GB2119196A - A direct view thermal imager - Google Patents

Abstract

An imager including a diode display 1-eg. a row of light emitting diodes 13A; a detector array 3; optics 7 arranged to focus an image on the surface of the detector array 3; an image scan mechanism 9 arranged to scan this image across the detector array 3; viewing optics 11A, 11B for collecting light emitted from the diode display 1, and a display scan mechanism 9 arranged to scan the diode display 1. In order to improve brightness in the field of view E, the output signal from each detector element 3A is used to drive a plurality of the row diodes 13A. The signal to these diodes 13A is fed via a delay network 15 and the diodes are spaced so that the diodes 13A, when viewed, appear superimposed. <IMAGE>

Description

SPECIFICATION Direct view thermal imagers and diode displays for use therein The present invention concerns direct view thermal imagers and diode displays for use therein.

In a direct view thermal imager, infra-red radiation is focussed onto a photosensitive detector array and the image so formed is scanned over the array by means of an image scan mechanism-for example a rotating polygon reflector, or a polygon and oscillating mirror combination. The detector signals are used to drive a light emitting diode array, so the light output of each diode varies with variations in image intensity. The array is observed through viewing optics. The diode array is scanned in a manner similar to that of the detector array, and the mechanism for this may be the image scan mechanism itself, a mechanism having a common component with the image scan mechanism (eg rotating polygon) or it may be a separate synchronised mechanism.The detector array may comprise a plurality of photosensitive elements arranged in a row, in a column, or in a matrix of rows and columns. Whatever the configuration of the detector array, it is conventional that the diode array configuration is the same it has as many diodes as there are photosensitive elements and these are arranged with the same row and column content as the detector array. Each detector element is connected to drive the corresponding diode. Where there is row content, it is also arranged by design that during line scan, as each row detector element is displaced relative to the radiation image, there is a similar displacement of row diodes in the viewed image plane such that the display images developed by each row diode are effectively superimposed in register so that they can be integrated constructively by eye.

The light intensity of each light emitting diode (LED) is limited, and despite the more recent advances in the development of more efficient and brighter LED's, it is still usually necessary to incorporate an image intensifier in the viewing optics. This adds to the cost, bulk and weight of the imager.

This invention is intended to provide a remedy. It is intended to provide improvement in the effective light output of imager displays.

According to one aspect of the present invention there is provided a diode display for use in a direct view thermal imager, the display including: a diode array comprising at least one row of light emitting diodes; at least one signal input for each row of diodes; and, at least one delay element providing connection between each input and a plurality of consecutive diodes in the row corresponding to the input, to present a detector signal sequentially to these diodes.

According to another aspect of the present invention there is provided a direct view thermal imager including: a display comprised in accordance with the invention as described above; a detector array comprising at least one photosensitive element for producing a signal, connected to the display; focussing optics for forming an image on the surface of the detecor array; an image scan mechanism, co-operative with the focussing optics, arranged to scan the image over the area of the detector array; viewing optics for collecting light emitted from the diode array of the display; and, a display scan mechanism, co-operative with the viewing optics, arranged to scan the diode display; the line scan rate of the image scan mechanism and the delays afforded by the delay elements being matched so that the viewed images of each of the row diodes are superimposed in register.

Advantage is obtained from the invention in that each detector signal is utilised to drive not only one but a plurality of diodes; the row content of the display array is not limited in exact number to the row content of the detector array and there is thus provided a display having an effective brightness increased by a factor determined by the number of diodes common to each detector signal input.

Embodiments of the invention will now be particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a simplified schematic plan of a direct view thermal imager constructed in accordance with the present invention; Figures 2, 3 and 4 illustrate alternative combinations of detector array and diode display to those incorporated in the imager of Fig. 1.

A direct view thermal imager is shown in Fig. 1, the electronic part of which comprises a diode display 1 and a detector array 3 interconnected through boost amplifiers 5.

The imager has an optical arrangement of conventional design, comprising: focussing optics 7-eg an infra-red telescope sight-arranged to focus an image of a thermal scene onto the detector array 3; and, a rotating reflector 9, part of the scan mechanism, arranged between the focussing optics 7 and the detector array 3, to scan the image in a horizontal direction across the detector array 3. The diode display 1 is arranged in position relative to the reflector 9 so that it is also scanned, and can be viewed by means of viewing optics 11 (11A, 11 B). This arrange ment provides a display image in the field of view of an observer E, using the eyepiece 11 B of the viewing optics 11. The reflector 9, shown schematically, may be a reflecting polygon, the faces of which are vertical.The scan mechanism may also include flapper mirrors (not shown) arranged relative to the reflector 9 to enable vertical (frame) scan of the scene S and of the display 1. The details of this arrangement are conventional, and, for simplification, are omitted from the drawing.

The detector array 3 comprises several infra-red photosensitive elements 3A arranged in a vertical column. The diode display 1 includes an array 1 3 of light emitting diodes 1 3A. These are arranged in horizontal rows, there being as many rows as there are elements 3A in the column of the detector array 3. Each row of diodes 1 3A is connected to a corresponding one of the photosensitive elements 3A of the detector array 3 by a delay network 15, the input I of which is connected to the output of a corresponding one of the boost amplifiers 5. The delay network 1 5 shown is in the form of a tapped chargecoupled device (CCD) delay line.The taps of this delay line are connected to the diodes 1 3A through drive amplifiers 1 9. Between each tap, the line 1 5 serves to increment the delay of the output signal of the detector element 3A, so that it is presented to each diode in sequence. The value of each delay increment is fixed in accordance with the diode spacing and display image line scan rate, to provide display image registration as discussed below.

During operation, the reflector 9 scans the infra-red image across the detector array 3 many times. Following each of these line scans, the infra-red image is displaced vertically, relative to the detector array 3, so to build up a two dimensional scan frame of the scene S. During each line-scan, line signals are developed as each photosensitive element 3A responds to the instantaneous intensity of infra-red radiation focussed upon it, the signal ampiitude varying with the variations of intensity across the image of the scene. Each row of diodes 1 3A responds to one line signal, and the intensity oY' the light emitted by these diodes follows these variations. When viewed through the viewing optics 11, each diode develops a horizontal trace, of variable intensity, in the field of view.As each diode in the row responds in turn, these traces become optically superimpsed, and with other parallel traces build up a two-dimensional display image. The traces are superimposed in register and add constructively when integrated by eye. Consider this operation when a hot spot can be observed in the thermal scene. As the scene is scanned across the detector array 3, one of the photosensitive elements 3A responds and produces a signal pulse as the image of the hot spot passes across it. This signal is relayed to the corresponding row of light emitting diodes, and each diode lights up, for an instant, in turn, in response to the pulse Each diode 1 3A thus appears as a bright spot in the image plane of the viewing optics 11. Since the signal pulse is subjected to incremented delays, each diode lights up in sequence.Since each increment is fixed in accordance with diode spacing and line scan rate, the point in the image plane at which each diode is seen to light up is the same, and the bright spots become superimposed one upon the other to appear as a single and much brighter spot.

In the combination of a detector array 3 and a display 1 shown in Fig. 2, the detector array 3 has four photosensitive elements 3A arranged uniformly in a row, and the display 1 has eight diodes 1 3A also in row formation and uniformly spaced. Consecutive diodes 1 3A are connected in pairs each to a single input A, B, C, D; the second diode of each pair being connected through a delay element 1 7. In this case, since the diodes 1 3A in the row respond to more than one input, the registration of diode images depends not only on the time delays introduced by the delay line 17, but also upon the signal generation delays arising as the row of detector elements 3 are scanned. The diode spacing, here, ultimately depends on the detector element spacing, as is the case in conventional direct view thermal imagers having row content.

In the combination of Fig. 3 this spacing to spacing limitation is obviated. The signals from each of the row detector elements 3A are combined to produce a single output signal. This is done using a delay-and-add network 21. Each signal is delayed an appropriate amount, according to image scan rate and element spacing, and is added to other signals to produce a constructive output signal. This integration has the further advantage that random noise is reduced on integration, the signal to noise ratio in the resultant signal being enhanced. This resultant output signal is then fed to a tapped delay line 23 to drive the eight diodes. Both the delay-and-add network 21 and the tapped delay line 23 may be charge coupled devices (CCD's).

An integrating detector may be used in place of the discrete element array 3 and delay-and-add network 21. In such a detector, photocarriers generated within the detector can be caused to drift at a velocity that is matched to the scan velocity of the image across the focal plane so that the carriers are generated and accumulated in synchrony with the image, to produce an integrated signal at one or more output taps along the length of the detector. An example of such a detector is described in UK Patent No. 1,488,258. Thus, in Fig. 4 the detector array 3 comprises two integrating detectors 3B each having a single tap output 0 at one end. These are arranged in parallel. The corresponding display array 1 3 has a multiplicity of diodes 1 3A arranged in two rows. Each row is arranged to respond to one of the two detector output signals, and for this it is connected via one of two delay lines 23. To give further improvement in signal above noise, the display array 1 3 may be provided with integrating detectors arranged in rows, or alternatively extended integrating detectors having multiple taps may be used. In such cases the row diodes are subdivided into groups of consecutive diodes, each group being connected to one of the detector outputs. Here, the spacing between the groups is dictated by the detector spacing, or, the detector tap spacing, respectively.

Claims (6)

1. A diode display for use in a direct view thermal imager, the display including: a diode array comprising at least one row of light emitting diodes; at least one signal input for each row of diodes; and, at least one delay element providing connection between each input and a plurality of consecutive diodes in the row corresponding to the input, to present a detector signal sequentially to these diodes.

2. A direct view thermal imager includ ing:- a diode display having a diode array comprising at least one row of light emitting diodes, at least one signal input for each row of diodes, at least one delay element providing connection between each input and a plurality of consecutive diodes in the row corresponding to the input, to present a detector signal sequentially to these diodes; a detector array comprising at least one photosensitive element for producing a signal, connected to the display; focussing optics for forming an image on the surface of the detector array; an image scan mechanism, co-operative with the focussing optics, arranged to scan the image over the area of the detector array; viewing optics for collecting light emitted from the diode array of the display; and a display scan mechanism, co-operative with the viewing optics, arranged to scan the diode display; the line scan rate of the image scan mechanism and the delays afforded by the delay elements being matched so that the viewed images of each of the row diodes are superimposed in register.

3. An imager as claimed in claim 2 wherein the detector array comprises at least one row of photosensitive elements; and the diode array comprises a corresponding number of rows of diodes, the row diodes being arranged in adjacent groups, a plurality to each group, the number of groups being equal in number to the number of row elements, the diodes in each group being connected to a corresponding delay element.

4. An imager as claimed in claim 2 wherein the detector array comprises at least one row of photosensitive elements; and, corresponding to each row, a delay and add network connected to the row elements, to produce an integrated signal.

5. An imager as claimed in claim 2 wherein the detector array includes at least one integrating detector, and associated therewith, a source of bias arranged to produce a drift of photocarriers in the integrating detector at a velocity that is matched to the velocity of image scan.

6. A direct view thermal imager constructed, arranged, and adapted to operate substantially as described hereinbefore with reference to and as shown in Fig. 1, or in Fig.

1 and Figs. 2, 3 or 4 of the acompanying drawings.