GB2111236A - Lens system having bullseye filter - Google Patents

Abstract

It is common practice for a pilot's Head Up Display (HUD) to be recorded, nowadays with a CCD video camera. Unfortunately, the spectral response of available CCD cameras- high at the red end of the spectrum- coupled with the fact that conventional HUDs employ a cathode ray tube having a high intensity "green" phosphor coating, causes problems. Placing in front of the camera a full field spectral filter of the green pass/red cut variety is an acceptable solution when ambient light levels are high, but not when they are low, for it causes the camera's low light sensitivity to be very much reduced. The present invention seeks to overcome the problem for lens (11, 12, 13) systems of the type having an iris diaphragm (14) by using not a filter covering the entire available aperture of the system but rather a "bullseye" or "spot" filter (15) of relatively small diameter so that when placed athwart the optic axis it is effective for only a small, centrally- located, portion of the system's full aperture. <IMAGE>

Description

SPECIFICATION Filters This invention relates to filters, and it concerns in particular spectral filters used as an item in a lens/diaphragm combination.

Modern aircraft often employ what is known as a Head Up Display system, or HUD, to enable the pilot to see vital instrument readings without looking away from the real world in front of his aircraft as seen through the front of his cockpit canopy. In its most common form, a HUD system comprises a light transparent (glass) plate mounted in the piiot's forward line of vision, and inclined towards him, coupled with means for generating an optical display of the instrument readings and projecting the display onto the plate so that it is reflected towards the pilot both along his forward line of sight and as through it originated "at infinity".To the pilot, then, the display seems to have its origin at the same place as whatever he can see ahead of him, so that he can perceive, properly in focus, whatever information the display provides without moving his head or eyes ... thus, "head up".

It is common practice for the pilot to share his view of the HUD (and of the outside world seen in conjunction therewith) with a camera system which can be used to record, for later examination, items of interest. In the past the cameras have usually been cine cameras, recording the view on light-sensitive photographic film, but because of the delay in seeing the film arising from the need to process it present-day cameras tend to be video (television) cameras, recording what they see onto magnetic tape (video tape) that can immediately be played back, allowing the full record to be seen, as soon as the pilot returns.Moreover, because of necessity the camera needs to look into the HUD more or less down the same line of sight the pilot uses (otherwise it would record a different scene), and thus inevitably block part of the pilot's field of view, and because video cameras of the vacuum tube variety (such as vidicon cameras) are rather bulky, much use has recently been made of cameras of the sort wherein there is no bulky vacuum tube structure but instead there is used a miniaturised fully solid state charge-coupled device (CCD) array (such cameras are now fairly well known, and need no detailed description here). Unfortunately, the spectral response of available COD cameras is such as to cause problems.

One conventional way of generating the display used in a HUD system employs a cathode ray tube (CRT) having a high intensity "green" phosphor coating which emits light radiation primarily in the region of 550 nanometers; the pilot can easily perceive the display against any likely "background" (blue sky, white clouds, even green/brown land) as seen ahead of him, regardless of the relative intensities of the two, both because the human eye is intrinsically a marvellous mechanism and because, being colour sensitive, it can readily detect the difference on that basis alone.However, this is not necessarily so for "artificial eyes" like television cameras, and is especially not so for the COD cameras now being used: in the simple forms presently employed these cameras are monochrome -- that is, though they are sensitive to radiation of different wavelengths (colours) nevertheless their output is based upon the intensity (brightness) of the input radiation, not its wavelength.Now, while the available COD cameras are sensitive to light over a wide range of wavelengths, from 450 nanometers (blue) to beyond 1060 nanometers (up to the end of the near infra-red, generally considered to fill the range 650 to 1100 nanometers), nevertheless they are especially sensitive to radiation in the longer wavelength region -thus, to light that is generally red in colour - and considerably less sensitive to radiation around the 550 nonometers region thus, to light that is green in colour.In the context of recording the HUD and the pilot's view of the outside world, this mismatch between the colour of the HUD (green) and the colour to which the OOD camera is most sensitive (red) means that under certain circumstances the camera is "blinded" by the light coming from the outside world, and cannot distinguish the light coming from the HUD - and as a result the recorded image lacks any visible trace of the HUD, and so may be of very little value. One such circumstance is when the forward view is white clouds or blue sky (which is, perhaps surprisingly, still rich in red and infra-red light).

It might be though that the problem could be dealt with simply by placing in front of the camera (usually as part of its lens system) a full field spectral filter of the green pass/red cut variety.

This is acceptable when ambient light levels are high, but not when they are low, for as it gets dark so in general the light from outside becomes relatively richer in red/infra-red. The enhanced sensitivity of OCD cameras to such light normally means that they provide a good output even in poor light conditions -- but if a red cut filter has been inserted into the light path in front of them then naturally their low light sensitivity is very much reduced. One solution to this problem is to allow the filter to be removed from the light path when the outside light levels drop (when it grows dark), and to be inserted when the outside light levels rise, and this can obviously be done either manually (by the pilot) or entirely automatically.

Unfortunately, however it is done the use of a mechanically insertable/removable filter necessarily makes the camera system more bulky and obtrusive and gives rise to a weak point, open to failure, in an otherwise rugged system. The present invention seeks to overcome the problem in quite a different way, by using not a full field spectral filter (covering the entire available aperture of the camera lens system) but a "bullseye" or "spot" filter (a filter generally circular, of relatively small diameter so that when placed athwart the optic axis it is effective for only a small, centrally-located, portion of the full aperture of the lens system).

In one aspect, therefore, the invention provides a lens and spectral filter combination which comprises an appropriate lens system of the variety employing an iris diaphragm to regulate the effective aperture, the lens system having, immediately adjacent its diaphragm, a bullseye spectral filter.

The combination of the invention operates as follows. Under bright light conditions the lens system is stopped down using the iris diaphragm to reduce the amount of light passing through the system, and so the bullseye spectral filter covers a relatively large proportion, possibly all, of the lens's effective aperture. As the light level drops, however, the diaphragm is opened up to let in more and more light, and as this happens so the proportion of the effective aperture covered by the filter becomes less and less -- until, with the lens at maximum aperture, the proportion covered is insignificant. Thus, when there is most light from outside the filter is at its most effective, while when there is least light - and the camera system's sensitivity is to be used to the full then the filter is "automatically" at its least effective.

Though both hereinbefore and hereinafter the invention is described primarily in connection with COD video cameras employable in a HUD-using aircraft to record both the HUD and the view seen therethrough of the outside world, nevertheless the concept of the invention -- the ability of a spectral bullseye filter effectiv6'.y to modify the use system's spectral sensitiv';y at high light levels (;.nall apertures) but not, or not significantly, at low light levels (large apertures) - is applicable in many other fields and therefore the invention is not initially limited to HUD cameras.

The lens system used in the combination of the invention may be any appropriate lens system of the sort having an iris diaphragm to regulate the amount of light passing therethrough. Indeed, arguably the system could be a "pinhole" lens, without any "lens" components operating by refracting light. Generally, however, the lens will contain one or more light-refractive elements, and these may be arranged in any convenient way (such as singlets, and doublets) and for any required purpose (such as focusing and correcting aberations). A typical lens system used in a COD video camera is described in more detail hereinafter with reference to the accompanying drawings.

The inventive combination contains a bullseye (or spot) spectral filter. A spectral filter is a filter that passes light of one wavelength (or wavelength range) while blocking, or "cutting", light of the complementary wavelength (or wavelength range). As explained hereinbefore, HUD systems generally employ green displays, while COD video cameras are generally especially sensitive to red and infra-red, so that for use with such a system a preferred spectral filter is a green yellow pass/red cut filter, meaning that it passes the green light from the HUD while blocking the red/infra-red components of the light from outside. Of course, "pass" and "cut" are relative terms, and spectral filters can be designed to pass or cut almost as much or as little of the specified light as required.For HUD system use a spectral filter that passes 80% or better of the light to be passed and blocks from 0.1 to 0.01% of the light to be cut is both satisfactory and possible.

The spectral filter of the inventive combination is a bullseye, or spot filter, and as such it is generally small in effective diameter relative to the maximum aperture of the lens system and is positioned on the optic axis of the lens system. As explained hereinbefore, when the lens is stepped down the filter covers all or most of the effective aperture, while when the lens is fully open the filter covers an insignificant proportion of the aperture. Clearly, the size of the filter relative to the minimum and maximum lens apertures is important for determining exactly what effect it will have, and equally clearly the desired effect will depend to some extent both upon the pass/cut characteristics of the filter and the general light sensitivity of whatever device the inventive combination is to be used with.Nevertheless, by way of a specific example, when used with a HUD system employing for recordal purposes a COD video camera of the Fairchild CTVS type having a lens varying in effective aperture from f3.8 to f1 6, it is preferred that the filter be of such a size that it covers all of the aperture for about one stop at the stopped down end of the range, thus from f1 1 to f1 6, thereafter covering less and less as the lens opens out further. However, this situation can be modified depending upon whether the filter is a uniform density one or not.

While in general filters are of a uniform density throughout their area - that is, the proportion of the light a filter passes/cuts is the same for any part of the filter -- nevertheless in some cases filters may be graded so that this is not the case.

For example, a filter may be graded so that it is densest at its centre and less dense at its periphery, the variation in density with distance from the centre being, say, linear, exponential or logarithmic. Although the use of a filter graded in this general fashion could be advantageous, for there would then be no likelihood of any abrupt change in the apparent spectral sensitivity of whatever apparatus the lens/filter combination is being used with, graded filters are in fact rather difficult to make. An alternative type of filter is one which is of uniform density but has a peripheral shape such that overall the filter appears to be graded. An example of such a filter is a star filter - one that has a "solid" central area but a multi-lobed outline like that spoked shape conventionally drawn by children to represent a star. Such a star-shaped filter having, say, eight or more lobes behaves very like a graded filter, and yet is relatively easy to construct (and within reason the lobes themselves may be shaped to give any desired distance/density relationship).

The bullseye spectral filter used in the inventive combination may be made in any suitable way (one such way involves the controlled evaporation of filter material onto a suitable substrate) and of any suitable materials. Where the filter is a layer of filter material upon a substrate, the latter may be either a separate transparent component --- for example, a piece of glass or plastics sheet - or one of the optical components already present in the lens system. Provided there is an optical component having a face immediately adjacent the lens's diaphragm, then it is preferred that the filter be formed directly on that face. An example of such an instance is further described hereinafter with reference to the accompanying drawings.

The bullseye spectral filter is in use positioned immediately adjacent the lens system's diaphragm so that in operation the filter can be of a reasonably small size and yet still truly extend across the light path through the lens system when the diaphragm is stopped down to or near its minimum aperture. Some latitude is allowed in the distance from the diaphragm, but too great a distance may cause the filter to have an undesirable penumbral effect.

The invention extends, of course, to an optical system, particularly a HUD/outside view recordal system using a COD video camera, whenever employing a lens system/bullseye spectral filter combination as described and claimed herein.

Various embodiments of the invention are now described, though only by way of illustration, with reference to the accompanying drawings in which the Figure shows diagrammatically in axial section the component parts o-F . n inventive ens system/filter combination suitable for used with a COD video camera.

The Figure shows the general arrangement of lenses and diaphragm used with a Fairchild CTVS camera, but in "block" form only (for the actual optical and mechanical design of the lens system per se is not germane to the invention).

The combination shown in the Figure comprises a cylindrical lens barrel (10; about 1.2 in. long and of about 3/4 in. internal diameter) within which are mounted 4 lenses in 3 groups (1 1, 12, 13; the rear group 13 is a doublet), an iris diaphragm (14; the broken line represents its range between fully open and fully "closed") located between the lens groups 12 and 13. The rear lens group 13 is mounted immediately adjacent the diaphragm 14 (about 0.04 in.

therefrom), and on the front face of the front element of the group is centrally mounted a spectral spot filter (15; about 0.2 in. diameter).

It will easily be appreciated that with the lens fully stopped down (the diaphragm 14 fully "closed") the filter is effective for all light passing therethrough, but that with the lens at maximum aperture (diaphragm 14 fully open - an effective diameter of about 0.6 in.) the filter is operative for only a small proportion of the light (about 10%).

Claims (9)

1. A lens and spectral filter combination which comprises an appropriate lens system of the variety employing an iris diaphragm to regulate the effective aperture, the lens system having, immediately adjacent its diaphragm, a bullseye spectral filter.

2. A combination as claimed in Claim 1, wherein the spectral filter is a green yellow pass/red cut filter.

3. A combination as claimed in either of the preceding claims, wherein the spectral filter passes 80% or better of the light to be passed and blocks from 0.1 to 0.01% of the light to be cut.

4. A combination as claimed in any of the preceding claims, wherein the spectral filter is of such a size that it covers all of the lens aperture for about one stop at the stopped down end of the range.

5. A combination as claimed in any of the preceding claims, wherein the spectial filter is of a t triform density thrcJgho -its art a

6. A combination as claimed in Claim 5, wherein the spectral filter is o ie which is of uniform density but has a peripheral shape such that overall the filter appears to be graded.

7. A combination as claimed in any of the preceding claims, wherein, where there is an optical component having a face immediately adjacent the lens's diaphragm, then the spectral filter is formed directly on that face.

8. A combination as claimed in any of the preceding claims and substantially as described hereinbefore.

9. An optical system, particularly a COD video camera, whenever employing a lens system/bullseye spectral filter combination as claimed in any of the preceding claims.