FIELD OF INVENTION

The present invention relates to an arrangement to reduce influence from diffuse and direct reflections in a display device based on a source of light emitting in a narrow band where the image is regarded directly or through an optical system.

DESCRIPTION OF PRIOR ART

In fast aircrafts the p lot in certain situations cannot move the eyes from the field of view through the front shield to the indicators which he has to read as the aircraft during the time of reading will move too far. In order to render it possible for the pilot to read these indicators without reducing his attention to the outside view a semi-transparent mirror has been placed in the field of view through the front shield. By means of this mirror symbols and indicator readings are reflected onto the field of view of the pilot so that he sees them clearly when the eye is focused on remote looking. An arrange¬ ment of this kind is usually called HUD, short for the English ex- pression "head up display".

The symbols can for example be reflected from a cathode ray tube the display of which is projected through a lens system onto the semi- transparent mirror. As this mirror is planar the display will be in focus of the lens system. Sun-light accidentally Rassing the mirror towards the lens system w ll consequently be focused onto the display as a shining point which, however, does not essentially disturb the image.

Semi-transparent mirrors that are not wave-length selective have certain disadvatages when reflecting images onto a field of view as the transmission of incoming light and the reflection of the sym¬ bols are performed with low efficiency. Lately phase holograms have been used instead of such serni-.ransparent mirrors for reflecting symbols into the field of view of a pilot for e ample. Such arrange¬ ments are described for example in the Sw ^e dish patent No 398 007 or

the US patent No 3 915 548. Besides a high efficiency when reflectin and transmitting a hologram has the property that it also can refrac the light in the same manner as lenses or spherical mirrors. In the above mentioned US patent the hologram has been made in such a way that.it works as a wave-length selective spherical mirror. Thus the .exit pupil of the optical system will not depend on the dimensions o the lens system as in a conventional HUD-system, but it is possible to have a symbol presentation of remarkable size also when having a small dimension lens system. The lens system, the so called relay lens generates a virtual image of the symbol of the display unit in the focal plane, of the spherical mirror enabling this image to be magnified as desired.

SUMMARY OF THE INVENTION

The contrast of the reflected symbol image, however, may be inferior as a diffuse reflection takes a big part of the field of view. This reflection also disturbs the transparence. In spite of the -fact that the hologram performs a complicated filtration of incoming and re¬ flected light the attenuation which is obtained is unsufficient. Sun-light canpass the hologram and fall towards the lens system which breaks the parallel sun-light towards a point in front of the display of the cathode ray tube. Consequently the display will be illuminated from this point and give a diffuse reflection which is thrown back via the lens system towards the hologram and further towards the observer who sees an unacceptable diffuse illumination of his field of view. There is a corresponding problem also in nor- rnal display units used for direct observation of a cathode ray tube where reflections are a big problem especially when they are to be used in sun-light.

According to the invention it is possible to reduce the influence of diffuse and direct reflections in a display device based on a light source emitting in a narrow band and the characteristics of the in¬ vention appear from the appended claims.

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BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described more in detail in connection with the accompanying drawings where Fig 1 shows a display device with a semi- transparent mirror according to the prior art.

Fig 2 shows a holographic display device according to the invention. 5 Fig 3 shows the filter characteristics of an absorption filter being part of the arrangement according to Fig 2.

PREFERRED EMBODIMENT

In Fig 1 11 designates a luminous symbol image being generated for example by a cathode ray tube. The symbol image is placed in the focal plane of a lens system 12, which collimates the emitted light. This

10. is reflected by a planar, partially reflecting mirror 13 towards an observer's 14 eye. The observer then interprets the symbols as being placed at an infinite distance within a field of view being limited by the dimensions of the lens system. Sun rays 15, which pass through the semi-transparent mirror 13 towards the lens system 12 are refracted

15 into one shining spot on the very symbol image. This shining spot is then reflected towards the observer who sees a shining point in the symbol image which is not disturbed otherwise.

When the partially reflecting mirror according to the US patent No 3 915 548 is replaced by a hologram £1, which besides a partly

20 semi-transparent mirror works as a concave mirror, the conditions will be different as is described in Fig 2. The luminous symbol image 11 is reproduced magnified in a plane 23 by a lens system (relay lens) 22, which can be simpler than the Lens system being part of the arrangement according to Fig 1. The plane 21 is a focal plane

25 for the hologram 21, why the symbol image will be reproduced. In order to reduce the dimensions of the optical system there is in this case a mirror 24 placed in the beam path. In front of the mirror 24 an absorption filter is placed with a narrow pass band having mainly the attenuation characteristics which is shown in Fig 3.

30 The symbol image in Fig 2 is generated by a narrow band light source.

OMPI _

In an embodiment a cathode ray tube with P-43-type phosphor is used. This has a brightness maximum at λ = 543 nm and a band width of 2 nm The hologram has a band width being of the magnitude 14-18 nm around the wave-length. Within this area the hologram is working as a con- cave mirror with high reflectance. Outside the area the hologram is to be considered alsmost as transparent so. that light coming from the outside can pass without any attenuation worth mentioning. The narrow wave-length band being rejected does hardly effect the colour of the light coming from the outside.

The hologram performs a complicated filtration of incident and reflected sun-light but the attenuation thus achieved is unsufficient to prevent the sunshine from disturbing the reproduction of the imag and the through-sight. Incident sunlight 27 can pass through the hol gram also within the active area of the hologram and is refracted in the focal plane 26 of the lens system 22. This gives rise to a diffu reflection from the symbol image, that is the display of the cathode ray tube. This reflection, which occupies a big part of the field of view is projected towards the observer 14. By means of the f lter 25 having the. characteristics being shown in Fig 3 with a half value band width of the magnitude 25 nm and a reflectance of approximately 0,7, in combination with the mirror 24 at λ = 543 nm a heavy limita¬ tion of the energy contents of the incident sun-light reflected from the display of the cathode ray tube 11 is achieved. Ideally the fil¬ ter and the light source should have the same band width but due to tolerances in the position of the pass band of the filter and the ^' phosphor material the filter pass band has to be made wider. It has been proven that the reflections are attenuated a factor 4 with the shown filter of which a factor 2 relates to the band pass filtering. The big advantage of using an absorption filter instead of using an interference filter, which is normal when narrow band filters are desired, is that the absorption filter is independent of the angle o incidence of the light. The filter used has a pass band for long wav light above 600 nm. Even if the sensitivity of the eye quickly declines for said domain the absorption filter may possibly be completed with another filter working for this part of the spectrum. This filter may be an interference filter.

Also in a presentation system for direct sight, for instance a display using a cathode ray tube as the light source the influence of the reflections can be considerably reduced. By using a cathode ray tube display with a narrow band phosphor and by placing a filter of the type described in front of the dispaly it is achieved τhat the re¬ flections from the display screen are considerably attenuated.

OMPI _