GB2480223A

GB2480223A - Optical sensing system

Info

Publication number: GB2480223A
Application number: GB1001029A
Authority: GB
Inventors: Graham Whale Smith
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 2010-01-22
Filing date: 2010-01-22
Publication date: 2011-11-16
Also published as: WO2011089398A2; GB201100996D0; WO2011089398A3; GB201001029D0; GB2480344A

Abstract

The present invention relates to improving testing and thus reliability of optical sensor systems. The invention describes methods of manufacturing such apparatus to provide alternative or additional surfaces which allow for the presentation of incident light which may differ in frequency to that intended for the main optical sensing system. The optical sensing system may comprise: at least one first photosensitive surface 4, i.e. a photocathode, configure to react to a first frequency range of incoming light, e.g. x-rays; and at least one second surface 3, 5, or 6, configured to either permit electromagnetic energy, e.g. visible light, to pass through all or part of the system, or to react to said electromagnetic radiation having a second frequency range.

Description

Improvement to optical sensor system Photoelectrically sensitive devices-photocathodes-are well known in the art as described in, for example, US Patent No. 3,254,253. Photocathodes are common photosensitive portions of numerous optical sensing systems, including but not limited to optical oscilloscopes, streak camera tubes, image intensifiers, arrays, x-ray detectors and night-vision apparatus. They are applied where incident light is received perhaps within, but typically beyond the visible spectrum, which to be employed more usefully must be converted into visible light by known means.

A photocathode emits photoelectrons via an electron emitting layer generated in response to an incident light! photons reached by a light! photon absorbing layer, allowing related apparatus, described below, to convert incoming photons into light which is within or reasonably close to the visible range. The emission provided by the photocathode, perhaps following further signal processing, can be detected by related apparatus which may be any recognised medium such as capture to film, or in most modern establishments is typically common optical sensor systems which can then generate electronic representations (typically digital 2 or 3D images) of the spatial distribution of incoming quanta. The imaging apparatus of such systems typically comprises known capture cameras, which more typically comprise electronic array sensors such as Charge-Coupled Devices, Charge Injection Devices, Metal Oxide and Complementary Metal Oxide and amphorous silicon devices, all of which will through this document be referenced as CCD for the reader's convenience, in conjunction with suitable apparatus to transmit information, such as wired or wireless connections, to suitable processing and! or receiving means such as desktop computers or dedicated terminals designed to manage graphics or images.

It is common for a photocathode to be formed on a supporting substrate, which may be optical glass, sapphire, fused silica, quartz or other known materials which have sufficient transparency, a low degree of reflectivity and the ability to effectively bond with any substance which forms a photocathode. If a supporting substrate is present it typically functions as a reinforcing member to support the photoelectron emitting layer, and incident light directly reaches the photoelectron emitting layer while avoiding the supporting substrate or with such a negligible effect from its introduction thanks to the generally high level of transparency of most common materials.

In some applications a photocathode may be only 100 angstroms thick or less, and in some applications is made from a material such as gold and may have no additional supporting substrate-the photocathode itself can act as its own support. Alternatively or additionally, apparatus which form part of the overall optical system, such as known imaging means, for example generation 1-4 tubes (or whatever version may supersede them if applicable), proximity focus types, etc, can act as a form of supporting substrate.

Such optical systems are typically manufactured to exacting tolerances in highly-sterile environments to minimise the risk of damage or disruption to fine, sensitive surfaces by dust or dirt etc. Ordinary testing of such apparatus involves directing an appropriate type of incident light to ensure that various aspects of the construction and elements contained within a sensor system are all functioning.

In some cases, such as applications of photosensitive areas of perhaps streak tubes or night vision apparatus, standard operations can take place with little resource expended to verify a reasonable quality of overall functionality post construction.

However, in the absence of the ability to make standard, simple, consistent diagnostic and testing methods, even if any such system costs relatively little to construct, the time and cost to diagnose faults in either transmitting or receiving devices, to repair or replace any component post integration to a system (which may be sealed) may prove disproportionately large compared to the cost of finding fault or error in the overall system after it has been used.

In some cases, it is not possible to make simple tests of the equipment as a whole, as there may be only a small number of sources of appropriate incident light, such as in high energy x-ray sources where potentially hazardous materials are to be analysed, cobalt for example. In many cases the cost of resources to prepare such an energy source is far greater than the cost of constructing and purchasing the relevant optical sensing system.

Moreover, when a system comprises a first part transmitting device and a second part receiving means, accurately diagnosing a fault with one of the two parts may require complete replacement of the other, which if equipment is bulky or integrated to other systems (even the fabric of buildings), could prove to be extremely resource intensive. It is possible that significant effort could be made to remove and replace one part only to find it functions acceptably, and the other part is at fault.

As a consequence, optical sensing systems may be dispatched or even employed with the assumption they are functioning correctly, with no method of verifying this is the case until resources have been deployed to use the system. It may cost 150% or more of the purchase price of an optical sensing system to prepare and use a material such as cobalt for a single day. As a consequence, if the optical sensing system is not functioning effectively or there is any doubt that is the case, the potential loss to the user is significant.

There is much prior art relating to the manufacture of effective particular photocathodes, such as Russian patent number 2346352. However they have as their object or effect the aim of reducing the cost or increasing the consistency and effectiveness of manufacture, there is no deliberate attempt to avoid fully covering a photosensitive area or indeed providing more than one. In general, it would be expected that any advances in creating photocathodes, such as in KR 20030022975, leaving a portion of a substrate without sufficient coating would be at best purely incidental to a malfunctioning manufacturing process, and at worst, clearly undesirable; it could lead to an inefficiency in the receipt of incident light, and the boundary between a coating and any deliberate partial or full void could also lead to reduced effectiveness of gathering and transmitting light.

It is an object of the present invention to overcome these problems by providing an improved optical sensing system.

The present invention allows for a plurality of additional surfaces in addition to the photosensitive portion of existing or new optical sensing systems, which allow for the presentation of incident light from sources alternative to those the photosensitive portion has been formed to analyse. Thus, rather than having to prepare and present specific or rare materials or events to a typically sealed system of unknown function (which a user would not ordinarily service themselves), common or alternative incident light sources which would transmit light at alternative frequencies to the main, expected subject which could be prepared at low cost, would additionally or alternatively be presented to an optical sensing system to allow for suitable testing prior to dispatch or employment. Thus a user will have enhanced confidence prior to incurring potentially significant expense, or routine maintenance, diagnostics or testing over the lifetime of equipment, that it functions as required.

The present invention provides an optical sensing system which contains, or alternatively a photocathode itself, manufactured from one suitable material or substrate, with at least two separate coatings, the first for the transmission of incident light suited to the subject to be analysed, the other for transmitting light suitable for testing the functioning of the system.

Or alternatively, the invention provides a photocathode which is made from at least two separate materials, the first to transmit incident light suited to the subject to be analysed, with further separate materials to transmit light suitable for testing the functioning of the system. Also separately the present invention describes methods of manufacturing these types of photocathode.

There are a number of possible methods of manufacturing the invention: In a first embodiment two or more separate materials are used to form a photocathode.

At least one of these surfaces is a substrate which forms a photocathode itself or alternatively is a substrate coated in a conventional manner known in the art, both of which result in surfaces suited to transmitting light to analyse a given subject. The other separate material or materials form separate surfaces suited to transmitting light which allows for the internal components of the system to be inspected.

Manufacturing such an embodiment requires the inclusion of one or more additional photosensitive surface to the overall optical sensing system.

In a second embodiment when forming a photocathode on to a substrate, one or more portions of the photocathode are fully or partially masked, such that said portions are rendered without any substantial additional coating.

In a third embodiment, a substrate is coated with two or more different materials, one forming a photocathode suited to transmitting light to analyse a given subject, the other coating or coatings suited to transmitting light which allows for the internal components of the system to be inspected (such coatings may or may not overlap the first). An example may be one substrate on which is formed a photocathode (perhaps formed from say, titanium) to convert! transmit photons of a particular range of frequencies, and additionally on the same substrate is formed a conventional multi-alkali surface, which would allow for alternative wavelengths to be transmitted.

Manufacturing the second and third embodiments involves an additional step to the known process of forming a photocathode; there are three possible options. Firstly ensuring that a plurality of portions of the substrate are kept free of the coating to be applied (termed "masked") which forms the photocathode, and thus leaving one or more areas "raw" and untreated (per the second embodiment). As an alternative additional step to this the masked portion or portions would then be treated with a different coating to achieve the third embodiment. Naturally there is a hybrid option wherein there are some coatings which when applied in different thicknesses allow different light wavelengths to be transmitted. Thus it is possible for one photocathode to have the same light-sensitive material added in two or more different thicknesses. Furthermore all of these options could be used in addition to the first embodiment.

The masking or separation of these two or more materials could be to any area of the substrate considered appropriate, which need only be of sufficient size to allow light through which would be capable of allowing contact with internal components-thus it could be very small indeed. Such a surface could form one, specified area-perhaps for ease of manufacture, being a reasonably concentric ring around the periphery of the substrate.

To illustrate the invention the following figures are provided: Figure 1 shows a front-end view of a first embodiment of the invention.

Figure 2 shows a front-end view of a second embodiment of the invention.

Figure 3 shows a front-end view of a third embodiment of the invention.

Figure 4 shows a front-end view of a fourth alternative embodiment of the invention.

Figure 5 shows a side-on view of Figure 4.

Figure 1 shows an embodiment of the invention in which is shown the total area 1 upon which incident light would be expected to fall, contained within an appropriate optical sensing system 2, which includes an appropriate substrate 3, shown with an area 4, where a substance has been applied to the substrate to form a surface photosensitive to particular light frequencies on a central portion of the substrate only. This would leave the remainder of the substrate 5, to either be the "raw" substrate itself with no photosensitive coating, or to have formed upon it an alternative photosensitive coating which would react to a different range of light frequencies to the photosensitive area 4.

Figure 2 shows an embodiment of the invention wherein a photocathode is employed to capture information derived from the periphery of an incident light source as opposed to the light being required to fall roughly on the centre of it (as would be the case in Figure 1), in which are shown the total area 1 upon which incident light would be expected to fall contained within an appropriate optical sensing system 2 which includes an appropriate substrate 3 shown with an area 4 where a substance has been applied to the substrate to form a surface photosensitive to particular light frequencies on the outer periphery of the substrate only. This would leave the remainder of the substrate 5 to either be the "raw" substrate itself with no photosensitive coating, or to have formed an alternative photosensitive coating which may react to a different range of light frequencies.

Figure 3 shows an embodiment of the invention wherein an appropriate substrate 3, is shown with an area 4, where a substance has been applied to the substrate to form a surface photosensitive to particular light frequencies on an irregular portion of the substrate. This would leave the remaining areas of the substrate 5, to either be the "raw" substrate itself with no photosensitive coating, or to have formed upon it an alternative photosensitive coating which would react to a different range of light frequencies to the photosensitive area 4. There is no limitation to the possible shape for area 5.

Figure 4 shows an embodiment of the invention wherein photosensitive areas of optical sensing systems are constructed from a plurality of materials. For example if a photosensitive area was not formed on any given substrate, it may prove challenging to simply leave an area without any relevant coating and for it to act as a surface which can transmit light for use suitable for testing of internal components, as well as for any given subject. In figure 4 an appropriate substrate 3, is shown with an area 4, where a substance has been applied to the substrate to form a surface photosensitive to particular light frequencies. A plurality of additional surfaces 6 have no photosensitive coating, or have formed upon them alternative photosensitive coatings which would react to a different range of light frequencies to the photosensitive area 4. In the example above where a photocathode is created from only 100 angstroms of any particular material, the additional surfaces 6 in this embodiment of the invention are formed from any appropriate thickness of optical glass, quartz etc, which may or may not find support from internal componentry as the context may require.

As figure 4 shows it is possible to manufacture an optical sensing system with a plurality of separate photosensitive areas 6 which are not necessarily immediately adjacent to the main photosensitive area 4. Such an arrangement functions provided incident light can still reach relevant internal components, howsoever achieved. Thus if the appropriate optical system 2 were reasonably cubic in nature (although they may be tubular or irregular) then said surfaces 6 could be located on the same or any other surface or surfaces of the system which would still allow for effective testing to take place using light sources which may differ from the main photosensitive area has been created to analyse.

Figure 5 shows a side view of figure 4, depicting not just the elements present in figure 4 but also a general representation of the internal components of an intensifier-wherein an appropriate light source would ordinarily be sighted around 7 which as above is the total area upon incident light would be expected to fall, and in this case also includes an appropriate substrate for that purpose. The area where a photosensitive area has been formed is 4. If area 3 has no coating thus the "raw" substrate is exposed (or indeed a separate coating is applied), at least two different frequency ranges of light, which may or may not overlap, could be transmitted by these two areas of what would ordinarily form a single photosensitive area. Exemplar surfaces are also shown as on the upper surface 10 or the same on the lower surface 11 of the optical sensing system 2. It can be seen that if incident light were provided to surfaces 10 or 11 directed through the to the rear of the overall system, in either case it would only pass through a portion of the included microchannel plate 8 common in intensifiers, shown by the dotted lines which would thus not test all of the system. As a consequence where this would still function and thus increase confidence, it is undesirable as it would allow only a subset of the overall possible tests.

On the same diagram it could be reasoned that an appropriate substrate could be provided as a complete void, thus allowing incident light shown as the dashed line to be passed through the microchannel plate 8 and on to the capture camera 9. In some systems, this would function perfectly adequately, as the microchannel plate could be directly adjacent to the photocathode, thus where the microchannel plate would be exposed, the overall system would remain "sealed". However, if the void left a physical gap between components (a space between a photocathode and microchannel plate has an impact on the spatial resolution of an intensifier thus certain contexts require this to be present), it would allow for dust and moisture ingress, which would clearly be undesirable.

It is important to note that an appropriate light source 7 could in fact be a plurality of light sources, including perhaps, concurrently both infra-red and ultra-violet light from two separate sources. This method of testing is applicable to all embodiments.

Among all relevant embodiments item 8 need not be microchannel plate: Generation 1 (or other) tubes or other suitable imaging means, such as proximity focused image intensifiers (including diode types), would be substitutable. Microchannel plate is used as an example only and it not to be construed as a limitation to the invention.

If optical sensing systems were provided with the invention included, an operator would be aware that the target area for specific incident light was not necessarily conventional, thus appropriate steps could be taken to ensure when any relevant source was presented the necessary care was taken to ensure desired results were achieved.

There are known means of segmenting photocathodes or perhaps by independently gating the portions of a photo-sensitive surface, such as in an image intensifier comprising a divided photocathode in which each divided portion may be gated independently. This approach could be used in conjunction with the invention.

Furthermore, to give a consistent method of checking the functioning of internal components, a CCD could be divided in to specific portions, down to a pixel-level. The address information of pixels is present in the transmission sent to an appropriate receiving means to allow the resulting picture to be built back up. However it is not known to be used for testing purposes. As such the information being provided by the COD could be verified on a pixel-by-pixel basis, such that functionality tests could be verified by comparing the responses received by individual pixels on a CCD.

It is of course possible that ordinary incident could be provided to an extant system which has a photosensitive area. However, if the photosensitive area is formed in such a way as to transmit only specific wavelengths, the results (if any) would not be sensible to benchmark, as such benchmarking would necessarily have to be against similar componentry. The present invention provides for a standard set of well-known tests to be employed, allowing consistency of approach.

Testing any newly-formed equipment prior to dispatch or as part of routine inspection! maintenance of such apparatus would comprise;

-Flat field (shading) tests;

-Gate walkthrough tests including alignment; -Calibration and measurement of apparatus gain; -Spatial resolution (for example Modulation Transfer functions, Spectral and absolute - -Sensitivity (Quantum Efficiency)); -Optical blemish criteria which may include fibre optic output testing; -Overall system functionality to include verification of any integral or separate capture -camera's functioning (eg data capture, functioning of transmission link(s), synchronisation of CCD is correct with respect to image capture, being single, multiple events or continuous readouts; -Function of any further data link including to associated data recording apparatus -Intensifier gain; etc. All of these tests when using the invention could be verified using low cost, common light sources, in addition to any rare and! or expensive light sources a photosensitive area has been created to cater for.

Claims

C LA'l MS 1. An optical sensing system comprising at least one first surface and at least one second surface and apparatus for converting electromagnetic energy in to a readable form in which at least one of the first surfaces is photosensitive and at least one of the second surfaces is configured to permit electromagnetic energy to pass through all or part of the system to enable verification of the system's function.
2. An optical sensing system according to claim 1 wherein one or more of the first surfaces is constructed from a material configured to react to a first frequency range of incident light and one or more of the second surfaces is constructed from a material configured to react to a second frequency range of incident light.
3. An optical sensing system according to claim 1 or claim 2 wherein at one or more of the first and one or more of the second surfaces are formed on the same substrate.
4. An optical sensing system according to claim 1 or claim 2 wherein a void in the wall of the optical sensing system performs the function of one or more of the second surfaces.
5. An optical sensing system according to any previous claim in which at one or more of the surfaces is formed on optical glass.
6. An optical sensing system according to any previous claim in which one or more of the surfaces is configured to react to light in the x-ray range.***"7. An optical sensing system according to any previous claim in which at one or more of the first or the second surfaces is configured to react to light in the visible range.* S. **S * S *... * SS S* *SSS SSS S **S S..