GB2250881A

GB2250881A - Radiation detector optics unit

Info

Publication number: GB2250881A
Application number: GB9021742A
Authority: GB
Inventors: Melvyn Douglas Sedgbeer
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-10-05
Filing date: 1990-10-05
Publication date: 1992-06-17
Anticipated expiration: 2010-10-05
Also published as: GB9021742D0; GB2250881B

Abstract

An optics unit for use in a radiation detector and direction finder comprises lens(es) 20 which form defocussed spot image(s) of distant point source(s) of radiation on a radiation sensitive array of diodes 1. In order to reduce distortions inherent in the use of a lens with a short focal length (necessary to give a wide field of view) the center of the lens is obscured 22 having a clear optical aperture 21 of annular shape. so enabling the position of such emission(s) within the viewing angle to be deduced with much higher accuracy than would otherwise be possible. <IMAGE>

Description

RADIATION DETECTOR OPTICS UNIT This invention relates to optics units for use in radiation detectors; the optics unit comprising a lens or lenses, where the clear optical aperture is arranged to give minimal distortion.

Focal plane staring array radiation detectors maybe used to detect emissions and enable their location within the detectors viewing angle to be deduced; a viewing angle ideally as wide as possible c consistent with accurate detection ot the emissions location; a viewing angle where width maybe maintained in a conventional system by the introduction of mechanical gimbal arrangements. A detector would comprise an optics unit, which forms a defocussed spot image of a distant point source of radiation on an array of radiation sensitive photo-diodes, which are connected to electronic buffer amplifiers and signal conditioning/processing.The purpose of the latter is to enable accurate determination of the location of emissions within the detectors viewing angle, with overall performance being highly dependent on the quality of the optics unit. In a conventional arrangement the optics unit can become complex, large and costly, with multiple lens units or mirror arrangements necessary to achieve satisfactory overall performance.Also deficiencies of tile optics unit Ipay be corrected for, to some degree, in. the electronics and signal processing with added complexity, size and cost; the magnitude of which depending upon the degree of correction necessary to achieve the required performance, always assuming that such correction is practically realiseable.

According to one aspect of the present invention there is provided an arjnular single lens optics arrangement for use in compact focal plane staring array radiation detectors, with 'no moving part.' where it is mounted a short distance in front of a radiation sensitive array of photo-diodes, offering improved quality due to the single lens being an annulus or having a clear ptic.al aperture in the form of an annulus, as opposed to for example a standard aspheric lens.

According to another aspect of the present invention there is provided optics units offering improved quality by means of multiple lens arrangements with the ist lens being an annulus or having a clear optical aperture in the form of an annulus, and subsequent lenses being either conventional or annular.Such an arrangement maybe at the expense of compactness, but wold still have 'no moving parts' According to another aspect of this invention there is provided single or multiple annular lens optics units offering improved quality, together with means of manual or automatic adjustement to give movement along the longitudinal optical axis of the lens/es, or relative movements between lenses, with respect to the focal plane staring array. Such an arrangement would be at the expense of compactness, and also possess moving parts.

A specific embodiment Of the invention will now be described by way of example only, with reference to the accompanying drawings where: Fig. 1 is a cutaway view of a conventional radiation detector arrangement with optics unit/array of diodes.

Fig. 2 shows an annular optics unit incorporated.

Fig. 3 is a diode array, with 5: 1 enlargement of a portion.

Fig. 4a is a ray tracing diagram for a conventional single lens arrangement, for a distant on-axis point. source of radiation, with 1#:1 I enlargement of image spot.

Fig. 4b is as 4a for an annulus lens arrangement.

Fig. 5a is as 4a for an off-axis distant point source of radiation Fig. 5b is as 5a for an annulus lens arrangement.

Fig. 6 is as 4b for a near on-axis extended source of radiation.

Flg. 7 shows an annular lens.

Referring to Figs. 1/3, the illustrated example conventional radiation detector arrangement comprises a single lens (1#) mounted in a fixed position at a distance (15) between the rear face of the 1 lens and the surface of the radiation sensitive photo-diode array (1), which is enclosed within the hermetically sealed package (2), with an optical window (12) and electrical input/output connections (3); the lens being held in a fixed position by a clamping ring (6) with 'O' rings (5) and retaining ring (4); the clamping ring being attached to a spacer ring (8) with distance (15) governed by shims (7); the complete arrangement having a clear optical aperture (9); and with there being 'no moving parts' to the arrangement so giving high reliability and low cost. Furthermore, to provide a compact arrangement with viewing angles (11) of up to 4# degrees, the lens possesses a very short focal length (approx. 3# mms), which inherently gives a high degree of distortion to the defocussed spot image (16) of a distant point source of radiation.

The r ray tracing diagram in Fig. 4a illustrates the spherical aberration phenomena which will occur with the above arrangement, so meaning that for any given distance (15) the spot of radiation from a distant point souce that falls on the surface of the sensitive photo-diodes, will exhibit a high degree of distortion; tis being clarified in Fig. 4a by the 10:1 enlargement showing the image spot falling on the surf ace of the array (1).

As the distant. point source moves across the viewing angle of the detector, the characteristics of the spot of radiation would alter, see Fig.5a. Furthermore, if the distant point source ere to become a nearer extended source, blurring of the spot would occur. These distortions and changes in characteristics could be corrected for to some degree by using more complex multiple lens arrangements, but at the expense of increased size, with additional transmission losses reducing responsivity, and at. higher cost.

The introduction of an annulus lens arrangement (2#) with opaque central region (22) and clear optical aperture (21) as in Figs. 2/7, modifies the ray tracing diagrams as in Figs. 4b,5b,6.

The distance (15) may now be defined and fixed, such that the surface of the radiation sensitive photo-diodes is as shown, in a region where distortion is minimal. This greatly simplifies the specification of the following electronics/signal conditioning and the task required of the signal processing. The overall performance of the detector in establishing the location of an emission, relative to its optical axis, is greatly enhanced with much improved accuracies being attainable.

In the on-axis distant point source case (Fig.4b), the spot image falling on the surface of the array has minimal distortion; noting that the array surface is fixed behind the focal point as opposed to in front; the latter would give considerable ring-like distortions toward the outer fringes of the spot. The effect of the annular lens with the array arranged behind the focal point in this manner, is to reduce the spot central region distortions that would occur with a conventional lens.

For the case of the off-axis point source (Fig.5b), the changes in the characteristics of the spot are evident with the illustrated single annular lens, where in particular an elongated spot is formod. However, the annular arrangement still gives minimum distortion with the array positioned as previously described.

In the case of the distant point source tending toward a nearer extended source (Fig.6), the effect would be to blur (17) the image spot, with the annular lens still providing minimal distortion.

The transmission through the annular lens will be reduced, compared to a similar diameter conventional lens. However, a 5 mm. increase in the outer diameter of the annular lens would more than regain the area lost by the central opaque region (22) of the annulus which might be 2#mm. diameter, so maintaining the the clear optical aperture (21) and overall detector responsivity, for minimal trade-off of viewing angle.

The lens as shown in Fig. 7 maybe refined further in terms of front face profile from the central opaque region across the clear annular aperture (21), in order to optimise distortion characteristics, together with rear face proiling. In addition, the transition from opaque to clear regions of the lens in terms of transmission could be a gradual change, as opposed to a sharp transition, again to optimise distortion characteristics.

The lens material would be a conventional glass where the clear aperture (21) would have good transimission properties for visible and near IR wavelengths, with the central region possibly being a suitable coating opaque at these wavelengths; alternative conventional materials would cater for other wavelengths such as the mid/far IR. The surface could be anti-reflective coated and an optical bandpass filter inserted in front of the lens to give detector selectivity in terms of radiation wavelength; both of which provided for by means of conventional technology.

A second embodiment of the present invention enables overall improvements in performance, for particular applications, where fixed multiple annular lens arrangements would enable correction of (t at increased transmission losses, size, complexity and cost), the above elongation of the spot toward tho edge of the vlewing angle, but still retaining the highly reliable 'no moving par-ts' aspect A third embodiment of the invention enables overall improvements in performance for particular applications where annular lens arrangements would be manually or automatically adjustable alonL the optical axis with relation to the focal plane staring array or each other (but at increased size, complexity and cost, and with reduced reliability, although the latter could remain high through good design).

Claims

An annular single lens optics arrangement for use in compact focal plane staring array radiation detectors, with rio moving parts' , where it is mounted a short distance in front of a radiation sensitive array of photo-diodes; the.

lens having a very short focal length to maintain reasonable width viewing angles.
2 A combination of lenses as in claim 1 to form annular multiple lens optics arrangements for use in focal plane staring array radiation detectors, with 'no moving parts', mounted a short distance in front of a radiation sensitive array of photo-diodes.
3 Single or multiple annular lens optics arrangements as in claims 1 & 2 for use in focal plane staring array radiation detectors, where the optics is mounted a short distance in fron of a radiation sensitive array of photo-diodes, but where the position of the lens/es in relation to the array, or to each other in the case of multiple lenses, is manually or automatically adjustable along the optical axis.