WO1996001440A1

WO1996001440A1 - Display system for superposing three images for obtaining a mixed image

Info

Publication number: WO1996001440A1
Application number: PCT/NL1995/000235
Authority: WO
Inventors: Godwin Jeroom Odo Ghislenus Van Hooreweder
Original assignee: B.V. Optische Industrie 'de Oude Delft'
Priority date: 1994-07-01
Filing date: 1995-07-03
Publication date: 1996-01-18
Also published as: NL9401110A

Abstract

Display system for superimposing three images in order to obtain a mixed image. The system is provided with an eyepiece made up of a first and second light-transmitting, reflecting element with angle dependent and frequency-dependent reflection and transmission characteristics. The first image is incident upon a first side of the first element, the second image is incident upon a second side of the first element, and the third image is incident upon a side of the second element. The above-mentioned characteristics lead to the three images emerging through the eyepiece as a mixed image.

Description

Title: Display system for superposing three images for obtaining a mixed image

The invention relates to a display system for superposing three images for obtaining a mixed image. A display system according to the invention can be used for various applications where there is a need for mixing three different images together and transmitting them in the same direction so that they can then be perceived or projected onto, for example, a screen. In this case the first image corresponds to, for example, a greenish- coloured partial image, the second image to a reddish- coloured partial image, and the third image to a bluish- coloured partial image. In such a configuration the system can be used in, for example, a colour television projection system. However, the system can also be used in, for example, a night vision. In this case the first image corresponds, for example, to a night image, the second image to an information image, and the third image to a day image. European Patent Application 0,475,790 discloses a display system suitable for fixing on the head or to a helmet, in which a day image, night image and video image are mixed together. Only one (holographic) element is used in this case, in order to mix a video image and night image. These two mixed images are then projected together with a daylight image through a combination optical system in the direction of the eye. A disad¬ vantage of such a display system is that all three of the images are of necessity always displayed at the same distance and are always simultaneously sharp for a user. This can have a particularly confusing effect, and such a system is generally not sufficiently accurate for these and other applications which will be discussed below. Another type of display system is known from European Patent Application 0,252,200. These display systems are used, inter alia, for the helmets of pilots particularly of military aircraft and helicopters. In these systems a clear transparency is mixed with a night image, the night image being an intensified image. For the purpose of obtaining a clear image of an environment in which the light intensity is low, this known display system provides a user with an extremely clear, high- resolution image. Even if the intensity of the ambient light increases, a clear image is still obtained, due to the fact that a user can also perceive the environment directly. A user wearing such a system can therefore perceive in daylight, twilight and darkness without having to put on or take off the helmet. In other words, the ambient light is projected by the display system directly in the direction of the eye. A head fastening system or helmet provided with such a display system also has the advantage that a centre of gravity can be achieved which is situated ideally for the user.

The need has now arisen for a display system which makes it possible to mix three images very accu¬ rately.

To this end, the display system according to the invention is characterized in that the display system is provided with an eyepiece made up of a first and second light-transmitting, reflecting element with angle- dependent and freguency-dependent reflection and transmission characteristics, the first image is incident upon a first side of the first element, the second image is incident upon a second side of the first element, and the third image is incident upon a side of the second element, and in which the abovementioned characteristics lead to the three images emerging through the eyepiece as a mixed image. A special embodiment according to the invention is characterized in that the system is also provided with at least one image intensifier unit with an input onto which an image can be projected and an output which retransmits the projected image at an increased intensity and within a predetermined first frequency band, for the purpose of obtaining an intensified image forming the first image, the abovementioned characteristics, combined with the abovementioned first frequency band, causing the intensified image to emerge through the eyepiece in the direction of the eye, and a picture tube which transmits a visible video image forming the second image, while the abovementioned characteristics of the eyepiece cause the video image to emerge through the eyepiece in the direction of the eye within a predetermined second frequency band.

A user, for example a pilot who is wearing a helmet provided with such a display system, therefore in principle does not have to take his eyes off the sur- roundings in order to perceive the required weapon system information or other information. This is because the information coming from the display unit can be displayed in the pilot's field of vision, while he is looking for example, straight out ahead. European Patent Applications 0,286,496 and

0,288,365 and US Patent Specification 5,257,094 also disclose display systems in which images are mixed together. However, in these systems it is always just two, instead of possibly three, images which are mixed together.

A special embodiment of the invention is characterized in that in succession the intensified image is reflected by the first element, is reflected and collimated by the second element, and is then transmitted by the first element. In this case the video image is reflected by the first element in the same direction as that in which the intensified image emerges. More par¬ ticularly, the intensified image and the video image respectively are incident upon the first element at different sides of the first element. According to a very advanced embodiment, the angle of incidence of the intensified image on the first element is different from the angle of incidence of the video image on the first element. An optical system working in this way can be achieved in particular if the first frequency band at any rate essentially comprises frequencies different from the second frequency band. The first frequency band com¬ prises, for example, at any rate essentially greenish- coloured light, and the second frequency band comprises at any rate essentially reddish-coloured light. Due to such a choice of frequencies, the abovementioned reflec¬ tions and transmissions of the first and second element can be achieved. Moreover, it is very advantageous for a user that the video image is perceived in a different colour from that of the night image. Confusion between the two images is consequently ruled out, while the colour contrast improves the separate perception of each of the two images.

The display system according to the invention has the further advantage that a daylight image is trans¬ mitted in succession by the second and first element and emerges from the eyepiece in the same direction as the intensified image and the video image. This means that the day image can be perceived directly.

In particular, the first element comprises at least one holographic layer. However, it is also possible for the first element to comprise at least one multilayer interference filter. The second element preferably comprises at least one holographic concave layer and herewith forms a concave collimating holographic mirror. Besides, it is, however, possible for the second element to comprise a curved multilayer interference filter. According to a more specific embodiment of the invention, the system is also provided with a lens part by means of which, for example, the night image is projected onto the input of the image intensifier unit. The abovementioned image display unit can consist of, for example an LCD or a CRT.

The invention also relates to a helmet provided with such a display system.

Such a helmet is preferably provided with an image intensifier unit (IIT) and an eyepiece for each eye of a user, and the helmet is also provided with a common image display unit for one of the eyes or both eyes. This means that a user can perceive both the day image and the night image in stereo, i.e. in three dimensions.

The invention will be explained further with reference to the drawing, in which:

Figure l shows a schematic drawing of a display system according to the invention;

Figure 2 shows a reflection characteristic of a first element according to Figure 1;

Figure 3 shows an emission spectrum of a phos¬ phor screen according to Figure 1;

Figure 4 shows a transmission characteristic of the first element according to Figure l; Figure 5 shows an alternative schematic drawing of a display system according to the invention;

Figure 6 shows a front view of a display system mounted on a helmet;

Figure 7 shows a side view of a part of the display system of Figure 6; and

Figure 8 shows a top view of a part of the display system of Figure 6.

Figure l diagrammatically shows a display system l which is suitable for, for example, mounting on a helmet. The display system l comprises a first light- transmitting, reflecting element 2, having angle-depend¬ ent and frequency-dependent reflection and transmission characteristics which are defined in greater detail below with reference to Figures 2, 3 and 4. In this example the first element 2 consists of a flat element which is known per se and has at least one holographic layer or multi¬ layer interference filter as will be explained in greater detail below.

In addition, the display system 1 comprises a second light-transmitting, reflecting element 4 which has angle-dependent and frequency-dependent reflection and transmission characteristics which are defined in greater detail below with reference to Figures 2, 3 and 4. In this example the second element 4 consists of a concave collimating element 4, which is known per se and has at least one holographic layer (concave holographic mirror) or multilayer interference filter, as will be explained in greater detail below. The two elements 2, 4 together form an eyepiece 6 of the display system 1. A light beam 8 from a day image in the direction of the eyepiece is transmitted both by the second element 4 and by the first element 2. A light beam 10 coming from a night image is incident upon a first side 12 of the first element 2 and is reflected by it in the direction of the second element 4. The light beam 10 is then reflected back by the second element 4 in the direction of the first element 2. The light beam 10 reflected by the second element 4 is transmitted by the first element in the same direction as the light beam 8. A light beam 14 from a video image is also incident upon a second side 16 of the first element and is also reflected by it in the direction of the light beam 8.

All this means that the day image, night image and video image are mixed together and can be perceived by an eye 18 of a user. An identical second display system 1 can be used for a second eye 20 of a user. This means that the user can perceive the day image and the night image in three dimensions. The video image can be reflected towards both eyes, or if desired to only one eye. In the latter case the reflection of the video image can be omitted for one of the two display systems l. The display system 1 is provided with means which are known per se for generating the night image and the video image, which will not be discussed in any further detail in connection with Figure l.

Figure 2 shows the reflection characteristic 22 of the first side 12 of the first element 2 for light beams which are incident at an angle HI relative to the normal of the first side 12. It can be seen from this that in this example the first side of the first element is designed in such a way that light beams with a wave¬ length of around 550 nm are reflected virtually complete¬ ly. This property is put to use by generating a night image with frequencies falling at any rate at least within a frequency band comprising wavelengths of around 550 nm. This can be achieved by, for example, using a phosphor screen which is known per se for generating the night image, which in this case is greenish. Figure 3 shows the emission spectrum of such a phosphor screen, which spectrum has a peak 24 around 550 nm. This explains the fact that the incident light beam 10 is in fact reflected at any rate essentially by the first element 2. However, the sidebands 26, lying at either side of the peak 24, are simply transmitted. For this purpose, the first side 12 can be provided with a holographic layer 13, which is known per se, or a multilayer interference filter 13, which is also known per se, on a light-transmitting medium, which is shown hatched in the drawing.

The characteristics of the second element are selected in such a way that a light beam which is inci¬ dent at an angle H2 and has a frequency spectrum compris- ing a peak around 550 nm is reflected. For this purpose, the second element 4 is provided with a holographic layer 25 or multilayer interference filter 25, known per se, on a light-transmitting medium which is shown hatched in the drawing. The above is comparable to what was discussed in relation to the first element and will not be explained in any further detail here.

The second side 16 of the first element 2 has reflection and transmission characteristics which are comparable to the characteristics of the first side 12 discussed above. In this example the second side 16 is designed in such a way that a light beam 14 which is incident at an angle H3 is reflected for frequencies containing reddish-coloured light. The second side 16 can be provided for this purpose with a holographic layer 17, which is known per se, or a multilayer interference filter, which is likewise known per se, on a light-trans¬ mitting medium shown hatched in the drawing. The reflected video image 14 accordingly comprises reddish- coloured light. This has the additional advantage that a user can easily distinguish the greenish night image in this example from the reddish video image. The video image which is incident upon the first element 2 will therefore comprise at least a frequency band which at any rate partially overlaps the frequencies or the frequency band for which, at an angle of incidence H3, reflections occur on the first element 2. This means that the inci¬ dent video image can be, for example, a black-and-white image. The reddish frequency components of this image are then reflected, and the remaining components are trans¬ mitted by the first element 2.

Figure 4 also shows the transmission characteristic of the first element 2 when a light beam is incident at a smaller angle which differs from the angle HI. It can be seen from this that transmission occurs when the wavelength deviates from about 625 nm. This explains the fact that the light beam 10 reflected by the second element 4 is transmitted by the first element. The second element 4 has transmission characteristics comparable to Figure 4, so that the daylight - which in fact comprises a continuous frequency spectrum -is transmitted virtually entirely by the second and first element. If it is desirable for the video image reflected by the first element 2 to be a different colour from that described above - such as, for example, a bluish reflected image 14' - this can be achieved by making the video image 14' be incident upon the first element 2 at an angle H4 which in this example is greater than HI.

At a greater angle H4 reflections of bluish frequency components (second frequency band) then occur, while other frequency components are transmitted by the first element 2. The reflected image 14' will then be bluish. Of course, the condition is that the video image 14' incident upon the first element 2 at least comprises frequencies which are reflected by the first element if the angle of incidence is H4. In other words, the last- mentioned frequencies fall at any rate partially within the earlier mentioned second frequency band.

In a particularly advantageous embodiment such as that shown in Figure 5 the first element 2 comprises only a single layer 17 with the abovementioned angle- dependent and frequency-dependent reflection and transmission characteristics, which means that the first layer 13 and the second layer 17 belong to one and the same layer, which in Figure 5 is provided with a reference number 13. Through the special construction of a holographic layer 13 or multilayer interference filter with the characteristics shown in Figure 2 and 4, the frequency-dependent reflection and transmission characteristics shown in those figures are achieved for one specific angle of incidence, for example the angle Hi. However, if the angle of incidence is other than HI, the corresponding centre frequencies of the peak in Figure 2 and of the trough in Figure 4 are also different. In particular, a smaller angle of incidence relative to the normal is accompanied by a greater wavelength of the centre frequency of the peak in figure 2 and of the trough in Figure 4, the above corresponding to the known Bragg equation. This special property of holographic layers or multilayer interference filters with angle-dependent and frequency-dependent reflection and transmission characteristics can now be used advantageously in the case of the first element 2 and the second element 4.

As can be seen from Figure 5, the angle of incidence relative to the normal for the light coming from the image intensifier unit must be different from the angle of incidence relative to the normal of the light from the video image, in order to be able to ensure that the direction of emergence of both the night vision image and the video image is the same. The angles con- cerned are indicated by Hi and H3 respectively in

Figure 5. If now in the case of element 2 only one holo¬ graphic layer 13 or multilayer interference filter 13 is used to obtain the angle-dependent and frequency-depend¬ ent reflection and transmission characteristics for both the reflection at one side 12 of the night vision image 10 and the reflection at the other side 16 of the video image 14, it then emerges from the above that those reflections occur at different wavelengths; and, since angle HI is greater than angle H3, the corresponding centre wavelength of the night vision image is smaller than the corresponding centre wavelength of the video image, or a reddish video image goes with a greenish night vision image. The embodiment with only one holo- graphic layer 13 or multilayer interference filter 13, described here and shown in Figure 5, has not only the advantage that only one layer with angle-dependent and frequency-dependent reflection and transmission char¬ acteristics need be applied to a substrate, but also the advantage that the night vision image and the video image are reflected in colours (greenish and reddish respectively) , in the case of which research has shown that it is those two particular colours, as contrasting colours, which in a composite image result in a particu- larly good separate perception of each of the two images (the greenish and the reddish respectively) . The embodi¬ ment described here can thus not only be manufactured less critically (only one holographic layer or multilayer interference filter need therefore be manufactured) , but one of the best-known colour contrasts (green-red) can also be achieved with it for the night vision image and the video image. In this example the layer or filter 13 is accommodated in a light-transmitting medium, which is shown hatched in the drawing. The layer 25 is likewise accommodated in a light-transmitting medium, which is shown hatched. In order to obtain, for example, a bluish reflected video image 14' , the angle of incidence can be, for example, equal to H4, as discussed in connection with Figure l. A possible embodiment of a display system for a helmet will now be discussed with reference to Figures 6, 7 and 8. In these figures the parts corresponding to those in Figures and 5 are provided with the same reference number. The display system comprises, in addition to the parts discussed in Figures 1 and 5, a CRT 28 by means of which an information-containing video image 14 is generated. The video image 14 is presented to the eye by way of a mirror 30, a lens 32, a mirror 34 and the first element 2. This means that after reflection at the second side 16 of the first element 2, this video image 14 can be perceived with the eye 18 at a certain distance. For each eye the display system is also provided with a lens 36 by means of which a night image can be displayed at an input of an image intensifier tube (IIT) . The IIT 38 increases the intensity of the image, which is then retransmitted at an output of the IIT 38. The night image thus obtained is guided in the direction of the eyepiece 6 by way of a mirror 40, a lens system 42, a mirror 44, a lens system 46, and a mirror 48. All this is synchronized in such a way that the night image 10 is displayed at infinity. This means that, after reflection at the first element 2 and the second element 4 and transmission by element 2, this night image can be perceived by eyes 18, 20 fixed on infinity. What this achieves is that night image 10 and day image 8 are perceived superimposed. This is particularly pleasant for a user, because day and night image merge with each other naturally.

For the sake of clarity, the CRT 28, mirror 30 and lens 32 are omitted in Figures 7 and 8.

In any case, a user will generally not see the video image which is displayed at a certain distance with the day and night image simultaneously sharp, because these two images are generally a long distance away. The IIT 38 in this example comprises a phosphor screen which generates a greenish image, while the video image gener¬ ated by the CRT is a reddish colour. This means that the chance of these images making a confusing impression on a user is particularly small.

An advantage of the display system is that the relatively high resolution of the IIT 38 is retained. An IIT can also be replaced by integral CCD cameras with, for example, CRT display.

However, it is also possible to use an LCD instead of a CRT. Since an LCD has a different frequency spectrum from a CRT, the first side 12 of the first element 2 and the second element 4 will have to be adapted accordingly. On account of the relation already described above between angle of incidence relative to the normal and centre frequencies of the reflected light, the position of the CRT or LCD can, of course, likewise be changed. It is also possible to produce a display system in a so-called mono version, according to which example the parts on the right of Figure 6 are omitted, so that in principle the display system according to Figure l or 5 remains. The display system according to Figure l is, of course, also suitable for mixing together any three images, these images not necessarily being a day image, a night image and a video image. The first image, which corresponds to night light, can then be, for example, at any rate essentially greenish, the third image, corresponding to daylight, at any rate essentially bluish, and the second image, corresponding to the video image, essentially reddish. These images are then mixed and can be perceived by the eye 20 or displayed on a screen if, for example, a lens which displays the images mixed on the screen is situated at the position of the eye. In this way it is simple to obtain a colour televi¬ sion projection system in which the three partial images have already been brought perfectly into register and only then are projected in register. Of course, the second image can likewise be bluish, as indicated by reference number 14' in Figures 1 and 5. In that case the image 8 can be, for example, reddish. In all these cases the mixed image can form a colour picture. The first, second and third images then contain the same picture, and after mixing they form a colour picture according to the principle which is used for, for example, colour televisions. These and other variants are all within the scope of the invention.

Claims

1. Display system for superposing three images for obtaining a mixed image, provided with an eyepiece made up of a first and second light-transmitting, reflecting element with angle-dependent and frequency-dependent reflection and transmission characteristics, the first image being incident upon a first side of the first element, the second image being incident upon a second side of the first element, and the third image being incident upon a side of the second element, and in which the abovementioned characteristics lead to the three images emerging through the eyepiece as a mixed image.

2. Display system according to Claim l, character¬ ized in that the second element is made so that it is collimating.

3. Display system according to Claim 1 or 2, characterized in that the third image is incident upon a side of the second element facing away from the second side of the first element.

4. Display system according to one of the preceding claims, characterized in that the system is also provided with a lens for displaying the mixed image on a screen.

5. Display system according to one of the preceding claims, characterized in that the first image emerging through the eyepiece is a greenish colour, the second image emerging through the eyepiece is a reddish colour, and the third image emerging through the eyepiece is a bluish colour.

6. Display system according to one of the preceding Claims 1 - 4, characterized in that the first image emerging through the eyepiece is a greenish colour, the second image emerging through the eyepiece is a bluish colour, and the third image emerging through the eyepiece is a reddish colour.

7. Display system according to one of the preceding claims, characterized in that the colour of the third image incident upon the second element is at any rate virtually identical to the colour of the third image emerging through the eyepiece.

8. Display system according to Claim 2, intended for fixing on the head or to a helmet, characterized in that the system is also provided with at least one image intensifier unit (IIT) with an input onto which an image can be projected and an output which retransmits the projected image at an increased intensity and in a predetermined first frequency band, for the purpose of obtaining an intensified image forming the first image, the abovementioned characteristics combined with the abovementioned first frequency band causing the inten¬ sified image to emerge through the eyepiece in the direc¬ tion of the eye, and a picture tube which transmits a visible video image forming the second image, while the abovementioned characteristics of the eyepiece cause the video image to emerge through the eyepiece within a predetermined second frequency band in the direction of the eye.

9. Display system according to Claim 8, character¬ ized in that in succession the intensified image is reflected by the first element, is reflected and collimated by the second element, and is transmitted by the first element.

10. Display system according to one of the preceding Claims 8 or 9, characterized in that the video image is reflected by the first element.

11. Display system according to Claims 9 and 10, characterized in that the intensified image and the video image respectively are incident upon the first element at different sides of the first element.

12. Display system according to Claim 11, characterized in that the angle of incidence (HI) of the intensified image on the first element is different from the angle of incidence (H3) of the video image on the first element.

13. Display system according to one of the preceding Claims 8 - 12, characterized in that a daylight image is transmitted in succession by the second and first element and emerges from the eyepiece in at any rate virtually the same direction as the intensified image.

14. Display system according to one of the preceding claims, characterized in that the first element comprises at least one holographic layer.

15. Display system according to Claim 14, characterized in that the first element comprises a holographic layer at each side.

16. Display system according to Claim 14, characterized in that the first element comprises only one holographic layer.

17. Display system according to one of the preceding claims, characterized in that the first element comprises at least one multilayer interference filter.

18. Display system according to Claim 17, characterized in that the first element comprises a multilayer interference filter at each side.

19. Display system according to Claim 17, characterized in that the first element comprises only one multilayer interference filter.

20. Display system according to one of the preceding claims, characterized in that the second element comprises at least one holographic concave mirror.

21. Display system according to one of the preceding claims, characterized in that the second element comprises at least one curved multilayer interference filter.

22. Display system according to one of the preceding Claims 8 - 21, characterized in that the first frequency band comprises frequencies which are at any rate essentially higher than those of the second fre¬ quency band.

23. Display system according to Claim 22, characterized in that the first frequency band comprises at any rate essentially greenish-coloured light, and the second frequency band comprises at any rate essentially reddish- coloured light.

24. Display system according to one of the preceding Claims 8 - 23, characterized in that the system is also provided with a lens part, by means of which the abovementioned image is projected onto the input of the image intensifier unit.

25. Display system according to one of the preceding Claims 8 - 24, characterized in that the image display unit consists of an LCD or a CRT.

26. Helmet provided with a display system according to one of the preceding Claims 8 - 25.

27. Helmet according to Claim 26, characterized in that the axis of the image intensifier unit is directed at any rate virtually horizontally during normal use.

28. Helmet provided with a display system according to Claim 24, characterized in that during use the lens part is situated at approximately the same height as the eyes of a wearer of the helmet.

29. Helmet according to one of the preceding Claims 26 - 28, characterized in that for each eye of a user the helmet is provided with an image intensifier unit and an eyepiece, and the helmet is also provided with a common image display unit for one of the eyes or both eyes.

30. Helmet according to one of the preceding Claims 26 -29, characterized in that the video image for a user is displayed at a different distance from the image pro¬ jected by the image intensifier unit.