Display source

A helmet mounted display 30 includes a display source 31 arranged to be directly imaged by a primary relay optical arrangement 32 having relay optical elements 33A, 33B and 33C. Light exiting the primary relay optical arrangement 32 indicated by ray traces 34A, 34B and 34C continue towards a visor 35 which is arranged to reflect incident light to a exit pupil located in a convenient position for a viewer 36. The display source 31 includes a light source, beam splitter, reflective liquid crystal display and a display source relay optical arrangement to provide an output image at an output diffuser screen. The image at the output screen is then directly imaged by the primary relay optical arrangement 32. Should the reflective liquid crystal display require modification or replacement with a newer model, then the display source 31 can be redesigned to accommodate the new reflective liquid crystal display rather than re-engineering the primary relay optical arrangement 32 at greater expense.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a display source which is particularly, but not exclusively, suitable for use in a helmet mounted display or head mounted display.

(2) Description of Related Art

Traditional, as illustrated inFIG. 1, a prior art helmet mounted display1can be constructed to directly image a transmissive digital display device2using only primary relay optical elements3a,3band3cto transmit light generated by the digital display device2onto a visor4, which is arranged to reflect light back to an exit pupil at the point of a viewer5. It will be understood that the viewer5observes a forward scene through the visor4and the image produced by the digital display device2is superimposed upon the forward scene. The problem with this approach is that transmissive digital display devices2have an inherent drop in contrast related to the angle of incidence between the transmissive digital display device2and the primary relay optical elements3ato3c. Accordingly, light on axis to the transmissive digital display device2, indicated by ray trace6, provides good contrast at the point of the viewer5. However, light off axis, indicated by ray traces7and8, have a larger angle of incidence than light on axis ray trace6and therefore provide poorer contrast at the point of the viewer5. Helmet mounted display primary relay optical elements3ato3c, by nature, provide a large numerical aperture and hence require a large angle of incidence to be present at the transmissive digital display device2. This in turn leads to large variations in contrast for the head mounted display at the exit pupil at the point of the viewer5.

FIG. 2, illustrates a way of improving the contrast for a helmet mounted display10according to the prior art using a reflective digital display device11, a beam splitter12and an illumination source13rather than the transmissive digital display device2ofFIG. 1. In operation, light from the illumination source13passes through illumination optic elements14and15prior to entering the beam splitter12for reflection within the beam splitter12to the reflective digital display device11. Light incident on the reflective digital display device11can either be reflected so as to pass back through the beam splitter12to be directly imaged by primary relay optical elements16A,16B and16C or to be prohibited from passing through the beam splitter12. Light allowed to pass back through beam splitter12and hence the primary relay optical elements16A to16C, as indicated by on axis ray trace17and off axis ray traces18and19, is incident on a visor which is arranged to reflect the light to an exit pupil for viewing by a viewer21.

However, to use a reflective digital display device11in such a manner so to be directly imaged by the primary relay optical elements16A to16C requires a large beam splitter12and associated illumination optics14and15with a separate illumination source13to provide the required sized exit pupil to a viewer21. This would require modification of the complex primary relay optical elements16A to16C to accommodate a large back focal length to allow room for the beam splitter12, illumination source13and illumination optic elements14and15.

Furthermore, directly imaging a digital display device2or11as described and illustrated either inFIG. 1or2, means that the primary relay optical elements3ato3cofFIG. 1or16ato16cofFIG. 2can only be designed for a specific digital display device2or11. The primary relay optical elements3ato3cor16ato16chave a fixed focal length and any change in digital display device2or11, for example size, will severely impact the optical performance of the helmet mounted display1or10. That is, if the size of the digital display device2or11is decreased then the field of view presented at the exit pupil for a viewer5or21will be lost. Conversely, if the size of the digital display device2or11is increased, then display pixels around the periphery of the digital display device2or11will be wasted resulting in a lower resolution as viewed by a viewer5or21at the exit pupil. As is well known, the technology in digital display devices2or11is rapidly advancing and hence such digital display devices2or11quickly become redundant and superseded, thus it would be necessary to redesign the primary relay optical elements3ato3cor16ato16cof the helmet mounted display1or10each time a digital display device2or11became redundant.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, a display source includes a light source generator arranged to generate light including linearly polarised light of a first predetermined type, a beam splitter being arranged to substantially transmit incident linearly polarised light of the first predetermined type through the beam splitter and to substantially reflect incident linearly polarised light of a second predetermined type from the beam splitter, an image generator including a plurality of display pixels, each pixel including a predetermined state being arranged to reflect and convert incident linearly polarised light of the first predetermined type into linearly polarised light of the second predetermined type, a display source relay optical arrangement, an output screen arrangement and the display source relay optical arrangement being arranged to transmit linearly polarised light of the second predetermined type reflected from the image generator via the beam splitter to the output screen arrangement.

Preferably, the light source generator may include a light source and an illuminator lens.

Advantageously, the image generator may include a reflective liquid crystal display. A field lens may be arranged between the reflective liquid crystal display and the beam splitter.

Preferably, the predetermined state of the display pixels of the image generator may reflect and convert incident linearly polarised light of the first predetermined type into linearly polarised light of the second predetermined type to form a desired image to be displayed on the output screen arrangement.

The linearly polarised light of the first predetermined type may be p-polarised light. The linearly polarised light of the second predetermined type may be s-polarised light.

Advantageously, the display source relay optical arrangement may include a relay lens arrangement. Preferably, a fold mirror may be arranged between the relay lens arrangement and the output screen arrangement.

Preferably, a compensator plate may be arranged between the beam splitter and display source relay optical arrangement. Alternatively, a compensator plate may be arranged between the reflective liquid crystal display and field lens.

Advantageously, the relay lens arrangement may include a first singlet lens, a first doublet lens, a second singlet lens and a second doublet lens arranged in series with respect to one another.

The output screen arrangement may include an output diffuser. Preferably, a field lens may be arranged between the output diffuser screen and the screen display source relay optical arrangement.

According to a second aspect of the invention, a helmet mounted display or a head mounted display including a display source includes a primary relay lens arrangement and a visor, the primary lens arrangement arranged to transmit light from the output screen arrangement to the visor and the visor arranged to reflect incident light to a viewer.

Preferably, the helmet mounted display or the head mounted display may include a projector lens arranged between the display source and the primary relay lens arrangement.

According to another aspect of the invention, a helmet mounted display or head mounted display includes a primary relay lens arrangement and a visor, the helmet mounted display or head mounted display further includes a display source, the display source being arranged to provide image bearing light to the primary relay lens arrangement which in turn is arranged to transmit the image bearing light to the visor, the display source also being interchangeable with a replacement display source without the need to alter the primary relay lens arrangement of the helmet mounted display or head mounted display.

In this manner, should it be necessary to replace the image generator of the display source with a different type of image generator, then it will only be necessary to redesign the arrangement of the display source, rather than the primary relay optical arrangement of the helmet mounted display or head mounted display. It will be understood that redesigning the primary relay optical arrangement of the helmet mounted display or head mounted display will incur a relatively large cost when compared to redesigning the display source, especially when one considers the speed at which advances are made in the development of image generators and that such devices can quickly become superseded. The primary relay optical arrangement for a helmet mounted display or head mounted display will have a fixed focal length and any change in size of the image generator will impact the optical performance of the primary relay optical arrangement. That is, if there is an increase in size of the image generator, then display pixels of the image generator will exist outside of the usable area as constrained by the primary relay optical arrangement, which will result in a lower resolution as observed by a viewer using the helmet mounted display or head mounted display. Conversely, if the image generator is reduced in size, then the constrains of the primary relay optical arrangement will result in a lost of field of view as observed by a viewer using the helmet mounted display or head mounted display. A redesign of the display source relay optical arrangement is cheaper and can be implemented so as to provide a correctly sized image to the primary relay optical arrangement of the helmet mounted display or head mounted display.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides a different philosophy to that of the prior art described with reference toFIGS. 1 and 2. In general, a primary relay optical arrangement of a helmet mounted display or a head mounted display is very complex and an expensive item to produce. If the primary relay optical arrangement were arranged to directly image a digital display device, then the primary relay optical arrangement would need to be a bespoke design suited to the specific digital display device utilised in the helmet mounted display or head mounted display. Any change in the mechanical or optical properties of the digital display device could render the entire helmet mounted display or head mounted display system obsolete. Therefore, a display source is employed in the invention to be directly imaged by the primary relay optical arrangement and that can be redesigned relatively simply and cheaply if the digital display device for the helmet mounted display or head mounted display needs to be changed. It will be noted that the primary relay optical arrangement of the helmet mounted display or head mounted display remains the same and hence does not require expensive and complex changes. Accordingly, the helmet mounted display or head mounted display is not rendered obsolete by variations in the digital display device employed to provide the image at the output of the display source.

Referring toFIG. 3, a helmet mounted display30includes a display source31arranged to be directly imaged by a primary relay optical arrangement32comprises primary relay optical elements33A,33B and33C.

Light exiting the primary relay optical arrangement32indicated by ray traces34A,34B and34C continue on towards a visor35of the helmet mounted display30which is arranged to reflect incident light to an exit pupil located in a convenient position for a viewer36utilising the helmet mounted display30such that the viewer36observes a forward scene through the visor35and the image provided by the display source31is superimposed upon the forward scene.

Referring toFIG. 4, the display source31in detail includes a light source generator40including a light source41, which can be a lumiled high brightness five watt light emitting diode with an integral lens, not shown, and an illuminator lens42. The illuminator lens42can be a plano-convex lens, which is arranged to allow a large angle cone of light to be collected from the light source41to provide a brighter image throughout the display source30. It is to be noted that this embodiment of the invention is designed to generate a daylight luminescence display at the exit pupil of the helmet mounted display30.

Light exiting the illuminator lens42passes to a beam splitter43, for example a wire grid polarising beam splitter, which is arranged to be highly transmissive to linear p-polarised incident light received from the illuminator lens42and highly reflective to other incident light including incident linear s-polarised light received from the illuminator lens42. Therefore, in this embodiment the randomly polarised light collected by the illuminator lens42from the light source41is polarised by the beam splitter43. That is, only p-polarised light is transmitted through the beam splitter43and all other polarisation states of light, for example s-polarised light, are reflected away from the beam splitter43. It will be understood that the beam splitter43is not perfect and that it is possible that the beam splitter43can reflect p-polarised light rather than transmit the p-polarised light through the beam splitter43. Equally, the beam splitter43can be transmissive rather than reflective to other than p-polarised light.

P-polarised light transmitted through the beam splitter43is then incident on a field lens44which is arranged to roughly collimate the light and to illuminate a reflective liquid crystal display45. The field lens44is arranged to illuminate the reflective liquid crystal display45and can be a meniscus lens.

The reflective liquid crystal display45is arranged to generate a display object to be formed at the output of the light source generator40. Any selected pixels in an activated state are arranged to rotate the polarisation of the incident p-polarised light such that reflected light from an active pixel is transformed into the s-polarisation state. Hence, reflected light from the reflective liquid crystal display45passes back through to field lens44to the beam splitter43.

S-polarised light incident on the reflective liquid crystal display45is reflected by the beam splitter43to a display source relay optical arrangement46via a compensator plate47. Accordingly, any p-polarised light reflected to the beam splitter43is transmitted through the beam splitter43and not to the display source relay optical arrangement46. The compensator plate47is arranged to correct any drop in contrast due to changes in angle of incidence across the beam splitter43so as to improve the contrast of the final display from the display source31. Alternatively, the compensator plate47can be positioned between the reflective liquid crystal display46and the field lens44to perform the same task.

The display source relay optical arrangement46includes a first singlet lens48, a first doublet lens49, a second singlet lens50and a second doublet lens51arranged in series with respect to one another and in the pathway of light exiting the compensator plate47and are arranged to correct aberration and relay light from the compensator plate47towards the output of the display source31. First singlet lens48is a plano-convex lens arranged as a substrate for cleaning up linear s-polarised light reflected from the beam splitter43. The first singlet lens48acts to remove any orthogonal polarisation of light that may have leaked through the beam splitter43and would otherwise reduce image contrast. First doublet lens49is arranged to colour correct the image reflected by the beam splitter43. It is to be noted that the entire optical design of the display source31of this embodiment is arranged to be colour corrected over the visible spectrum. This accommodates the spectral bandwidth of the light source41and also allows the use of colour reflective liquid crystal displays45. Second singlet lens50is a bi-concave lens forming part of the imaging optics for the display source31. Second doublet lens51is a colour correcting doublet lens forming part of the imaging optic for the display source31.

It will be understood, that although this embodiment of the invention utilizes four lenses,48,49,50and51, that the display source relay optical arrangement46can comprise other arrangements to provide the correct image at the output of the display source31.

Light exiting display source relay optical arrangement46then proceeds to a fold mirror52, which is arranged to reflect the light to an output screen arrangement53. It will be understood, that the present display source31has been specifically designed to fit a particular helmet mounted display30system. Use of a fold mirror52in the optical path of the light can allow the display source31to be fitted between an inner helmet shell and a helmet module with minimal mechanical interference.

The output screen arrangement53includes an output diffuser screen54and a field lens55arranged between the output diffuser screen54and the display source relay optical arrangement46. The field lens55ensures that the image at the output diffuser screen54is telecentric to allow matching of the numerical aperture of the display source relay optical arrangement46to be best matched to the numerical aperture of the primary relay optical arrangement32of the helmet mounted display30. This gives the best display brightness to the eye of a viewer36. Output diffuser screen54is arranged to increase the small numerical aperture of the display source relay optical arrangement46so as to match the numerical aperture of the intended primary relay optical arrangement32of the helmet mounted display30.

Accordingly, the display source31uses a small numerical aperture display source relay optical arrangement46to image the reflective liquid crystal display45, thereby allowing a relatively small and cheap display source relay optical arrangement46to be used in conjunction with an output diffuser screen54to generate the required numerical aperture at the focal point of a primary relay optical arrangement32so as to be directly imaged by the primary relay optical arrangement32. This approach is far simpler than integrating a small display source31directly into the primary relay optical arrangement32. This also allows for the obsolescent nature of the reflective liquid crystal display54over time. The exit pupil of the helmet mounted display30as seen by a viewer37does not suffer from large contrast variations as the reflective liquid crystal display45is not directly imaged. The illumination of the reflective liquid crystal display45is carefully designed such that sufficient light is collected from the light source41to provide a display having a luminance bright enough for daytime use of the helmet mounted display30.

In this manner, should it be necessary to replace the image generator of the display source with a different type of image generator, then it will only be necessary to redesign the arrangement of the display source, rather than the primary relay optical arrangement of the helmet mounted display or head mounted display. It will be understood that redesigning the primary relay optical arrangement of the helmet mounted display or head mounted display will incur a relatively large cost when compared to redesigning the display source, especially when one considers the speed at which advances are made in the development of image generators and that such devices can quickly become superseded. The primary relay optical arrangement for a helmet mounted display or head mounted display will have a fixed focal length and any change in size of the image generator will impact the optical performance of the primary relay optical arrangement. That is, if there is an increase in size of the image generator, then display pixels of the image generator will exist outside of the usable area as constrained by the primary relay optical arrangement, which will result in a lower resolution as observed by a viewer using the helmet mounted display or head mounted display. Conversely, if the image generator is reduced in size, then the constrains of the primary relay optical arrangement will result in a lost of field of view as observed by a viewer using the helmet mounted display or head mounted display. A redesign of the display source relay optical arrangement is cheaper and can be implemented so as to provide a correctly sized image to the primary relay optical arrangement of the helmet mounted display or head mounted display.

Although the invention has been described with reference to a helmet mounted display, it will be understood that the invention can also be incorporated within a head mounted display and that such a display source31can be used in a helmet mounted display or head mounted display system.

It will be noted that such a display source31can be used in a helmet mounted display or head mounted display as a cathode ray tube replacement. In this manner, the helmet mounted display or head mounted display can be rendered lighter and/or more compact.