Lighting device

Abstract

A lighting device 1 adapted to enhance the visibility of objects comprising fluorescent materials to the human eye in scotopic and mesopic lighting conditions comprising a first light source 7 emittin

Description

The present invention relates to a lighting device, in particular a lighting device that is adapted to enhance the visibility of objects to the human eye in low light or night conditions. In well-lit conditions (a luminance level of 10 to 108 cd / m2) photopic vision in humans allows colour perception, mediated by the cone cells present in the human eye. This is known as photopic vision, with the peak photopic human eye response occurring at a wavelength of approximately 555nm (green light). The photopic span extends between approximately 400nm and 700nm, with luminous intensity normalised to 1 based on the peak value of 555nm in accordance with the CIE (Commission Internationale de I'Eclairage) luminosity function. In low-light level conditions (10-3 to 10-6 cd / m2)the cone cells do not function at all, and sight is exclusively through the rod cells. The rod cells are most sensitive to light of wavelengths of around 498nm in the green-blue portion of the visible region of the electromagnetic spectrum. This is known as scotopic vision, with the transition between photopic and scotopic vision known as the Purkinje Effect. The CIE 1951 scotopic luminosity function indicates that the maximum scotopic efficacy of the human eye is 1700lm / W at 507nm. Whilst the ratio between the scotopic and photopic efficacies is only around 2.5 at peak sensitivity, this increases strongly below 500nm. At dusk and in early mornings the human eye sees using mesopic vision (10-3 to 1005 cd / m2), which is combination of photopic and scotopic vision in which both the cones and rods are activated resulting in poor visual acuity and colour perception. Unfortunately, this causes traffic crossings and junctions to be dangerous for cyclists, pedestrians and other vulnerable road users in low lighting conditions, particularly at night, or during times of the year where dawn and dusk occur at peak commuting times. To try to combat this, cyclists are advised to wear high visibility garments to make them more conspicuous to other road users. Such garments should meet either the EN 1150 standard or the EN 20471 standard for professional high visibility clothing. Typically, two materials are used in the construction of the garments: firstly, a fluorescent fabric for daylight and dusk visibility; and secondly, a retroreflective material for night-time visibility. The fluorescent material functions by absorbing radiation at a short wavelength and re-emitting at a longer wavelength, for example, by absorbing radiation in the UV (ultraviolet) bandwidth and re-emitting in the visible portion of the electromagnetic spectrum. Retroreflective materials comprise a plurality of micron-scale prism reflectors, either in the form of glass or plastic, that reflect incident light back out in the direction it was emitted from - 180° reflection. This works well for objects illuminated by a light source positioned a relatively short distance away, such as a headlamp, but not if the external illumination is provided in the axis of the observer. A combination of both materials therefore represents the safest option. These issues also apply in areas where workers are exposed to danger at night or in low-level ambient lighting, such as road workers, rail track workers and emergency services personnel. All of these individuals can be classed as vulnerable at night. One issue with visibility of fluorescent clothing in particular is its reduced effectiveness under street lighting. Older-style sodium street lighting lacks a blue wavelength component and therefore fails to excite the fluorescent material. Even newer white LED (light emitting diode) street lighting has only a small blue wavelength component. This is because a white LED is created from a blue or short wavelength UV source that is covered in a phosphor coating. The phosphor acts to convert the light emitted by the LED to longer wavelengths that have a distribution that resembles white light to the human eye. Consequently, even these street lights have a low blue wavelength component as a large proportion of the blue light is absorbed by the phosphor. The resulting effect is that the fluorescent materials may actually end up being more visible during daylight, since natural daylight comprises a more significant blue component than the white light emitting diodes are able to represent, as well as ultraviolet components. There is therefore a problem in ensuring that vulnerable personnel (pedestrians, cyclists, workers and emergency personnel) are visible in scotopic and mesopic lighting conditions, as the fluorescent material they may be wearing can become less effective under artificial light. Indeed, testing shows that fluorescent materials appear dim and no brighter than non-fluorescent materials when illuminated by artificial lighting. The present invention aims to address these issues by providing, a lighting device used to illuminate an object comprising fluorescent materials, as defined in claim 1. There is also provided a system comprising the lighting device and the object comprising fluorescent materials, as defined in claim 6. The lighting device is adapted to enhance the visibility of objects comprising fluorescent materials to the human eye in scotopic and mesopic lighting conditions, and comprises: a first light source adapted to emit light at a wavelength in the blue and / or ultraviolet portion of the electromagnetic spectrum; and a second light source adapted to emit white light; wherein, in use, the first and second light sources are adapted to emit light to illuminate an object comprising fluorescent materials simultaneously or sequentially. Unlike merely using a brighter or less yellow-hued street light in an attempt to illuminate objects, including vulnerable personnel, in low light conditions, using a blue-hued light in combination with a white or white-appearing light provides enhanced visibility of fluorescent and / or retroreflective materials. This results in the human eye having an improved perception of such objects even in scotopic and mesopic lighting conditions, as the fluorescent material stands out from the background with improved perception. The first light source comprises one or more light emitting diodes emitting light at a wavelength in the range 450nm to 480nm. Preferably, the first light source comprises a light source emitting light in a broad bandwidth and a filter adapted to filter wavelengths in the range 315nm to 525nm. Preferably, the light emitted by the first light source causes the illuminated objects to fluoresce. The second light source comprises at least one white light emitting diode that may emits light in the wavelength range 400 to 700nm. Preferably, the light emitted by the second light source causes the illuminated object to retroreflect. Preferably, the light source further comprising: a housing adapted to contain the first and second light sources; and a mounting device coupled to the housing and adapted to mount the housing on a support. The light emitted by the second light source may generate background illumination to the fluorescent material The invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure lisa schematic perspective view of a lighting device in accordance with a first example; Figure 2 is a schematic perspective view of a lighting device in accordance with a second example; Figure 3 is a schematic graph illustrating the white and blue spectral responses of the first and second light sources; Figure 4 is a comparison of the spectral response for supplementary white lighting (a) and supplementary white and blue lighting (b); and Figures 5a to 5c are schematic illustrations of possible illumination areas created by embodiments of the present invention. The present invention takes the approach that increasing the proportion of blue light incident in regions where vulnerable personnel will be present enhances the visibility of objects comprising fluorescent materials in both scotopic and mesopic lighting conditions. Therefore, a lighting device adapted to enhance the visibility of fluorescent materials to the human eye in scotopic and mesopic lighting conditions is particularly advantageous. Such a lighting device comprises a first light source adapted to emit light at a wavelength in the blue and / or ultraviolet portion of the electromagnetic spectrum. A second light source adapted to emit white light is also provided. In use, the first and second light sources are adapted to emit light to illuminate an object simultaneously or sequentially. This simultaneous or sequential illumination significantly enhances the visibility of fluorescent materials to the human eye in both scotopic and mesopic lighting conditions. Figure lisa schematic perspective view of a lighting device in accordance with a first example. A lighting device 1 comprises a housing 2, which is generally cuboid in shape, the housing 2 being formed from a plastics material such as PET (polyethylene terephthalate), PP (polypropylene), PE (polyethylene) or PC (polycarbonate). The housing 2 is substantially hollow in order to accommodate the various components required for the lighting device 1 to function. An aperture 3 is provided in a front surface 4 of the housing 2 to enable light to be emitted from the housing 2 by at least one light source. A protective front shields, formed from clear polycarbonate, is provided to cover the aperture 3, with a waterproof seal 6 placed between the protective front shield 5 and the front surface 4 of the housing 2. A first light source 7, adapted to emit light at a wavelength in the blue and / or ultraviolet portion of the electromagnetic. Blue wavelengths in particular have the advantage that any fluorescent material present on an object, such as a cyclist, will fluoresce under the incident light. Preferably the first light source 7 comprises at least one light emitting diode emitting light at a wavelength in the range 380nm to 525nm. The first light source 7 comprises at least one light emitting diode emitting light at a wavelength in the range 450nm to 480nm. This covers a blue wavelength spectrum that enhances the visibility to the human eye in scotopic and mesopic lighting conditions. Examples of suitable first light sources 7 include blue light emitting OSLON™ SSL 80 and OSLON™ SSL 120 light emitting diodes available from Osram. However, other equivalent blue / blue-violet light emitting diodes are also suitable. The lighting device 1 is also provided with a mounting device 8 coupled to the housing 2 and adapted to mount the housing 2 on a support (not shown). The mounting device 8 is designed to enable the lighting device 1 to be either permanently or removably mounted on a support, such as a lighting pole. The mounting device 8 is preferably adjustable, such that the angle the lighting device 1 makes to the support to form an illumination area may be varied. Suitable mounting devices 8 include clamps (either sprung or using a nut and bolt arrangement as shown), screws, bolts, caps (for mounting at the top of a support) and jubilee clips or other ratchet and strap arrangements. In the example shown in Figure 1 a second light source 9 is provided. This second light source 9 is to emit white light, which is in the photopic span. The second light source 9 comprises at least one white light emitting diode, with an array of white light emitting diodes being particularly useful. Suitable light sources include a white light emitting diode plate having eight light emitting diodes emitting at 5000K such as the white OSLON™ high efficiency white surface mount light emitting diode, also available from Osram. One particularly useful light source is a phosphor conversion white light emitting diode, as illustrated in Figure 3. The provision of a second light source 9 is to enable to embodiments of the present invention to be used in areas where there is no lighting already provided, or to enhance the visibility of retroreflective materials and to provide general scene lighting to illuminate surfaces and materials that don't fluoresce. For example, whist it may be common to provide auxiliary lighting on zebra crossings, it is less common to do so on pelican or toucan crossings, or at road junctions. For road and rail workers auxiliary lighting is used as a matter of course. Figure 1 shows the first 7 and second 9 light sources as physically separate arrays for clarity. Each of the first 7 and second 9 light sources comprises at least one light emitting diode. Whilst an array of light emitting diodes is useful, there may be circumstances where a full array is not required. In a final implementation it would be beneficial for these sources to be as close together as possible, and in accordance with the invention, mixed. Mixing the light source elements merges the blue and white light sources, and in accordance with the invention arrays of light emitting diodes are placed alternately on the same support plate. Such a merged light is beneficial in particular at short illumination distances to avoid unnatural shadowing from two separated light sources of different wavelengths. In order for the lighting device 1 to work effectively the first light source 7 is adapted to emit light at a wavelength in the blue and / or ultraviolet portion of the electromagnetic spectrum. The second light source 9 is adapted to emit white light, and in use the first and second light sources are adapted to emit light to illuminate an object simultaneously or sequentially. This can be done in a number of ways. The first light source 7 and the second light source 9 may illuminate the fluorescent material at the same time, either, for example, by both being turned on together or the first 7 of the second 9 light source being turned on at a short interval after the other of the second 9 or the first 7 light source is turned on. The time interval is short to benefit from the human eye's persistence of vision response to avoid unnatural or distracting flickering or flashing. Alternatively, the first and second 9 light sources may be illuminated sequentially with the illumination time of both the first 7 and the second 9 light sources being determined by the persistent vision response time of the human eye. The first 7 and second 9 light sources may be pulsed on and off. Firstly, for a housing 2 that comprises a first light source 7 but no second light source 9, a controller (not shown) may be included in the housing 2, and an optical sensor (also not shown) placed on the exterior of the housing. The optical sensor may either detect when the ambient lighting changes between photopic and mesopic / scotopic, or when a separate white light source in the vicinity comes on. At this point the first 7 and second 9 light sources are turned on by the controller and supplied with electricity by cabling contained within the support on which the lighting device 1 is mounted. For the embodiment shown in Figure 1, where both a light source 7 and a second light source 9 are present, each light emitting diode array is linked to the same controller and therefore illuminated at the same time. The role of the dual illumination is described further below. Figure 2 is a schematic perspective view of a lighting device in accordance with a second example which is not in accordance with the present invention. The main differences between Figures 1 and 2 is that in Figure 2 only the first light source 7 adapted to emit light at a wavelength in the blue and / or ultraviolet portion of the electromagnetic spectrum is contained within the housing 2. A separate housing 10 is provided to contain the second light source 9 adapted to emit white. This means that the (blue) first light source 7 and the (white) second light source 9 can be installed separately, either at different positions on a support or on different supports, or the second light source may be a street light. The other aspects of the housing 2 and mounting device 6 are the same as those depicted in Figure 1. Alternatively, the housing 2 may be provided with the same optical sensor as surrounding street lights, such that the controller turns the first light source 7 on at the same time as the surrounding street lights. As a further alternative, a controller may be placed in a central control cabinet linked to local street lighting, or the lighting device 1 may share a controller with the local street lighting, such that the first light source 7 and second light source, if provided, is turned on at the same time as the local street lights. Figure 3 is a schematic graph illustrating the white and blue spectral responses of the first and second light sources. In this example, the second light source 9 is a phosphor conversion white light emitting diode, which exhibits a small blue peak in the region of the peak generated by the first light source 1. The photopic eye response is overlaid onto these two spectral responses to illustrate the relationship between the behaviour of the human eye and the spectral content of the first 7 and second 9 light sources. To determine the effectiveness of the embodiments in accordance with the present invention, a prototype lighting device 1 was made in the design depicted in Figure 1. The lighting device 1 comprised a plastic housing 2 of the type usually employed to contain radar detection equipment. Within the housing 2 were included a plate mounting for eight Osram GW CSSRM3.PM-N6N8-A333-1 light emitting diodes, each provided with a 42° circular lens in a 3-D printed holder. A heatsink was included to remove heat from the light emitting diode light source and to promote reliability. For the first series of tests outlined below the first light source 7 included was an Osram CSSPM1.14-UOVJ-W4-1-350-R18 455nm light emitting diode, and for the second series of tests an Osram LB CP7P-GZHX-1 470nm light emitting diode was used. A polycarbonate protective shield 5 was used with a thickness of 3mm, along with a 3-D printed anti-glare shield. A waterproof seal was provided between the protective shield 5 and the front surface 4 of the housing 2. The brightness of the light emitting diodes was controlled using a pulse wide modulation technique, with Ledil FP11002 LISA2 lenses, having a beamwidth of 41° were used with Osram SSL 150 grade light emitting diodes (including the blue light emitting diode products described above). Each lighting device was tested at two distances, 9m and 14.27m, with a target of a fluorescent yellow cycling garment meeting the ENI 150 standard mounted at the same height as the lighting device 1. Camera settings were 100 ISO, exposure time 1 second, f / 3.1 and fixed focus of 5m. It was determined that white light alone increased the overall visibility of the target due to the retroreflective material included on the cycling garment, since this was positioned in the axis of the viewer (camera). The use of blue light alone was seen to increase the visibility of the target due to the fluorescence of the yellow material of the cycling garment. However, combining both the blue light and the white light yielded the most effective results, since this increased visibility due to both retroreflection and fluorescence. This can be seen clearly in the image histograms shown in Figure 4. Figure 4 illustrates the spectral response for supplementary white lighting (a) and supplementary white and blue lighting (b). The histograms shown were recorded using the 470nm light emitting diode, with the y-axis showing the total number of tonal pixels and the x-axis showing the tone of image pixels, proportional to image luminance on a scale of 0% to 100%, where 0% represents black and 100% represents white. The response of the target to white and blue light simultaneously compared with white light alone shows an increased amplitude of the histogram due to the increase in reflection and increased brightness as indicated by a shift to the right of histogram b) compared to histogram a). Comparison of the empirical optical response by a human to the 455nm and 470nm testing indicated that the 455nm light emitting diode created an overall illumination that was bluer in hue than the 470nm light emitting diode. Positioning the lighting device 1 relative to the axis of potential viewers results in enhancement of different optical properties. For example, regardless of the angle the lighting device illuminates an object at, an increase in fluorescent response will always be seen. However, positioning the lighting device 1 such that it lies on the axis of a viewer will also enhance the visibility of a retroreflective material. The lighting device 1 described above is suitable for use as an auxiliary lighting unit adapted to enhance the visibility of fluorescent or retroreflective objects to the human eye in scotopic and mesopic lighting conditions. In this case use of a first light source 7 adapted to emit light at a wavelength shorter than that which induces the peak photopic human eye response alone as illustrated in Figure 2 may be sufficient to create enhanced visibility, since it may be combined with existing lighting, such as white light emitting diode street lights or on temporary lighting towers for road and rail work. Alternatively, where no additional lighting is provided, such as a pedestrian crossing, a lighting device 1 as illustrated in Figure 1, having both a first light source 7 and a second light source 9 is suitable as this will provide both light having a shorter wavelength than that which induces the peak photopic human eye response and a light in a broad bandwidth of the photopic response. Use, in scotopic and mesopic lighting conditions, of the first light source 7, adapted to emit light at a wavelength shorter than that which induces the peak photopic human eye response simultaneously with an existing light source adapted to emit light in a broad bandwidth within the photopic span will also enhance the visibility of fluorescent objects to the human eye. Figure 5a is a schematic illustration of a possible illumination area created by embodiments of the present invention. A lighting device 1 in accordance with the embodiment illustrated in Figure 1 is mounted at the top of a pole 11 that is also used to support a set of traffic lights 12 at a junction. The lighting device 1 is angled, with respect to the pole 11, to create an illumination area A that extends across the junction, thus enhancing the visibility of fluorescent objects (cyclists, pedestrians, motorcyclists wearing fluorescent items) within the illumination area A. Only one set of lights 12 is illustrated for clarity, but given the number of lights 12 usually present at a junction or crossing the illumination areas A created by lighting devices 1 at each set of lights 12 would overlap. Figure 5b is also a schematic illustration of a possible illumination area created by embodiments of the present invention. Here a lighting device 1 in accordance with the embodiment illustrated in Figure 2 is mounted at the top of a pole 11 by means of a support 13 connected orthogonal to the pole 1 itself. The pole 11 supports the traffic lights 12 at a junction, as before, with only a single set of lights 12 illustrated for clarity. The housings 2, 10 of the lighting device 1 are angled to create a coincident illumination area B that again will enhance the visibility of fluorescent objects within the illumination area B. Figure 5c is also a schematic illustration of a possible illumination areas created by embodiments of the present invention. Here a lighting device 1 in accordance with the embodiment illustrated in Figure 1 is mounted on a pole 14 at a distance d away from a pole 11 supporting a set of traffic lights 12 at a junction (again, only one set of lights 12 is illustrated for clarity). This enables the lighting device 1 to be positioned and angled in such a manner as to enhance the visibility of fluorescent objects by creating an illumination area C in the region of the junction. In addition, the positioning of the lighting device 1 on the pole 14 and the distance d of the pole 14 from the junction it is possible to create an illumination area that also serves to enhance the visibility of retroreflective objects to oncoming vehicles. Furthermore it is possible to mount more than one lighting device 1 in accordance with any of the embodiments of the invention on a single pole 11 or support, or to mount the lighting device 1 in accordance with the embodiment of Figure 2 with the housings 2, 10 one above the other vertically on a support rather than on an additional horizontal support 13 as illustrated in Figure 5b. The present invention employs light emitting diodes emitting light at a wavelength in the range 450nm to 480nm. More preferably, and in order to avoid excessive blue-hued illumination, embodiments of the present invention employ light emitting diodes emitting a light at a wavelength in the range 450nm to 475nm. As an alternative to such light emitting diodes, alternative light sources may be used. For example, a broad spectrum light source, one emitting light in a broad bandwidth may be employed with a filter adapted to filter wavelengths in the range 315nm to 525nm. This may be a light emitting diode array. The light emitted by the first light source 7 causes the illuminated objects to fluoresce. The first light source 7 is therefore adapted to enhance the visibility of fluorescent objects. The light emitted by the second light source 9 causes the illuminated object to retroreflect. The second light source 9 is therefore adapted to enhance the visibility of retroreflective objects. A further advantage of embodiments of the present invention is that the lighting device 1 may be employed to enhance the visibility of objects that may include vulnerable personnel (cyclists, pedestrians, workers, emergency personnel) and stationary or moving objects, such as gates at level crossings, barriers, bollards and other road and street furniture. Although in the examples and embodiments described above the support on which the lighting device 1 is mounted is a pole 11, embodiments of the present invention may also be configured, by use of suitable mounting devices 8, to be mounted on various other supports. For example, the lighting device 1 may be mounted on a moving object, such as a train or vehicle (on-road or off-road and including bicycles), or on an object being carried or worn by a person, such as a high-visibility garment or helmet.

Claims

1. Lighting device adapted to enhance the visibility of an object comprising fluorescent materials to the human eye in scotopic and mesopic lighting conditions, the lighting device comprising: a first light source comprising one or more light emitting diodes adapted to emit light at a wavelength in the range 450nm to 480nm in the blue and / or ultra-violet portion of the electromagnetic spectrum; and a second light source comprising one or more light emitting diodes adapted to emit white light; wherein, in use, the first and second light sources are adapted to emit light to illuminate an object comprising fluorescent materials simultaneously or sequentially, wherein the lighting device is for emitting light at a wavelength in the range 450nm to 480nm in the blue and / or ultra-violet portion of the electromagnetic spectrum by the first light source and white light by the second light source on an object comprising fluorescent materials simultaneously or sequentially, so that the visibility of the object comprising fluorescent materials to the human eye in scotopic and mesopic lighting conditions is enhanced, and wherein the first and second light sources are mixed in that arrays of light emitting diodes are used, which are placed alternately on a same support plate.

2. Lighting device as claimed in claim 1, wherein the first light source comprises a light source emitting light in a broad bandwidth and a filter adapted to filter wavelengths in the range 315nm to 525nm.

3. Lighting device as claimed in any preceding claim, wherein the second light source comprises at least one white light emitting diode that emits light in the wavelength range 400 to 700nm.

4. Lighting device as claimed in claim 3, wherein the second light source is halogen bulb or gas discharge tube.

5. Lighting device as claimed in any preceding claim, further comprising: a housing adapted to contain the first and second light sources; and a mounting device coupled to the housing and adapted to mount the housing on a support.

6. System comprising a lighting device and an object comprising fluorescent materials for illumination by the lighting device to enhance the visibility of the object to the human eye inscotopic and mesopic lighting conditions, the lighting device comprising: a first light source comprising one or more light emitting diodes adapted to emit light at a wavelength in the range 450nm to 480nm in the blue and / or ultra-violet portion of the electromagnetic spectrum; and a second light source comprising one or more light emitting diodes adapted to emit white light; wherein, in use, the first and second light sources are adapted to emit light to illuminate the object comprising fluorescent materials simultaneously or sequentially and wherein the first and second light sources are mixed in that arrays of light emitting diodes are used, which are placed alternately on a same support plate.

7. The system of claim 6, wherein the light to be emitted by the first light source causes the object to fluoresce.

8. The system of claim 6, wherein the light to be emitted by the second light source causes the object to retroreflect.

9. The system of claim 6, wherein the light to be emitted by the second light source generates background illumination to the fluorescent material.

10. Use of the system of claim 6, comprising emitting the light from the first and second light sources to illuminate the object comprising fluorescent materials simultaneously or sequentially.

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