GB2365144A - Lamp optical filter that absorbs and fluorescently emits light - Google Patents

Abstract

An optical filter which can be used in a lamp has low attenuation in a selected frequency range and comprises material which absorbs light outside the selected frequency range and fluorescently emits light in the selected frequency range. Hence the fluorescent omission supplements brightness of light from the filter. The filter can absorb infra red frequencies 3 so as to emit light (such as the yellow light in the figures) 2. The filter may consist of a polymer material doped with a fluorescent dye.

Description

2365144

DESCRIPTION

The present invention is concerned with lamps and with optical filtering.

It is often necessary to filter light in order to obtain light at a required frequency or range of frequencies. For example a light source such as an incandescent bulb or a fluorescent tube emits light across a broad range of visible frequencies, and indeed outside the visible range particularly in the infra red. In order to use such a light source to provide light of a selected frequency range or colour, its output can be directed through an optical filter.

Railway signals serve as a good illustration. Traditionally these use high brightness incandescent bulbs with coloured filters to give precisely defined colours and brightness levels. The glass filters used also provide environment protection and seal the signal cover against ingress of rain, dust etc.

Numerous other examples of lights using coloured filters will be known to the reader, e.g. traffic signals, dashboard and instrument lighting for vehicles, coloured room lighting etc.

A particular filtering requirement arises where infra red viewing systems are used. Military pilots for example use so-called Night Vision Goggles (NVGs) to enable them to see at night, using infrared frequencies, without use of visible illumination. Many conventional (visible) light sources cause dazzling of NVGs because as well as visible light they emit unacceptably high levels of infra red.

To overcome this problem it is known to filter light sources thereby preventing or at least reducing emission of the relevant infrared frequencies while allowing emission of the required visible frequencies, thereby rendering the sources 2 NVG compatible. The applicant's own granted U.S. Patent No:60114933 describes an external aircraft light having such a filer. The US military specification MIL-L85762A details internal cockpit lighting - for instruments, switches etc, whose output is likewise filtered to remove infra red, thus enabling N-VGs to be used in the cockpit without being dazzled by infra red from the cockpit interior.

Filters used to reduce the unwanted infrared emission are typically of interference type. Although these are relatively efficient, a portion of the desired visible light (perhaps ten percent) is removed by such filters. This can undesirably reduce visibility of the light, especially under high ambient light conditions (e.g. in sunlight). The problem also arises with other types of filter, which typically block a portion of even the desired output frequencies.

The present inventors have additionally recognised that conventional optical filters are inefficient in energy terms. Part of the light source's output (that part lying in a frequency range which is not required) is blocked and so wasted.

As a result, filtered lights can lack brightness.

The present invention seeks to provide a way to maximise, or at least to increase, the output of an optical filter and/or a light source in a desired frequency range.

The present inventors have recognised that fluorescent dyes can be used in solving this problem.

Reference is now directed to published International (PCT) Patent Application WOOO/07039 (Application No: PCT/GB99/02482) filed by The Court of Napier University. This discloses among other things a range of materials each comprising 3 a transparent polymer doped with a carefully selected fluorescent dye. The material consequently fluoresces, at a selected frequency. when subject to ambient light. This document teaches use of such materials in optical fibres and display elements. It is not, however, concerned with light filtration.

It should be noted that this prior art document discloses various dye molecules which may be used in the present invention. For the US, WOOO/07039 is hereby incorporated by reference.

In accordance with a first aspect of the present invention there is an optical filter having low attenuation in a selected ftequency range, the filter comprising material which absorbs light outside the selected frequency range and fluorescently emits light in the selected frequency range.

In accordance with a second aspect of the present invention there is an optical filter adapted to output light of a chosen colour, the filter comprising material which fluorescently emits light of the chosen colour and thereby increases the brightness of the filter's output.

Thus, for example a red filter for use on a railway signal light may have material which absorbs light of other colours and emits red light by fluorescence. In this way the brightness of the signal light, and its energy efficiency, can be improved. Photons of light outside the required output frequency range can be absorbed by the fluorescent material, the energy thereby imparted to the material being released in light photons within the desired frequency range.

A particular advantage of this application is that the outside coloured glass filter of the signal need not be changed when the existing incandescent light sources 4 are upgraded to high reliability LED's whilst keeping the specification of the light output within the prescribed colours. The glass cover filter also acts as an envirom-nental seal and advantageously this should not be changed.

Fluorescence typically involves absorption by the fluorescent material of higher energy photons (i.e., since photon energy is proportional to frequency, higher frequency light) and emission of lower energy (lower frequency) light. For example blue/green light may excite a fluorescent material to emit red light. However, there are fluorescent materials which, by so-called anti-Stokes mechanisms can absorb lower frequency light to fluoresce at higher frequencies, e.g. absorbing red light to fluorescence green. Physically, two lower energy photons (and hence energy transitions in the material) produce one emitted higher energy photon. Either type of material (Stokes or anti-Stokes) may be used in the present invention.

It is particularly preferred that the filter comprises a polymer material combined with a fluorescent dye. The polymer material may be doped with the dye. The polymer can be selected for constructional convenience, e.g. to be capable of beina moulded or machined. In this way the fluorescent dye can be incorporated in a filter which is straightforward to manufacture.

The filter can be such as to block or substantially attenuate selected infra red frequencies. Such filters can as described above be used to render infrared emitting light sources, such as incandescent bulbs, compatible with NVGs. The present invention makes it possible to compensate for the problem described above of reduction in brightness of the desired (normally visible) frequencies/colours by using the fluorescent material to emit light, and hence increase output brightness, at these frequencies/col ours..

In accordance with a third aspect of the present invention there is a lamp comprising a light source, a filter adapted and arranged to filter light from the source and so provide output light of a chosen colour, and a body which is exposed to light from the source and is stimulated thereby to fluoresce with the chosen colour, increasing the brightness of the lamp's output.

It must be understood that the "colour" in question could in some enibodiments be a frequency range outside the visible range, eg. in the infra red.

The fluorescent body and the filter may be formed by the same component.

However the fluorescent body may in accordance with a further embodiment of the present invention be a separate component from the filter, e.g. a reflector of the lamp, arranged such that light therefrom contributes to the lamp output.

In such embodiments the filter may be arranged to filter light emitted from both the light source and the fluorescent body.

The light source is typically electrically driven. An incandescent bulb may be used.

It is parficularly preferred that the light source comprises one or more LEDs. In such embodiments the fluorescent material may be selected to absorb light at a first emission frequency of the LED and to fluoresce at or close to a second emission frequency of the LED. Brightness of light at the second frequency is thus increased. The filter can be such as to output light at or around the second frequency with low attenuation.

Such an LED based lamp can be designed for receipt by a socket for an 6 incandescent bulb. Hence the lamp can be used to replace an incandescent bulb with the minimum of cost and description, during routine maintenance. The LED based lamp offers improved reliability over an incandescent bulb.

It is especially preferred that the filter is such as to remove selected infra red frequencies for compatibility with NVGs. Such lamps may, according to a preferred aspect of the present invention. be indicator lights. These may for example be for use in aircraft cockpits In accordance with a fourth aspect of the present invention, there is a light emitting diode comprising fluorescent material which is stimulated by light in a first emission band of the LED to fluoresce at or adjacent to a second emission band of the LED such as to increase the LED's brightness.

Again, by transferring from one (undesired) frequency (or rather band of frequencies) to another (desired) frequency, the brightness of the desired frequency is improved.

Specific embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a graph of the emission spectrum of a known incandescent light source (bulb), the vertical axis being normalised spectral radiance and the horizontal axis being wave length in nanometres; and Figs. 2a and 2b are both graphs of the absorption and emission characteristics desired for a lamp according to the present invention, normalised absorption emission being on the vertical axis and wavelength in nanometres being on the horizontal axis; and 7 Fig. 3 is a greatly simplified diagram of an arrangement which can be used for testing a transmission filter according to the present invention.

Fig. 1 illustrates the broad spread of frequencies, both visible and infra red, obtained from a known incandescent bulb.

In Fig. 2, the peak labelled 1 represents a desirable spectrum for a yellow indicator light.

In implementing the present invention, dye molecules can be selected which absorb light from the incandescent bulb in either of the absorption ranges labeled 2 and 3 in order to fluoresce in the region 1. This can be by Stokes or anti-Stokes fluorescence mechanisms.

While Fig. 2 is concerned with emission characteristics for a yellow light. other colours - green, red etc - can be provided, the corresponding emission spectra being centred on different frequencies.

Where the lamp is to be NVG compatible, it is desirable that the fluorescent emission of the dye at wavelengths longer than those of the required emission spectrum should be very limited, to avoid dazzle of NVGs. Where this cannot be achieved, a filter to remove such frequencies is required in front of the body of fluorescent dye.

The fluorescent dye of the invention can be incorporated in a body of optically transparent or translucent polymer. As explained in Napier University's application PCT/G1399/02482, the polymer can be doped or blended with organic fluorescent dye molecules. Suitable polymers include P11HA, polycarbonate and polystyrene. Suitable dye molecules include PBD, Bis-MS13, 'J'-3'diethyloxycarboxyanine-iodide and cresyl violet 670 percholate. However, other materials, selected for appropriate optical (and indeed physical) properties can be used.

To test a transmission filter according to the present invention, the filter 10 can as seen in Fig. 3 be exposed to light from a source 12, the filter's output being detected by unit 14 for analysis.

Claims (12)

1. 9 CLAIMS

   An optical filter (10) having low attenuation in a selected frequency range (1), the filter comprising material which absorbs light outside the selected frequency range and fluorescently emits light in the selected frequency range.
2. 2. An optical filter as claimed in claim 1, wherein the filter comprises a polymer material combined with a fluorescent dye.
3. An optical filter as claimed in claim 2, wherein the polymer material is doped with the dye.
4. 4. An optical filter as claimed in any preceding claim, wherein the filter is such as to block or at least substantially attenuate selected infra red frequencies.
5. 5. A lamp comprising a light source, a filter adapted and arranged to filter light from the source and so provide light of a chosen colour, and a body which is exposed to light from the source and is stimulated thereby to fluoresce with the chosen colour, thereby increasing brightness of the lamp's output.
6. 6. A lamp as claimed in claim 5, wherein the fluorescent body and the filter are formed by the same component.
7. 7. A lamp as claimed in claim 5, wherein the fluorescent body is a reflector arranged such that light reflected therefrom contributes to the lamp output.
8. 8. A lamp as claimed in any of claims 5 to 7 wherein the light source comprises at least one LED.
9. 9. A lamp as claimed in claim 8, wherein the fluorescent material is selected to absorb light at a first emission fr,equency of the LED and to fluoresce at or close to a second emission frequency of the LED.
10. 10. A lamp as claimed in claim 9 wherein the filter is such as to output light at or close to the second frequency with low attenuation.
11. 11. A larrip as claimed in any one of claims 5 to 10 which is adapted to substitute for an incandescent bulb.
12. 12. A lamp claimed in any one of claims 5 to 11 wherein the filter is such as to remove selected infra red frequencies for compatibility with NVGS.