Abstract:
An optical warning device comprising a light source located within a housing having a wall, a top and a base, the base supporting the light source positioned in front of a reflector to transmit light through a light transmitting surface forming part of the wall of the housing, the top of the housing including an absorber comprising an outwardly facing light shielding surface and an inwardly facing surface profiled to entrap light.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims the benefit of the filing dates of Australian Application No. 2008900428 filed on 31 Jan. 2008 and U.S. Application No. 61/026,569 filed on 6 Feb. 2008, the contents of which are incorporated herein by reference in the entirety. 
     
    
     INTRODUCTION 
       [0002]    This invention relates to optical warning devices for use in a wide range of environments. 
       BACKGROUND OF THE INVENTION 
       [0003]    Optical warning devices are widely used in road traffic, industrial environments as well as by police and other emergency services to alert people of possible hazards and dangers. 
         [0004]    Research has indicated that the speed at which a human being identifies an object in his/her visual field is related to 3 primary criteria namely the luminance or brightness of the object, the size of the object and the contrast between the object and its background. Optical warning beacons exploit luminance and contrast to maximise their visibility and thus speed of recognition. Traffic signals demonstrate this concept by being very bright and also being placed inside a black background shielded from ambient light by a hood. Unfortunately, it is not always practical to place warning signals in the same controlled environment as traffic signals. 
         [0005]    It is these issues that have brought about the present invention. 
       SUMMARY OF THE INVENTION 
       [0006]    According to one aspect of the present invention there is provided an optical warning device that comprises a light source located within a housing, the housing providing optical access to the light source, and an absorber to absorb stray light from external sources. 
         [0007]    Preferably, the absorber includes an outwardly facing light shielding surface to shield the device from external light and an inwardly facing light entrapment surface to absorb internally scattered light. 
         [0008]    Preferably, the shielding surface is dark, opaque or painted in a dark colour. 
         [0009]    According to a further aspect of the invention, there is provided an optical warning device comprising a light source located within a housing having a wall, a top and a base, the base supporting a light source positioned in front of a reflector to transmit light through a light transmitting surface forming part of the wall of the housing, the top of the housing including an absorber comprising an outwardly facing light shielding surface and an inwardly facing surface profiled to entrap light. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: 
           [0011]      FIG. 1  is an illustration of a conventional optical warning device showing the effects of sunlight on such a device; 
           [0012]      FIG. 2  is a side elevation view of an optical warning device in accordance with the present invention, with a light shield as a separate component; 
           [0013]      FIG. 3  is a sectional view of part of the shield forming part of the device of  FIG. 2 ; 
           [0014]      FIG. 4  is a cross sectional view of an optical warning device with an integral shield; 
           [0015]      FIGS. 5   a, b  and  c  are perspective views of shield designs; and 
           [0016]      FIGS. 6   a  and  b  illustrate forms of the shield as a separate and an integral part of the housing. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    A typical optical warning device is shown in  FIG. 1 , in which a light source A is positioned centrally of a parabolic reflector B and mounted centrally of a base C. A cylindrical housing D usually made of clear or coloured glass or plastics, sits on top of the light source A. The wall E of the housing may be smooth or contain an annular array of Fresnel reflectors F and the end of the housing defines a top G. The base structure is designed to facilitate mounting the device and may contain control gear and/or motors to, if necessary, rotate the light source and reflector. The light source can be a single lamp or a plurality of LEDs. Although the reflector is shown as parabolic, it is understood that many other shapes of reflector can be incorporated within the device. Alternatively, the reflector may be omitted. 
         [0018]    The device shown is designed to produce an optical warning signal visible to observers on the axis K. 
         [0019]      FIG. 1  also illustrates the effect of sunlight on an optical warning device. The sunlight can enter the warning device in several distinct ways that are illustrated as follows:
       Light  62  can pass through the optics of the wall E and illuminate the base with the light reflecting off the base being scattered through the unit.   Light  64  can enter through the top G and enter the reflector B to be scattered throughout the optical system. This stray light substantially increases the significant chance to create a ghost signal or dilute the effect of the signal produced by the optics.   Light  66  can enter through the top G and illuminate the base with the light reflecting off the base being scattered throughout the unit.   Light  68  can enter through the top G and illuminate the optics  25  in the wall E, again causing scattering light that will create a ghost signal or dilute the effect of the signal produced by the device.         
         [0024]    In the optical warning device  10  of the preferred embodiment shown in  FIG. 2 , the housing  11  is of similar design to the housing D of  FIG. 1 , that is with a circular top  12  and cylindrical wall  13  which may be either planar or in the form of Fresnel refractors  25 . A similar light source  40  with parabolic reflector  50  is mounted centrally of the housing on a base structure  30  in the same manner as  FIG. 1 . It is understood that the light source and reflector could be rotatable. However, in this embodiment an absorber  80  is positioned on the underside of the top  12  of the housing  11  at a position that it does not impede the exeant beam. The absorber  80  is constructed of light absorbing material such as black thermoplastics and defines an outwardly facing shielding surface  81  and an inwardly facing entrapment surface  82  that is profiled to scatter and capture light. 
         [0025]    In this embodiment, when sunlight enters the warning device  10 , the detrimental effect is reduced in several ways. Referring to the same sources of light as  FIG. 1 , light  62  can pass through the optics of the lens  12  and illuminate the base  30 . Light  72  reflected off the base  30  is scattered through the unit and light illuminating the surface  82  of the absorber  80  is absorbed before reaching the top  12 . The shielding surface  81  blocks light  64   a  incident on the top  12  at angles that would have entered the optics. The shielding surface  81  blocks light  66   a  incident on the top  12  at angles that would have illuminated the base. Furthermore, the shielding surface  81  blocks light  68   a  that could have entered through the top  12  of the device and illuminate the optics  25 . The shielding surface  81  blocks much of the incoming light falling on top  12  of the warning device and the entrapment surface  82  also absorbs most of the extraneous light that enters the light from the side and then is scattered throughout the device  10 . 
         [0026]    The impact of sunlight on the visibility of the signal to an observer on axis K is significant, the effect of uncontrolled light  64 ,  66 ,  68  reduces the contrast and may overpower the signal from light source A. The major problem being the magnitude of difference in luminance between light source A and sunlight. 
         [0027]    Due to practical limitations and, in particular, the road traffic regulatory requirements it is not possible to increase the intensity of the light signal beyond approximately 2×10 3  cd/m 2 . 
         [0028]    The luminous intensity of sunlight itself (light rays  60 ,  62 ,  64 ,  68 ) is generally accepted to be 10 9  cd/m 2  under a clear sky. 
         [0029]    A practical matte black surface can absorb 90% of incident light; the reflected light of such a surface would thus still have an intensity of approximately 10 8  cd/m 2  if reflected into a narrow beam. 
         [0030]    The Lambertian nature of light reflected by a matte diffuse surface broadly reduces the observed luminous intensity of an incident light beam by a factor of 4π; which, in combination with the impact of the black surface treatment in this example, still represents a luminous intensity close to 10 7  cd/m 2  for an observer from any angle 
         [0031]    If  FIG. 2  is analysed within the context of the foregoing, it is clear that the reflected light  72  can still be of considerably higher intensity than the signal produced by the light source  40  even if the light source luminous intensity towards observer K is amplified by with the aid of reflector  50 . 
         [0032]    Under ideal conditions, the upper surface of the base  30  will be a matte black surface and thus be able to attenuate the first reflection of an incoming sunlight ray  62  intensity from 10 9  cd/m 2  to 10 8 cd/m   2  through absorption and then to 10 7  due to Lambertian scattering effect. Should a subsequent cycle of reflections direct the inter reflected sunlight  72  towards the observer K the same combination of absorption and scattering will produce “ghost signals” at a residual luminous intensity of 10 5  cd/m 2 . Thus after the incident sunlight intensity has been attenuated by 99.99%, the “ghost signal” would still be 100 times stronger than the legal maximum of the signal produced by light source A. 
         [0033]    This makes it clear that required light absorption efficiency of the absorber  80  cannot be realized by a simple painted shield or opaque area G of the housing. 
         [0034]    As shown in  FIG. 3 , the entrapment surface  82  of the absorber  80  absorbs light  72 ,  76  reflected off the internal surface of the device. By way of a matte surface, the light  78  becomes scattered and not focused reducing its observed intensity by 4         . As discussed above, the dark colour/surface of the absorber  80  ensures that 80-90% of the light is absorbed. To further improve the absorption efficiency, the internal face  82  defines a geometry designed to entrap light, and specifically avoid directing light towards observers of the warning beacon on axis K (in  FIGS. 2 and 3 ). The surface  82  of the absorber  80  facing the base of the device  10  consists of a multitude of suitably proportioned cavities  84  which trap most of the reflected light  79  through multiple additional reflections each cycle of which absorbs 80-90% of the light. Preferably, the surface of the cavity  84  will be a matte surface finish to produce diffuse reflections 
         [0035]    In this manner of capturing the stray light into a multitude of inter-reflection cycles, the resultant observed intensity is substantially reduced below the signal intensity. The visual effect being that the warning device provides a crisp highly illuminated signal with the required contrast to make it readily visible to the human eye in day light conditions without having to resort to increasing the warning signal intensity. Because the absorber  80  is located outside the beam produced by light source  40  and reflector  50  it has no detrimental effect on night time signal visibility. 
         [0036]    Three key aspects are required for the light entrapment surface  82  to function as effective light absorber:
       A. The roof of the cavity  84  must not visible from observer axis K ( FIG. 2 );   B. A minimum of the inner wall surface of the cavity  84  is visible from observer axis K;   C. The remaining surface  82  should produce a diffuse reflection.       
 
         [0040]      FIG. 4  shows how the absorber  80  can be incorporated into the housing  11 . The outer surface  81  of the top  12  of the housing  11  can be fabricated from a dark opaque material or painted a solid dark colour to shield and absorb light. The light entrapment surface  82  can be moulded or machined directly into the housing  11 . The substantially vertical sides  25  of the device remains transparent. 
         [0041]    It would normally be required to adjust the size and shape of the housing  11  to optimize the function of the integral absorber  80 —it being better to bring the absorber in close proximity to the light source and reflector. 
         [0042]      FIGS. 5   a, b  and  c  show some of the multitude of possibilities that a skilled practitioner may use to create the light entrapment cavities in the absorber  80 , the cavities may be produced as concentric rings  90  ( FIG. 5   a ), round  91  or hexagonal pits  92  ( FIGS. 5   c  and  b ) and many other shapes, or mixtures of shapes to suit. The Figures show the devices in an inverted position for clarity. 
         [0043]      FIGS. 6   a  and  b  show the section of another possible design approach for the absorber. The absorber in  FIG. 6   a  comprises a dark coloured hollow device  200  that is a separate item that sits under the top  12  of the housing  11 . The outer surface  201  defines the shielding surface whilst holes  202  cause light entering through the holes  202  to be entrapped in the single cavity  210 . In  FIG. 6   b , a similar absorber  200  is integral with the top  12  of the housing  11 . 
         [0044]    In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.