GB2254959A

GB2254959A - Lens mounting

Info

Publication number: GB2254959A
Application number: GB9108079A
Authority: GB
Inventors: Ian Kenneth Pasco
Original assignee: Combined Optical Industries Ltd
Current assignee: Combined Optical Industries Ltd
Priority date: 1991-04-15
Filing date: 1991-04-15
Publication date: 1992-10-21
Also published as: GB9108079D0

Abstract

In a lens mounting for a light source (2) and a lens (1), the lens (1) is mounted on a first plastics sleeve (3) and the light source (2) is mounted on a second plastics sleeve (4). The first sleeve (3) extends beyond light source (2) and is anchored to point (A) rearward of the light source. The second sleeve (4) extends beyond lens (1) and is anchored to point (8) forward of the lens. The first and second sleeves (3, 4) are movable with respect to each other due to thermal expansion or contraction to compensate for changes in the focal length of the lens due to temperature variation. <IMAGE>

Description

LENS MOUNTING The present invention relates to a lens mounting.

More particularly, but not exclusively, it relates to a lens mounting for a light emitting or laser diode and a collimating lens therefor which is automatically adjustable for changes in ambient temperature.

There are many applications, ranging from bar code readers to anti tank missile systems where a parallel beam of light is required to be generated from a point source.

For these purposes, a collimating lens is used with the light source at its focal point.

However, such lenses are often made of plastics material such as polymethylmethacrylate or polystyrene and these materials have physical properties which change with ambient temperature. For example, the coefficient of thermal expansion for polymethylmethacrylate is 6.5 x 10-5 mm/mm/ C. This compares to the coefficient of thermal expansion of a metal such as aluminium which is 1.5 x 10'5 mm/mm/ C. Thus, as the ambient temperature increases, the lens will expand and the thicker parts of the lens will expand more than the thinner parts, thereby leading to a new lens shape with altered curve cap height.

Furthermore, the refractive index of polymethylmethacrylate and similar plastics materials changes with temperature and in the case of polymethylmethacrylate it changes by -8.5 x 10'5 per OC.

The overall effect of these changes in physical properties is that the focal length of the lens may change by 80 um for a 500C temperature change. Since the light source should be at the focus of the lens, such a change will effect the degree of parallelism of the collimated light beam. In order to correct for this, the lens and the light source should be moved apart so that the light source remains at the focal point of the lens.

In order to compensate for temperature changes, it would be ideal if the lens or the light source could be mounted on a material which expanded longitudinally in response to a change in ambient temperature. If the mount were to made of plastics material, such as polymethylmethacrylate, the separation between the mounting point and the point of anchoring of the plastics material would need to be 24.62 mm to accommodate an 80 um expansion in the focal length of the lens. It is not practical to provide a lens mount of this size for a lens of, say, 5 mm diameter and therefore it is an object of the present invention to provide a lens mount which enables the separation between lens and light source to change automatically with temperature but which is not impractically long.

According to the present invention there is provided a lens mounting for a light emitting or laser diode and a collimating lens therefor, wherein said lens is mounted to a first plastics sleeve extending beyond said diode and anchored to a point rearward of said diode, and said diode is mounted to a second plastics sleeve extending beyond said lens and anchored to a point forward of said lens, said first and second plastics sleeves being movable each with respect to the other under the influence of thermal expansion or contraction.

The first and second plastics sleeves are contained within and fixed to an outer sleeve of metallic material.

In one embodiment said first and second plastics sleeves may each comprise a plurality of longitudinally extending and circumferentially spaced fingers, the fingers of one sleeve passing between the fingers of the other sleeve.

In another embodiment, the sleeves may comprise concentric tubes.

The preferred plastics material for the lens mounting is polymethylmethacrylate, although other plastics material such as polystyrene may be used.

The invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: FIGURE 1 is a schematic cross sectional view of a lens mounting embodying the invention; FIGURE 2 is a cross sectional view taken along the line II-II of Figure 1; and FIGURE 3 is a ray trace diagram to show the difference in focal length at different temperatures.

Referring now to the drawings, there is shown a lens mounting assembly including a collimating lens 1 and a laser diode light source 2. The lens 1 is preferably of polymethylmethacrylate, which has a coefficient of thermal expansion of 6.5 x 10'5 mm/mm/ C and a temperature coefficient of refractive index of -8.5 x 10'50C.

Accordingly, as the ambient temperature changes, the focal length of the lens will change due to both of the above factors. For a change in temperature of 500C, the focal length will change by 80 um. This is shown in Figure 3.

In order to produce beam of parallel light, the light source 2 should be at the focus of the lens 1. As the ambient temperature changes, this spacing must be changed to accommodate the change in focal length. For an additional spacing of 80 um in the change in the focal length, the mounting of either the lens or light source must be moved by an equivalent amount. This requires, for thermal expansion of polymethylmethacrylate a length between anchor point and mounting of 24.64 mm. Such a length is impractical and therefore the present invention envisages mounting each of the lens 1 and light source 2 to a respective sleeve of plastics material, such as polymethylmethacrylate.A first sleeve 3 holds the lens 1 and is anchored at point A, while a second sleeve 4 holds the light source 2 and is anchored at point B. Thus, expansion of the plastics material by virtue of increase in temperature will cause both the sleeves to increase in length and thereby separate the lens 1 from the light source 2.

The sleeves 3 and 4 are configured over their cooperating length as a plurality of circumferentially spaced fingers, the fingers of one sleeve being interspaced between the fingers of the other sleeve.

The sleeves 3 and 4 are anchored at A and B to a sleeve of metal such as aluminium, for which the coefficient of thermal expansion is only 1.5 x 10-5 mm/mm/ C. This aluminium sleeve 5 is connected to a heat sink 6 to reduce the effects of any change in ambient temperature.

Thus, as the focal length of the lens changes with changes in temperature, its spacing from the light source is automatically changed by virtue of the thermal expansion of the sleeves 3 and 4 and by selection of materials having appropriate physical properties, the lens and light source can be maintained at a spacing close to the focal length of the lens, at whatever temperature it may be.

Claims

CLAIMS:

1. A lens mounting for a light emitting or laser diode and a collimating lens therefor, wherein said lens is mounted to a first plastics sleeve extending beyond said diode and anchored to a point rearward of said diode, and said diode is mounted to a second plastics sleeve extending beyond said lens and anchored to a point forward of said lens, said first and second plastics sleeves being movable each with respect to the other under the influence of thermal expansion or contraction.

2. A lens mounting according to claim 1, wherein said first and second plastics sleeves are contained within and fixed to an outer sleeve of metallic material.

3. A lens mounting according to claim 1 or 2, wherein said first and second plastics sleeves each comprise a plurality of longitudinally extending and circumferentially spaced fingers, the fingers of one sleeve passing between the fingers of the other sleeve.

4. A lens mounting according to claim 1, 2 or 3, wherein the sleeves comprise concentric tubes.

5. A lens mounting according to any of the preceding claims, wherein the plastics material is polymethylmethacrylate or polystyrene.

6. A lens mounting substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.