GB2163542A

GB2163542A - Indicator lamps

Info

Publication number: GB2163542A
Application number: GB08515435A
Authority: GB
Inventors: Timothy George Bushell; Geoffrey Stephen Edwards
Original assignee: Oxley Developments Co Ltd
Current assignee: Oxley Developments Co Ltd
Priority date: 1984-08-21
Filing date: 1985-06-18
Publication date: 1986-02-26
Also published as: DE3529803A1; GB2163542B; GB8421185D0; FR2572496B1; FR2572496A1; GB8515435D0

Abstract

An indicator lamp comprising a first, thin-film interference filter (14) located in front of a light source (10) for rejecting undesirable near infra-red radiation originating from the light source, and a second filter (12) located on the opposite side of the filter (14) to the light source for absorbing radiation of predetermined undesired visible wave lengths from the ambient external light and ambient external light reflected by the filter (14). The second filter can be a neutral density filter, a circularly polarising filter or simply an absorbtion filter. <IMAGE>

Description

SPECIFICATION Indicator lamps The invention relates to indicator lamps.

Indicator lamps, when in operation, emit visible light but most such devices also emit a near infra-red component. This is an inherent feature of known devices. Ideally, for some applications, an indicator lamp of this sort would have a high energy output of visible light, and no output in the near infra-red spectral region.

The need for such an indicator lamp has been brought about by the increasing use of sensitive night viewing apparata which are designed to enable the user to view objects in dark light. Such apparata are able to detect the low levels of near infra-red radiation emitted or reflected by these objects and are, thus, able to "see" in the dark. Commonly, night viewing apparata are sensitive within the spectral region 600 to 900 nanometres, the red/near infra-red region.

Indicator lamps used in proximity to night viewing apparata and on most other electronic equipment, use light emitting diodes. L.E.D.'s have a relatively narrow spectral output, in the region of 40 nanometres, but inherently, also emit energy in the near infra-red region. The levels of near infra-red energy emitted are generally sufficient to "blind" the night viewing apparata. This is because the L.E.D.'s can flood the night viewing apparata with a much higher level of near infra-red energy than can the objects which are being observed by the apparata.

Whilst it is obviously important to reduce the near infra-red radiation from the lamp to a minimum, it is also important to maintain the visible energy output of the indicator lamp at as high a level as possible.

It is an object of the present invention to mitigate the above problems by providing an indicator lamp whose emission is substantially free from radiation in the near infra-red spectral region while still providing a high level of visible light.

In accordance with the present invention there is provided an indicator lamp comprising a light source, a first filter means located in front of the light source for rejecting undesirable near infra-red radiation originating from the light source, and a second filter means located on the opposite side of the first filter means to the light source for absorbing radiation of predetermined undesired visible wave lengths from the ambient external light and ambient external light reflected by said first filter means.

Preferably, the light source is a light emitting diode. In one preferred embodiment the first filter means is a thin film interference filter fabricated on a glass substrate. The second filter means comprises a neutral density filter and/or a circularly polarising filter located between the observer and the interference filter.

The interference filter can be fabricated onto one face of the neutral density glass filter or circularly polarising filter, the interference filter being located on the glass face away from the observer and the glass acting as the lens for the light source.

The second filter means may, alternatively, comprise an absorption filter which has for a green lamp, for example, light transmission in the green region of the spectrum only.

In another embodiment, the indicator lamp comprises a laminated structure of a clear glass lens, a neutral density filter and an interference filter fabricated on a separate glass substrate. Alternatively, the neutral density filter may be replaced by an absorption filter.

In another embodiment, the interference filter is fabricated on a glass substrate and is fixed to the light emission surface of the light source.

Preferably, in order to eliminate the danger of emission of unfiltered light, all light source surfaces from which emission could occur and not pass through the filter means are blackened as well as the internal surfaces of the housing.

The invention will now be described further hereinafter, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of one embodiment of an indicator lamp in accordance with the present invention; Figure la illustrates diagrammatically the operation of the embodiment of Fig. 1; Figure 2 is a schematic diagram of a second embodiment of an indicator lamp; and Figure 3 is a schematic diagram of a third embodiment.

The indicator lamp illustrated in Fig. 1 comprises an L.E.D. 10, a housing 16, and a neutrai density glass filter 12 which has one face coated with an infra-red blocking filter 14. The infra-red blocking filter 14 is located in front of the light emitting surface of the L.E.D. 10.

Filter 14 filters out the undesirable infra-red energy, but also, disadvantageously, tends to attenuate the visible light output. In order to reduce the latter attenuation to a minimum, it is preferred to use a thin film interference filter as the infra-red filter 14. This allows a very rapid transition from transmission in the visible region, to rejection in the infra-red region. Although the interference filter is shown fabricated on a glass substrate, it is possible to use other substrates such as sapphire.

A disadvantage of the interference filter is that it is highly reflective in the red or orange/red spectral region, particularly if the infrared blocking is required to begin in the 600 to 650 nanometres region. Such a filter, if used alone in the indicator lamp can give spurious "on" indication if the lamp is to be used in bright sunlight. For this reason the neutral density filter 12 is used to reduce the level of reflection from the infra-red blocking filter 14.

The operation of the arrangement shown in Fig. 1 is illustrated diagrammatically in Fig. la to which reference is now made.

Reference numeral 30 indicates light incident on the IR filter 14 from the light source (LED 10). A portion 30a of this light, corresponding to the unwanted iR, is reflected by the IR filter back towards the general direction of the light source. The remaining portion 30b, corresponding to the required visible light, is passed by the IR filter. However, it undergoes a certain amount of attenuation by the neutral density filter before emerging as visible light 30c.

On the other hand, ambient illumination 34 incident on the front face of the neutral density filter 12 enters the filter 12, undergoes some attenuation by the neutral density filter, is reflected by the IR filter, undergoes further attenuation by the neutral density filter 12 and emerges again as indicated at 34a.

In addition to the components shown in Fig.

1, a circularly polarising filter may be used, positioned between the observer and the infra-red block filter to further reduce reflection.

Alternatively, a glass or plastics film/dye absorption filter may replace the interference filter and a further absorption filter may replace the neutral density filter. An absorption filter for a green lamp, for example, has a high transmission in the green region and a low transmission in the undesirable red orange/red region.

Fig. 2 shows an indicator lamp having a laminated structure comprising a separate clear glass lens 18, a neutral density filter 20, and an infra-red blocking filter 22 fabricated on a separate glass substrate. Again, in this embodiment, the neutral density filter could be replaced by an absorption filter or the infra-red blocking filter could be coated onto the glass substrate of the neutral density filter.

Fig. 3 shows an indicator lamp having a neutral density filter 24 which is of glass/neutral density filter laminate construction and which acts as the lamp lens. An infra-red blocking filter 26, coated onto a glass substrate 28 is attached to the front face of an L.E.D. 30 which is so shaped so as to accommodate the filter.

Whichever indicator lamp configuration is used, it is essential to ensure that all the energy emitted from the light source is either incident upon the infra-red blocking filter, or otherwise prevented from being emitted from the indicator. L.E.D.'s are particularly suited for this as the light emitting surface area is relatively small and it is, therefore, relatively easy to ensure that all the emitted energy passes through the infra-red blocking filter. In the case of interference infra-red blocking filters, the reflection characteristics are optimised by having the light source incident normally to the filter. For an L.E.D. this can be relatively easily achieved by a simple lens arrangement.

In order to eliminate the danger of emission of unfiltered light all L.E.D surfaces from which emission could occur and not pass through the IR blocking filter are preferably blackened.

All internal surfaces of the indicator which do not emit filtered light are preferably blackened to reduce further the chance of spurious 'on' indications occurring in bright light conditions.

Although in the preferred embodiments L.E.D.'s are used as the light source, other sources such as incandescent bulbs can be used. Also, although glass lenses are used, it is possible to use other materials such as sapphire and fused silica.

Claims

1. An indicator lamp comprising a light source, a first filter means located in front of the light source for rejecting undesirable near infra-red radiation originating from the light source, and a second filter means located on the opposite side of the first filter means to the light source for absorbing radiation of predetermined undesired visible wave lengths from the ambient external light and ambient external light reflected by said first filter means.

2. An indicator lamp as claimed in claim 1, wherein the first filter means is a thin film interference filter fabricated on a glass substrate.

3. An indicator lamp as claimed in claim 1 or 2, wherein the second filter means comprises a neutral density filter.

4. An indicator lamp as claimed in claim 1 or 2, wherein the second filter means comprises a circularly polarising filter.

5. An indicator lamp as claimed in claim 3 or 4, when appended to claim 2, wherein the thin film interference filter is fabricated on one face of the neutral density glass filter or circu larly polarising filter, the interference filter being located on the glass face facing the light source and the glass acting as a lens for the light source.

6. An indicator lamp as claimed in claim 1, wherein the second filter comprises an absorption filter.

7. An indicator lamp as claimed in claim 1, comprising a laminated structure of a clear glass lens, a neutral density filter serving as said second filter means, and an interference filter, serving as said first filter and fabricated on a separate glass substrate.

8. An indicator lamp as claimed in claim 1, comprising a laminated structure of a clear glass lens, an absorbtion filter serving as said second filter means, and an interference filter, serving as said first filter and fabricated on a separate glass substrate.

9. An indicator lamp as claimed in claim 1 wherein the first filter means comprises a thin film interference filter fabricated on a glass substrate and fixed to the light emission sur face of the light source.

10. An indicator lamp as claimed in any of claims 1 to 9 wherein all internal surfaces of the lamp through which light emission is not required are blackened.

11. An indicator lamp substantially as here I inbefore described with reference to and as illustrated in the accompanying drawings.