Reflective thin sheet and night vision system

Abstract: A night vision system 10 is provided for detecting objects at relatively low visible light levels. The system 10 includes an infrared light source 14. The system 10 further includes a thin sheet optical element 16 extending along a first axis 27 receiving light from the infrared light source 14 and reflecting the light generally in a first direction. Finally, the system 10 includes an infrared camera for receiving the light reflected off objects in the environment and generating a video signal responsive to the received light.

Description: This invention relates to a night vision system for detecting objects at relatively low visible light levels. In particular, the invention may be applied to a night vision system utilising a thin sheet optical element to emit infrared light and an infrared camera that detects the infrared light reflected from objects in the environment. Night vision systems are utilised to allow a user to see objects at relatively low visible light levels. Further, night vision systems may be classified as either passive night vision systems or active night vision systems. In known passive night vision systems used in automotive vehicles, mid-infrared cameras are used to image objects using the ambient infrared light emitted by the objects in the environment. Mid-infrared cameras generally used in automotive applications have a relatively small number of pixels. Accordingly, images formed using such cameras have a low video resolution and a relatively narrow field-of-view. Further disadvantages of night vision systems using mid-infrared cameras include (i) a relatively high manufacturing cost, (ii) generated images that are generally harder for a user to interpret than images formed using CCD or CMOS cameras, (iii) a potentially poor image contrast, and (iv) operational restrictions on the location of the camera resulting in a negative impact to the vehicle styling. A known active night vision system utilises an infrared diode laser or an incandescent light source and relatively large diameter lenses or reflectors to emit infrared light. The infrared light is subsequently reflected off objects in the environment and is received by an infrared camera. The infrared camera generates a video signal responsive to the received light. A disadvantage of the known active night vision system is that the lens or reflector-based illuminator package is relatively large. Accordingly, the known active system cannot be packaged in a small volume and placed on top of a dash or in a rear view mirror assembly of an automotive vehicle. There is thus a need for a night vision system and a method related thereto that minimises for reduces one or more of the above-mentioned deficiencies. Our EP 0 940 625, filed 22 February 1999, describes a vehicle lighting system includes a light source for generating light and a unitary optical element having a light collimating surface, a plurality of stepped surfaces projecting from the collimating surface, each stepped surface having a reflective surface angled with respect to the collimating surface for receiving collimated light, a dispersal surface angled with respect to the stepped surfaces, the dispersal surface having a plurality of dimples aligned with respect to the reflective surfaces for dispersing collimated light, and a kicker having a surface with a plurality of reflective facets adapted to receive and redirect the dispersed light. Our US 5890796, dated 6 April 1999, describes a lamp assembly for an automotive vehicle includes a remote laser light source coupled with a light transmitting fiber optic light guide which illuminates a unitary thin sheet optic having a diffractive optical element portion, a manifold section, and a kicker section. EP 0 490 029 of Daimler Benz AG, filed 5 September 1991, describes an arrangement for improving visibility in vehicles which utilises the different polarisation characteristics of useful signals and interference signals in the reflected light of infrared illumination optics in a vehicle. The present invention provides a night vision system and a method related thereto for detecting objects at relatively low visible light levels. The invention is defined by the appended claims. The night vision system and the method related thereto in accordance with the present invention represents a significant improvement over conventional systems and methods. In particular, the system may be packaged in a relatively small volume since the thin sheet optical element is extremely thin as compared with conventional lenses/illuminators. Accordingly, the inventive system may be located in a greater number of locations in an automotive vehicle. Further, the inventive system is less expensive, has a larger field-of-view, and provides higher quality images than known passive night vision systems. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which; Figure 1 is a combination schematic and block diagram of a night vision system in accordance with the present invention. Figure 2 is a perspective view of a thin sheet optical element utilised in the night vision system of Figure 1. Figure 3 is a front view of the thin sheet optical element of Figure 2. Figure 4 is an enlarged fragmentary sectional view of the thin sheet optical element of Figure 2 taken along lines 4-4. Figure 5 is an enlarged fragmentary sectional view of the thin sheet optical element of Figure 2 taken along lines 5-5. Figure 6 is a graph showing the operational performance of a night vision system in accordance with the present invention. Figures 7A-7C are flow charts illustrating a method for detecting objects in accordance with the present invention. Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, Figure 1 illustrates a night vision system 10 for detecting objects at relatively low visible light levels. The system 10 may be utilised in a plurality of applications. For example, the system 10 may be used in an automotive vehicle (not shown) to allow a driver to see objects at night that would not be visible to the naked eye. Further, the system 10 could be utilised as part of a security system wherein a user could see objects at night in a monitored area. As illustrated, the system 10 includes a housing 12 which holds the remaining components of the system 10. It should be understood, however, that the components of system 10 contained in housing 12 could be disposed at different locations wherein the housing 12 would not be needed. For example, the components of the system 10 could be disposed at different operative locations in the automotive vehicle so that a single housing 12 would be unnecessary. Referring to Figure 1, the system 10 further includes an infrared light source 14, a thin sheet optical element 16, a fibre optic cable 17, a holographic diffuser 18, an infrared camera 20, and an optical bandpass filter 22. As will be discussed in more detail hereinbelow, the system 10 may be used to detect any reflective object, such as an object 24, in operative proximity to the system 10. The housing 12 is provided to enclose and protect the various components of the system 10. The housing 12 may be constructed from a plurality of materials including metals and plastics. The infrared light source 14 is provided to generate infrared light. Many objects in the environment that are dark in the visible light spectrum are brightly illuminated in the infrared light spectrum. Accordingly, it is advantageous to use an infrared light source when trying to detect objects at night. The light source 14 is conventional in the art and may comprise an infrared diode laser. As illustrated, the light source 14 receives an operational voltage V CC from an external voltage source (not shown). Further, the light source 14 receive a control voltage V C from an external controller (not shown) and generate the infrared light responsive to the control voltage V C . In one constructed embodiment, the light source 14 was a Single Stripe Diode Laser, Model No. S-81-3000-C-200-H manufactured by Coherent, Inc. of Santa Clara, California. As illustrated, the light source 14 may be disposed in the housing 12. Further, the light source 14 may be connected to a first end 25 of the fibre optic cable 17 using a conventional light coupler (not shown) as known by those skilled in the art. The second end 26 of fibre optic cable 17 is operatively disposed adjacent to the thin sheet optical element 16. Although the system 10 preferably utilises an infrared light source 14, an alternate embodiment of system 10 may utilise a conventional light source that emits visible light (including infrared light), such as a halogen bulb, in lieu of the infrared diode laser 14. The fibre optic cable 17 is utilised to transmit light from the light source 14 to the thin sheet optical element 16. Because of the high brightness (candela per unit area) of the light source 14, cable 17 preferably is a relatively small diameter (0.1-1.0 mm) glass fibre. The use of a small diameter glass fibre provides several benefits over monofilament plastic pipes and glass fibre bundles used in non-laser based remote lighting systems. A small diameter glass fibre is less bulky than plastic pipes or glass fibre bundles that typically are 10-12 mm in diameter. Further, a small diameter glass fibre is significantly less expensive than monofilament plastic pipes or glass fibre bundles. Still further, a small diameter glass fibre is easier to package, handle, and to install than monofilament plastic pipes or glass fibre bundles. The thin sheet optical element 16 is provided to reflect and expand light (represented by arrows A) generated by the light source 14 generally in a first direction from the element 16. In a preferred embodiment, shown in Figures 2 and 3, the element 16 comprises a unitary sheet of optical material extending generally along a first axis 27. The element 16 preferably has a thickness range from 3-9 mm. It should be understood, however, that the element 16 may have a thickness less than 3 mm or greater than 9 mm. The element 16 is preferably constructed from a transparent, solid piece of plastic such as polycarbonate and utilises the principle of total internal reflection (TIR) to reflect light. TIR is explained in more detail hereinbelow. The element 16 may also be constructed from other transparent materials such as acrylics. Referring to Figures 1, 2 and 3, the element 16 includes a front surface 28, a back surface 30, a bottom surface 32, a top surface 34, side surfaces 36, 38, and an aspheric lens 40. Referring to Figures 3 and 4, the bottom surface 32 of element 16 defines a first plurality of reflective steps 42 extending generally along the axial length of the element 16. Each of the reflective steps 42 includes a reflective facet 44 and a tread portion 46. As illustrated, each tread portion 46 is generally parallel to the axis 27. Each reflective facet 44 is oriented at approximately a 45° angle relative to the adjacent tread portion 46. It should be understood, however, that the angle of each reflective facet 44 may vary depending upon the critical angle (discussed further hereinbelow) of the respective facet 44. Further, the reflective facet 44 may have a curved shape (not shown) to further direct the infrared light in a desired direction. Still further, the number of reflective steps 42 may vary, and correspondingly, the number of reflective facets 44 may vary. The reflective facets 44 utilise the principle of TIR to reflect infrared light received from the aspheric lens 40 towards the reflective facets 50. Total internal reflection of the light occurs when the incident angle  exceeds the critical angle  C given by the equation  C =sin -1 (n 1 /n 2 ) wherein n 1 is the index of a refraction of air and n 2 is the index of a refraction of the polymeric material used to construct the element 16. In an alternate embodiment (not shown), the reflective facets 44 can be metalled if the light strikes the facets 44 at an angle less than the critical angle. Referring to Figures 1, 2 and 5, the back surface 30 defines a second plurality of reflective steps 48 extending generally perpendicular to the axis 27. Each of the reflective steps 48 includes a reflective facet 50 and a tread portion 52. As illustrated, each tread portion 52 is generally perpendicular to the axis 27 and parallel to the front surface 28. Each reflective facet 50 is oriented at approximately a 45° angle relative to the adjacent tread portion 52. It should be understood, however, that the angle of each reflective facet 50 may vary depending upon the critical angle of the respective facet 50. Further, each reflective facet 50 may have a curved shape (not shown) to further direct the light in a desired direction. Still further, the number of reflective steps 48 may vary, and correspondingly, the number of reflective facets 50 may vary. Referring to Figures 4 and 5, the facets 50 are aligned to receive light reflected from one or more reflective facets 44, and, like the facets 44, utilise the principle of TIR. The facets 50 reflect the received light through the front surface 28 of the element 16 as will be described in further detail hereinafter. In an alternate embodiment (not shown), the reflective facets 50 can be metalled if the light from the reflective facets 44 strikes the facets 50 at an angle less than the critical angle. Referring to Figure 2, the aspheric lens 40 is provided to collimate the light exiting the fibre optic cable 17. The axial distance between the cable 17 and the lens 40 is chosen such that the light diverging from the cable 17 fills the aperture of lens 40. The lens 40 is preferably constructed to have a surface of revolution about the axis 27 with a circular or hyperbolic cross section. As illustrated, the element 40 is disposed on the side surface 36 of the element 16 and may be integral with the element 16. In an alternate embodiment of the element 16, the lens 40 may comprise a separate lens disposed in operative proximity to the element 16. Referring to Figures 1 and 2, the infrared light generated by the infrared light source 14 is received by the element 16 from the second end 26 of the fibre optic cable 17. The light being emitted from the second end 26 preferably has a spread angle between 20-50°. It should be understood, however, that the spread angle may be less than 20° or greater than 50° depending upon the light characteristics of the cable 17. The emitted light enters the element 16 through the aspheric lens 40 disposed on the side surface 36 of the element 16. As discussed previously, the element 40 collimates the light, which then propagates toward the reflective facets 44. The reflective facets 50 receive the light reflected from the facets 44 and further reflect the light through the front surface 28 of the element 16 generally in a first direction. Referring to Figure 1, the holographic diffuser 18 is provided to receive infrared light emitted from the thin sheet optical element 16 and to diffuse the light prior to being emitted into the environment. Further, the diffuser 18 in conjunction with the element 16 allows the emitted light to meet a Maximum Permissible Exposure light level requirement as specified in an ANSI standard described in more detail hereinbelow. Still further, the diffuser 18 allows the system 10 to spread the infrared light over the required field-of-view. The diffuser 18 is conventional in the art and may be disposed proximate to or integral with the front surface 28 of the element 16. In a constructed embodiment of the system 10, the diffuser 18 has a vertical spread angle of 5° and a horizontal spread angle of 20°. However, in an alternate embodiment (not shown), the reflective facets 50 of element 16 may be oriented at varying angles with respect to one another to diffuse the infrared light. Accordingly, in the alternate embodiment, the diffuser 18 would not be needed. In another alternate embodiment, an array of lenses (pillow optics) disposed across the exit surface 28 of the element 16 may be used to diffuse the light instead of the diffuser 18. The infrared camera 20 is provided to generate a video signal V V responsive to reflected infrared light received by the camera 20. The camera 20 is conventional in the art and may comprise a CCD camera or a CMOS camera. In a constructed embodiment of the system 10, the CCD Camera Model No. WAT-902HS manufactured by the Watec American Corporation of Las Vegas, Nevada was utilised. As illustrated in Figure 1, the infrared camera 20 receives an operational voltage V CC from an external voltage supply (not shown). Infrared light emitted from the thin sheet optical element 16 is reflected off of an object 24 in the environment and is received by the infrared camera 20. The video signal V V may be applied to a television monitor (not shown) or a heads-up display (not shown) in an automotive vehicle to allow a user to see the object 24. The optical bandpass filter 22 is provided to filter the infrared light reflected from the object 24. In particular, the filter 22 only allows light within the infrared light spectrum to be received by the camera 20. Preferably, the filter 22 allows a maximum transmission of light at a wavelength equal to the wavelength of light generated by the infrared light source 14. An advantage of using the filter 22 is that the filter 22 prevents blooming (i.e., saturation of the pixel elements) in the camera 20 by visible light emitted from the head lamps (not shown) of other automotive vehicles. The filter 22 is conventional in the art and is preferably disposed proximate to a receiving lens (not shown) of the camera 20. The American National Standards Institute has implemented the American National Standard for Safe Use of Lasers (ANSI Z136.1), hereinafter referred to as the ANSI Standard. The ANSI Standard sets forth guidelines for systems (such as night vision systems) that utilise lasers. In particular, the ANSI Standard specifies that for a given laser wavelength, viewing time, illuminator size, and beam divergence, the irradiance (power per unit area) at a given distance from the illuminator (i.e., element 16) must be less than the Maximum Permissible Exposure (MPE) as stated in the ANSI Standard. Referring to Figure 6, a graph illustrates the operational characteristics of the night vision system 10 as compared with the ANSI Standard. In particular, line 53 graphically illustrates the MPE in accordance with the ANSI Standard. Further, dashed line 55 illustrates the light irradiance as a function of distance emitted by the thin sheet optical element 16 when (i) the area of the front surface 28 is 7x7 cm 2 , (ii) the horizontal and vertical divergence angles of emitted infrared light are 20° and 5°, respectively, (iii) the wavelength of the infrared light is 810 nm, and, (iv) the emitted power of the infrared light is 3 Watts. As illustrated, the irradiance of the element 16 is less than the MPE at all distances. Accordingly, the night vision system 10 meets the MPE requirements of the ANSI Standard. Referring to Figure 7A, a method 54 for detecting an object 24 utilising a night vision system 10 in accordance with the present invention is illustrated. The inventive method 54 may include a step 56 of transmitting infrared light to the thin sheet optical element 16. As previously discussed, the infrared light is transmitted to the element 16 via the fibre optic cable 17. The method 54 may further include a step 58 of reflecting the light from the thin sheet optical element 16 generally in a first direction from the element 16. Referring to Figure 7B, the step 58 may include the substeps 60, 62, and 64. The substep 60 involves collimating the light entering the element 16. The substep 62 involves reflecting the light from the reflective facets 44 to the reflective facets 50. The substep 64 involves reflecting the predetermined number of light beams from the reflective facets 50 generally in the first direction through the front surface 28 of the element 16. Referring to Figure 7A, the method 54 may further include a step 66 of diffusing the light emitted from the element 16. The method 54 may further include a step 68 of receiving the light reflected off of an object 24 in the environment. The object 24 may be one or more reflective objects that are disposed generally in the first direction from the element 16. For example, if the system 10 is utilised in an automotive vehicle (not shown), the object 24 could be a road surface, a pedestrian, or an animal. Referring to Figure 7C, the step 68 may include a substep 72 of filtering the light through an optical band pass filter 22. Referring again to Figure 7A, the method 54 may further include a step 70 of generating a video signal V V responsive to the received light. The night vision system 10 and the method related thereto in accordance with the present invention represent a significant improvement over conventional night vision systems and methods. In particular, the system 10 may be packaged in a relatively small package volume since the thin sheet optical element 16 is extremely thin as compared with conventional lenses/illuminators. For example, the system 10 may be packaged on the dashboard or inside a rear-view mirror assembly in the interior of the automotive vehicle. Thus, the system 10 provides for greater design flexibility in automotive vehicles and in other applications where a small package volume is required. Another advantage of the system 10 is that the laser light emitted from the element 16 meets the ANSI Standard described hereinabove. Further, the inventive system 10 is less expensive, has a larger - field-of-view, and provides higher quality images than known passive night vision systems.

Claims: A night vision system, comprising: a light source (14); and a thin sheet optical element (16) extending along a first axis (27) and disposed to receive light from said light source (14), the optical element (16) being adapted to reflect said light internally and to transmit said light generally in a first direction; characterised by said light source (14) being an infrared light source and by incorporation of an infrared camera (20) sited to receive said light after reflection from an object in the field of view of the camera (20) and adapted to generate a video signal responsive to said received light, wherein said thin sheet optical element (16) comprises; a sheet of optical material extending along a first axis (27) having a front surface (28), a back surface (30), a bottom surface (32), and a side surface (36, 38) connected between said front (28), back (30), and bottom (32) surfaces, said bottom surface (32) defining a first plurality of reflective steps (42) extending along an axial length of said thin sheet optical element (16) and being aligned to receive light from said infrared light source (14) said back surface (30) defining a second plurality of reflective steps (48) extending generally perpendicular to said first axis (27), said second plurality of reflective steps being aligned to receive infrared light reflected from said first plurality of reflective steps (42) and to further reflect said light through said front surface (28) in said first direction. The night vision system as claimed in claim 1, wherein said first (42) and second (48) plurality of reflective steps include a first and second plurality of reflective facets (44, 50), said first plurality of reflective facets (44) receiving said light from said infrared light source (14) and reflecting said light to a second plurality of reflective facets (50) that further reflect said light from said thin sheet optical element (16) generally in said first direction. The night vision system of claims 1 or 2 wherein said infrared light source (14) is an infrared diode laser. The night vision system as claimed in any one of the preceding claims, wherein said thin sheet optical element (16) is comprised of a polymeric material. The night vision system as claimed in any one of the preceding claims, wherein said thin sheet optical element (16) further includes an aspheric lens (40) disposed integral with or on said thin sheet optical element (16), said lens (40) receiving said light from said infrared light source (14) and collimating said light, said light being transmitted from said lens (40) through said thin sheet optical element (16) to said first plurality of reflective facets (44). The night vision system as claimed in any one of the preceding claims, further comprising a fibre optic cable (17) transmitting said light from said infrared light source (14) to said thin sheet optical element (16). The night vision system as claimed in any one of the preceding claims, further comprising an optical bandpass filter (22) for filtering said light entering said infrared camera (20). The night vision system as claimed in any one of the preceding claims, further comprising a holographic diffuser (18) or lens array disposed proximate said thin sheet optical element (16) to diffuse said light being emitted from said thin sheet optical element (16). A method of detecting objects utilising a night vision system, comprising the steps of: transmitting infrared light to a thin sheet optical element (16) extending along a first axis (27) and disposed to receive light from a light source (14), the optical element (16) being adapted to reflect said light internally and to transmit said light generally in a first direction; reflecting said light from said thin sheet optical element (16) generally in a first direction from said thin sheet optical element (16); receiving said light reflected off an object; and generating a video signal responsive to said received light; characterised in that said thin sheet optical element (16) comprises; a sheet of optical material extending along a first axis (27) having a front surface (28), a back surface (30), a bottom surface (32), and a side surface (36, 38) connected between said front (28), back (30), and bottom (32) surfaces, said bottom surface (32) defining a first plurality of reflective steps (42) extending along an axial length of said thin sheet optical element (16) and being aligned to receive light from said infrared light source (14), said back surface (30) defining a second plurality of reflective steps (48) extending generally perpendicular to said first axis (27) and said second plurality of reflective steps being aligned to receive infrared light reflected from said first plurality of reflective steps (42) and to further reflect said light through said front surface (28) in said first direction. The method as claimed in 9, wherein said first (42) and second (48) plurality of reflective steps include a first and second plurality of reflective facets (44, 50), said first plurality of reflective facets (44) receiving said light from said infrared light source (14) and reflecting said light to a second plurality of reflective facets (50) that further reflect said light from said thin sheet optical element (16) generally in said first direction, and said step of reflecting said light includes the substeps of: collimating said light entering said thin sheet optical element (16); reflecting said light from said first plurality of reflective facets (44) to said second plurality of reflective facets (50); and reflecting said light from said second plurality of reflective facets (50) generally in said first direction. The method as claimed in claims 9 or 10, wherein said step of receiving said light reflected from said object includes the substep of: filtering said light through an optical bandpass filter (22). The method as claimed in claims 9, 10 or 11, further comprising the step of diffusing said light reflected from said thin sheet optical element (16).

Inventors: FOHL TIMOTHY, REMILLARD JEFFREY THOMAS, WEBER WILLES H, FOHL, TIMOTHY, REMILLARD, JEFFREY THOMAS, WEBER, WILLES H.
Applicants: FORD GLOBAL TECH LLC, FORD GLOBAL TECHNOLOGIES, LLC
Application year: 2001
Main classification: G02B5
IPC codes: H04N5, G02B5, G02B23