GB2320721A - Luminescent Indicator - Google Patents

Abstract

A luminescent indicator (10), for example for use in a gun sight, comprises a cylindrical tritium-containing luminous element (18) and a transparent window (16). These are secured within a plastic sleeve (12) with an internal lip (14) near one end. The window (16) abuts the lip (14) and is a close fit in the sleeve (12), while the element (18) is a sliding fit, and may be fixed with glue (20). Pre-assembling the indicator (10) in this way simplifies the installation of the window (16) and element (18) into a housing of a gun sight.

Description

Luminescent Indicator This invention relates to a luminescent indicator, and to a method of making such an indicator.

Luminescent indicators are known for providing a continuous source of light without the use of any external power supply. For example such indicators are used in gun sights and in instruments such as compasses.

The light source in such an indicator may be a polymer composition, for example as described in GB 2 242 908 it may be a mixture of polymer labelled with tritium, and an organic compound which emits light when subjected to radiation emitted by the tritium. Alternatively it may comprise gaseous tritium in a glass capsule, as described for example in GB 2 202 312, with walls coated with a material such as zinc sulphide which emits light when irradiated. It will be appreciated that tritium gas is potentially hazardous if released, so that it is desirable to minimise the risk of damage to such a capsule during assembly or use of the luminescent indicator.

According to the present invention there is provided a method of assembling a luminescent indicator comprising a cylindrical tritium-containing luminous element and a transparent window, the method comprising the steps of pushing the window into a tubular sleeve of plastics material with an internal lip at one end, until the window abuts the lip, and then inserting the luminous element into the sleeve until it abuts the window, the luminous element being a sliding fit in the sleeve.

The method may also include the step of providing an adhesive to secure the luminous element to the sleeve.

The sleeve may be such that the window is held securely once inserted, for example it may be an interference fit at least in that part adjacent to the internal lip.

The invention also provides a luminescent indicator comprising a tubular sleeve of plastics material with an internal lip at one end, a transparent window within the sleeve and abutting the lip, and a cylindrical tritiumcontaining luminous element within the sleeve and abutting the window, the element being a sliding fit within the sleeve.

The term interference fit means that the bore of the sleeve has a diameter about 0.02 mm less than that of the window so the sleeve deforms slightly when the window is inserted, and holds it securely. The term sliding fit, which may be referred to as clearance fit, means that the bore is about 0.05 mm (preferably between 0.04 and 0.15 mm) larger than the diameter of the luminous element, so the element can be slid into the sleeve easily without subjecting the element to any stress.

The sleeve, or at least that part of the sleeve at the end where the lip is, is desirably white, so that the lip and that end of the sleeve define a white ring around the exposed face of the window. The preferred material for the sleeve is an ABS resin (acrylonitrile-butadienestyrene) as this is a fairly high tensile strength engineering plastic, which is thermoplastic, and can be moulded to the desired shape. The window may be of glass or sapphire, for example. It may be parallel-faced, or may be a lens for example a converging lens.

The luminescent indicator can be safely handled without fear of damaging the tritium-containing luminous element. If it is to be used in a gun sight, it would then be installed in a housing with only the external face of the window and the lip of the sleeve exposed.

For such a use the sleeve might typically have a diameter in the range 1 mm to 5 mm, preferably between 1.5 mm and 3 mm, for example 2mm. It thus avoids the problems involved in cementing a window directly into a housing.

The invention will now be further described, by way of example only and with reference to the accompanying drawing which shows a longitudinal sectional view of a luminescent indicator for use in a gun sight.

Referring to the drawing, a luminescent indicator 10 includes a sleeve or tube 12 moulded of white ABS plastic, of diameter 2 mm and length 5 mm. At one end the tube 12 defines an internal lip 14, with an inclined external face and a square inner face. The tube 12 has a wall thickness of 0.2 mm. A parallel-sided glass window 16 of thickness 1 mm abuts the inner face of the lip 14, and in this position is a close fit with the tube 12 (with nominally zero clearance). A cylindrical glass capsule 18 locates in the tube 12 behind the window 16; it is of diameter 1.5 mm so as to be a sliding fit in the tube 12 and of length about 3 mm. It has glass walls coated with zinc sulphide phosphor, and contains tritium gas, so it continuously emits light. A drop 20 of adhesive secures the capsule 18 in the tube 12.

The luminescent indicator 10 can subsequently be installed in a housing (typically of metal or of rigid plastic) of a gun sight, the tube 12 locating in a correspondingly sized blind aperture and being glued into position with just the outer face of the window 16 and the end of the tube 12 being visible. The light hence emerges through the window 16, providing an illuminated dot surrounded by the white ring or iris defined by the external face of the lip 14.

Assembly of the luminescent indicator 10 as shown involves pushing the window 16 along the tube 12 until it abuts the lip 14. During this operation the tube 12 is supported in a tubular two-part housing to ensure the tube 12 does not buckle. The capsule 18 can then be slid into the tube 12, and then the drop of adhesive 20 is applied to the rear of the capsule 18 to ensure it does not fall out again.

It will be appreciated that a luminescent indicator may differ from that described above while remaining within the scope of the invention. For example the dimensions may differ, for example to fit a different gun-sight housing. The window may be replaced by a lens, for example a plano-convex lens, to increase the apparent brightness of the light. The capsule 18 might be replaced by a different luminous element, for example a capsule containing a different phosphor coating, or a light emitting polymer rod. Furthermore the tube might be long enough to accommodate additional items for example a rear glass window might be an interference fit or a close fit into the tube behind the luminous element.

In one alternative the tube or sleeve has a slightly tapering bore, widest at the end remote from the lip 14, so the window 16 can be easily slid into the tube, and is an interference fit as it reaches its final position so as to slightly stretch the tube there, so ensuring the window 16 is held securely. In another alternative no adhesive 20 is used to secure the capsule 18 into the tube 12, but instead that portion of the tube 12 which extends behind the end of the capsule 18 is thermally deformed to engage the rear edge of the capsule 18; this may be achieved using an ultrasonic or microwave welding technique. It will be appreciated that although it is not easy to mould plastic to provide a predetermined size of bore, nevertheless the dimensions of plastic tubes 12 made from a given mould can be expected to be consistent, and windows 16 can be accurately made to fit the tubes 12.

It will be appreciated that use of a luminescent indicator as described above simplifies the assembly of the gun sight, and improves the safety of the assembly process. After a period of some 10 or 12 years the light emission from the tritium-containing element will have decreased so that the indicator is no longer useful, and at that stage too the use of a luminescent indicator as described above means that the indicator can easily and safety be removed, facilitating the recovery and disposal or recycling of the remaining tritium.

Claims (7)

Claims

1. A method of assembling a luminescent indicator comprising a cylindrical tritium-containing luminous element and a transparent window, the method comprising the steps of pushing the window into a tubular sleeve of plastics material with an internal lip at one end, until the window abuts the lip, and then inserting the luminous element into the sleeve until it abuts the window, the luminous element being a sliding fit in the sleeve.

2. A luminescent indicator comprising a tubular sleeve of plastics material with an internal lip at one end, a transparent window within the sleeve and abutting the lip, and a cylindrical tritium-containing luminous element within the sleeve and abutting the window, the element being a sliding fit within the sleeve.

3. A luminescent indicator as claimed in Claim 2, wherein the sleeve is white, at least at the end with the lip.

4. A luminescent indicator as claimed in Claim 2 or Claim 3, wherein the window is a converging lens.

5. A luminescent indicator as claimed in any one of Claims 2 to 4, wherein the bore of the sleeve tapers along its length, being least adjacent to the lip.

6. A method of assembling a luminescent indicator substantially as hereinbefore described with reference to the accompanying drawing.

7. A luminescent indicator substantially as hereinbefore described with reference to the accompanying drawing.