electronic of the DLP is necessary. Colour Substitution Other methods managing to produce a night vision with reduced quality that needs only one projector and one image generator are mainly realised with special image content and special ratios of the LEDs light levels. Using only one data channel the projector would not be able to display a separated infrared content like halos and noise 4-6

5 effects without displaying them in the visible as well. The idea is to generate an image which shows bright regions like windows unmodified, but dark surroundings in a monochromatic colour, for example blue, see Figure 6. single one-dimensional micromirror which is hit by a plane wave at an angle θ i. In normalized form this can be written as [2] i I = j k sina π b 8sin Hθ r βl + sin Hθ i βl<e y λ π λ b 8sin Hθ r βl + sin Hθ i βl< Figure 7 shows this angular distribution for a square sized mirror, length b = 9 µm, in the mirrors On state at a tilt angle of β = -12 in dependence on the aperture angle θ r. The light incidence angle is θ i = 2 β. 1 2 z { (1) wave 464 nm 0.8 wave 528 nm Figure 6: Special image used for colour substitution When the projector displays this image the blue LED is disabled and the infrared LED is turned on. That leaves bright regions bright for the eye with a yellow touch but the surrounding is only detectable by night vision devices. The disadvantage is that artificial blue lights at airports for example can t be displayed with their real colour. An improvement of this method is called Partial Colour Substitution. A similar blue image (Figure 6) is used with the only difference that regions which shall appear blue in the night vision are modified to have an enhanced intensity. Upon displaying this image the blue LED is not disabled but strongly dimmed while the infrared LED radiates at full power. The advantage is that each colour can be displayed, however looking closely on the screen the infrared content which is shown in the blue image may be noticeable with the naked eye. The implementation and perfection of this method is currently under development. RESULTS The projector prototype shows an optical performance less than expected because the designed near infrared lens, mentioned above was not yet evaluated and the mirrors built in are of lower quality than defined in the final specification. The near infrared has an average radiant flux of about 22 mw and a sequential contrast ratio, without dimming the LED of about 250:1. Compared to the visible contrast ratio this near infrared ratio is only about 15%. This low value is caused by a combination of two effects. First the wavelength dependent diffraction behaviour of the DLP chip and second the lens design of the projection lens which has a threshold at 700 nm. A simple approach to quantify the first effect leads to the calculation of the angular distribution of the diffraction intensity from a I ntensity H n ormalize dl wave 624 nm wave 750 nm Aperture D Figure 7: Angular distribution of the diffractive intensity for a onedimensional micromirror illuminated by a plane wave Figure 8 shows the Off state angular distribution for β=12 and θ i =-2 β. The 0 aperture angle corresponds to the direction to the projection lens. Compared to the red wavelength the diffraction intensity for the near infrared at θ r =0 increases both in the amplitude and in the full width at half maximum about 30% and 40%, respectively. I ntensity H n ormalize dl wave 464 nm wave 528 nm wave 624 nm wave 750 nm Aperture D Figure 8: Angular distribution of the diffractive intensity for a onedimensional micromirror in off state illuminated by a plane wave The enhanced diffraction intensity leads to an enhanced black level in the near infrared and hence to a poorer contrast. More detailed investigations addressing the contrast ratios detectable through night vision devices will be subject of further work. 5-6

6 CONCLUSION In summary the concept using an additional infrared channel to stimulate night vision is the logic next step regarding training and simulation using individual night vision devices. A near infrared LED was successfully integrated into a standard LED projector. Its optics and internal electronics were upgraded and modified to handle the additional light source. The LEDs electronic control perfectly suits the requirements regarding enhanced brightness and colour control needed for the projection modes described above. Their biggest disadvantage is the constraint in maximum brightness around 600 lumen on screen which is good for simulating dusk or dawn but not sufficient to simulate a bright day scene as well. Critically comparing the described solutions the 120 Hz dual channel and the 60 Hz single channel with partial colour substitution cover most of all thinkable simulation requirements thus further developments will concentrate on these two systems. The 120 Hz dual channel is the most cost efficient solution achieving the qualitatively best performance. The only disadvantage is that the electronics will not be available until the end of The decrease of brightness to 50% due to the bisection of the visible frame actually has no effect on night and day simulation. This is because when bright scenes are displayed the IR content can be omitted and the whole 120 Hz frame is used for the visible image. So there is no change in maximum brightness, on the contrary the 120 Hz frequency has the advantage of an additional reduction in motion blur. The method of partial colour substitution for one 60 Hz projector is the favourable solution if only one DVI signal is present. This system is already available but needs the special image input which can be included into any image generation process. Another possibility, making this method independent of the image generation, is a hardware tool which operates a DVI signal directly and converts a night vision image into a partial colour substitution image. In conclusion, the final task of the present work will be the speedy implementation of one or more specific projection modes using this hardware tool. This will allow to present a fully operating night vision simulation system based on a single projector in due time. ACKNOWLEDGMENTS The authors wish to thank adhoc, a hard- and software Development Company located in Austria for granting the schematic diagram of Figure 5. REFERENCES [1] J. O Reilly, M. Kochmann, B. Chladny, J. Clark, B. Colbert, 2007, Common Sensor Model Common Components A Design Approach, Proceedings of the I/ITEC Conference, CAE USA Inc., PEO STRI, Renaissance Science Corporation. [2] D. Armitage, I. Underwood, S.-T. Wu, 2006, Introduction to Micro Displays, John Wiley & Sons Inc. [3] S. J. Williamson, H. Z. Cummins, 1983, Light and Color in Nature and Art, John Wiley & Sons Inc. [4] Cyrus M. Smith, September 2001, Detection of Spectral Operations Forces Using Night Vision Devices, OAK Ridge National Laboratory, p [5] M. Armstrong, D. Flynn, M. Hammond, S. Jolly, R. Salmon, September 2008, High Frame-Rate Television, BBC Research White Paper WHP