Diffraction device

Abstract: The present invention provides a diffraction device having a high diffraction efficiency with respect to a wide-band radiation beam. A diffraction device according to the present invention comprises a substrate and a diffraction grating formed on said substrate, wherein said diffraction grating is formed so that a full-width of wavelength at half maximum of a diffraction efficiency of a diffracted beam by said diffraction grating is not less than about 8% of a value of a center wavelength of the diffracted beam.

Description: The present invention relates to a head-up display system comprising a display element emitting a light beam and a relaying element relaying said light beam from said display element to a diffraction grating of a beam combiner which diffracts the relayed light beam and directs it to a viewing position, wherein said relaying element is a diffraction device comprising a diffraction grating. Surface relief type diffraction gratings are known, and it might be considered to use a surface relief type diffraction grating as diffraction grating of the relaying element of a head-up display system. Fig. 1 shows a grating shape of a relief type diffraction grating prepared by machine work and its use state. A stepwise grating structure 21 is formed on the surface of a diffraction grating layer 20 on a substrate 19. When monochromatic light 22 is incident in a direction almost perpendicular to the grating surface 21, and a diffracted light beam 23 emerges in a direction almost perpendicular to the grating surface 21, a diffraction efficiency of this diffraction grating is maximized, and a wavelength at this time is called a blazed wavelength. Fig. 2 shows measurement results of the diffraction efficiency of the diffraction grating. Although there is a slight difference between TE light (solid curve) and TM light (broken curve) depending on a polarized state of incident light, high diffraction efficiencies can be obtained. When the diffraction grating obtained by machine work is used for a conventional beam splitter, it has excellent characteristics. However, when a reflection type diffraction grating of this type is used in an relaying element of a head-up display system, various problems are posed as in other diffraction gratings. In this case, a light beam incident on the diffraction grating and a light beam reflected and diffracted by the diffraction grating have a given spatial range so as to display a two- or three-dimensional image. As a result, in order to spatially separate the light beam incident on the diffraction grating and the light beam reflected and diffracted by the diffraction grating, a certain angular separation is necessary between the two light beams. The drawback of the conventional diffraction grating is a decrease in diffraction efficiency when an angle defined by the incident light and the diffracted light is increased. For example, in Fig. 1, when an angle defined by the incident light 22 and the reflected/diffracted light 23 is increased, a certain light component is lost at the corner of the grating inclined surface 21, thus decreasing the diffraction efficiency. The drawback of the relief diffraction grating obtained by machine work is difficulty in etching the grating on a non-flat surface and in formation of a non-linear grating. However, in order to achieve these requirements using machine work, machine control with higher precision than in a normal diffraction grating working technique is required, and this is impossible to achieve in practice. As a method of solving the problem in machine work, a method of forming a diffraction grating by a holographic exposure method is known. In this method, a substrate coated with a photosensitive material such as a photoresist is exposed with two light beams from a laser to form interference fringes, thus preparing a diffraction grating. A diffraction grating prepared by the holographic exposure method has many advantages. That is, the diffraction grating can be formed on an arbitrarily curved surface such as a concave surface, convex surface, and the like, and can have various aberration characteristics such as focusing characteristics. However, this diffraction grating prepared by the holographic exposure method has a problem in diffraction efficiency. Fig. 3 shows diffraction efficiency characteristics of a diffraction grating prepared by the holographic exposure method. The diffraction efficiency has large polarization dependency, and TE light and TM light have different wavelengths corresponding to peak diffraction efficiencies. For this reason, the diffraction grating cannot be effectively utilized except for an optical system using perfectly linearly polarized light. A head-up display system comprising the features of the preamble of claim 1 is known from document US-A-46 13 200. The two diffraction gratings of this known head-up display system are diffraction gratings prepared by the holographic exposure method. These two diffraction gratings are identically constructed. When the viewing position of an observer is slightly varied, uniformity and brightness of the viewed image correspondingly vary. It is an object of the present invention to provide a head-up display system which displays a uniform and bright image despite some variations in the viewing position. This object is achieved by the head-up display system according to claim 1. Document EP-A-278 395 belongs to the state of the art under Article 54(3) EPC and describes a head-up display comprising all features of claim 1 except that the full-width of wave lengths at half maximum of the diffraction efficiency of said relaying diffraction grating is not less than 8% of the value of the center wave length of said light beam from said display element. Other features and detailed aspects of the present invention will be apparent from the following description of preferred embodiments. Fig. 1 is a view showing a relief diffraction grating prepared by machine work; Fig. 2 is a graph showing wavelength characteristics of a diffraction efficiency of the relief diffraction grating; Fig. 3 is a graph showing wavelength characteristics of a diffraction efficiency of a diffraction grating prepared by a holographic method; Fig. 4 is a graph showing wavelength characteristics of a diffraction efficiency of a diffraction device used with the present invention; Fig. 5 is a view showing a printing optical system for forming the diffraction device used with the present invention by a holographic method; Fig. 6 is a view showing a head-up display system of the present invention; Fig. 7 is a graph showing the relationship of wavelength characteristics of diffraction efficiencies of two holograms in the system shown in Fig. 6; Fig. 8 is a view showing another printing optical system for forming the diffraction device by the holographic method; and Fig. 9 is a graph for explaining wavelength characteristics of a diffraction efficiency of a diffraction device prepared by means of the system according to Fig. 8. Diffraction efficiency characteristics of a volume-phase type diffraction grating which is used as a relaying diffraction grating of a head-up display according to the present invention will be described below with reference to Fig. 4. Fig. 4 shows measurement results of a diffraction efficiency obtained such that monochromatic light is caused to become incident on the volume-phase type diffraction grating in a direction perpendicular thereto, and an amount of primary reflected/diffracted light components is measured. In Fig. 4, a diffraction efficiency equal to or higher than 80% can be obtained within the range of ±20 nm to have a center wavelength λ c = 555 nm as the center. Since almost no change in diffraction efficiency depending on a polarization direction occurs unlike in Figs. 2 and 3, the characteristics in Fig. 4 are illustrated by a single curve. As shown in Fig. 4, since the full-width at half maximum of a diffraction efficiency of diffracted light is 65 nm or more, i.e., 8% or more of the value (555 nm) of the center wavelength of the diffracted light, the diffraction grating exhibits a high diffraction efficiency for a very wide wavelength range. Fig. 5 is a view showing an optical system for printing the volume-phase diffraction grating of a diffraction device 100 having the characteristics shown in Fig. 4. Light having a wavelength of 488 nm from an argon laser 1 is reflected by a reflecting mirror 2 and is then split by a half mirror 3 into light beams 4 and 5. The light beam 4 passing through the half mirror 3 is diffused by a microscope objective lens 6 to be converted to a light beam 7, and the light beam 7 is incident on a hologram photosensitive material 11 on a substrate 10. On the other hand, the light beam 5 reflected by the half mirror 3 is diffused by a microscope objective lens 8 to be converted to a light beam 9, and the light beam 9 is incident on the hologram photosensitive material 11 through the transparent substrate 10. These two light beams 7 and 9 interfere with each other in the hologram photosensitive material 11 and form interference fringes. Incident angles of the two light beams with respect to the substrate 10 are selected to be, e.g., 70° and 30°. A parallel light beam or a converged light beam may be used as a printing light beam as needed. In this embodiment, a polymer as a major component of the hologram photosensitive material 11 is selected from a group consisting of polyvinyl carbazole, and its alkyl- or halogen-substituted derivative. More specifically, examples of the polymer include polyvinyl carbazole, a 3-chlorovinyl carbazole polymer, 3-bromovinyl carbazole polymer, 3-iodovinyl carbazole polymer, 3-methylvinyl carbazole polymer, 3-ethylvinyl carbazole polymer, chlorinated polyvinyl carbazole, brominated polyvinyl carbazole, and the like. Of these polymers, an unsubstituted polyvinyl carbazole is most preferable since it is easily available and a resultant hologram has particularly excellent performance. The vinyl carbazole-based polymer may be copolymerized with another monomer to control characteristics such as a mechanical strength or flexibility when it is formed as a film, as needed. Examples of the other monomer used in such an application include a vinyl ester such as vinyl acetate, esters of acrylic acid and methacrylic acid, styrene and a styrene derivative, a vinyl-based monomer such as N-vinyl pyrrolidone, N-vinyl phtalimide or the like which can be copolymerized by a copolymerization method using radical polymerization in addition to the above vinyl carbazole group. For example, other polymers such as polystyrene, a styrene-butadiene copolymer, styrene hydrogenated butadiene copolymer, and the like can be blended within a range wherein a hologram image (interference fringes) can be recorded. Note that addition ratios of these materials are selected to obtain desired characteristics. A polymer component as a major component of the photosensitive material 11 must be activated in advance by an iodine compound with respect to a radiation beam. The iodine compound coexists in the polymer component to constitute a photosensitive material which has sufficient sensitivity in a visible wavelength range. More specifically, examples of the iodine compounds are those containing carbon tetraiodide, iodoform, ethylene tetraiodido, triiodoethane, tetraiodoethane, pentaiodoethane, hexaiodoethane, and the like as their major components. The hologram photosensitive material 11 is prepared as follows. That is, the above-mentioned polymer and iodide compound bare dissolved in a solvent at a predetermined ratio or are dispersed to prepare a dispersion, and the resultant solution or dispersion is coated on a carrier such as a glass or transparent resin film or is formed into a film by itself. An interference pattern is exposed by the above method on the vinyl carbazole photosensitive material with the above structure using object light and reference light formed by laser light from an Ar laser or coherent light having a proper wavelength within a wavelength range of 400 nm to 560 nm. Thereafter, the resultant material undergoes a developing process by utilizing swelling and contraction phenomena by a solvent, thus obtaining a volume-phase hologram with a high resolution and a high diffraction efficiency. With the conventional method, a volume-phase diffraction grating having transparency and a high diffraction efficiency can be formed. However, it is impossible to form a volume-phase diffraction grating having a high diffraction efficiency over a wide wavelength range like in the diffraction device of the present invention. For the conventional volume-phase diffraction grating, only conditions for a transmission type diffraction grating having high transparency or a reflection type diffraction grating (beam combiner) for a head-up display having high transparency have been studied. Characteristics of the conventional reflection type volume-phase diffraction grating will be described below. When the reflection type volume-phase diffraction grating is prepared using a transparent hologram photosensitive material such as bichromic acid gelatin, polyvinyl carbazole, or the like, the following relationship is approximately established between its full-width at half maximum Δλ and center wavelength λ o : where T is the effective thickness of the diffraction grating (hologram), and d is the surface pitch of the grating. If the thickness T = 6 µm, the center wavelength λ o = 555 nm, and a mean refractive index = 1.65, the grating has the surface pitch d = 1.68 nm. As a result, Δλ = 16 nm. In this case, if the thickness of the hologram is decreased to about 2 µm to widen the full-width at half maximum to about 45 nm, since the modulation degree of the refractive index of the hologram photosensitive material is normally as low as Δn = 0.05, the maximum-diffraction efficiency is only about 8%. Thus, such a material cannot be put into a practical application. If the reflection type volume-phase diffraction grating used with the present invention causes light scattering more or less, this does not pose a functional problem, and it is more important to provide a high diffraction efficiency over a wide wavelength range. As a result of extensive studies, the present inventors found that a reflection type volume-phase diffraction grating which can provide a high diffraction efficiency over a wide wavelength range can be obtained by changing conventional exposure conditions and developing conditions. The diffraction device used with the present invention will be described more in detail by way of its example and a comparative example. (Example 1) 2.0 g of polyvinyl carbazole, 0.2 g of carbon tetraiodide, and 20 mg of 2,6-di-tert-butylphenol were dissolved in 25 g of monochlorobenzene, and the resultant material was spinner-coated (Mikasa Spinner, 1H-2) on a 1.3-mm thick glass plate with a polished surface in a dark place, and was dried in a dark place to prepare a 6-µm thick hologram photosensitive material layer. Printing was then performed using the optical system shown in Fig. 5. Light beams 7 and 9 were formed by monochromatic (bluish green) light having a wavelength of 0.488 µm from an argon ion laser 1, and a photosensitive material 11 was exposed with the light beams 7 and 9 at a light amount of 50 mJ/cm². The exposed photosensitive material was dipped in a trichloroethylene solution at 25°C for 5 minutes to remove an iodine compound component. Thereafter, the resultant material was dipped in a xylene solution at 36°C as a swelling solution (first bath) for 60 seconds, and was then dipped in an n-hexane solution at 36°C as a second bath for 60 seconds. Thereafter, the material was naturally dried. As a result, the volume-phase diffraction grating having the desired properties was obtained. (Comparative Example 1) For the purpose of comparison, a diffraction grating sample of the comparative example was prepared following the same procedures as in Example 1 except that the exposure amount was 6 mJ/cm². The diffraction grating had a peak wavelength of 520 nm, a full-width of wavelength at half maximum of 15 nm, and a peak diffraction efficiency of 50%, i.e., had a sharp peak. The reflection type volume-phase diffraction grating of the diffraction device prepared as described above Example 1 has advantages which cannot be obtained by those formed by the machine work method band the conventional holographic exposure method. More specifically, with the described reflection type volume-phase diffraction grating, various devices which can provide a high diffraction efficiency, i.e., one which is designed to diffract perpendicularly incident light in a 15° direction to one which has a diffraction direction as large as 80°, can be prepared. More specifically, in a state satisfying the Bragg condition, each light beam can satisfy a positive reflection condition on each surface of the volume-phase diffraction grating, and since layers which overlap in a depth direction serve to enhance reflected/diffracted light, a high diffraction efficiency of 80% or more can be realized with respects to any preset incident angles and reflection/diffraction angles. The described diffraction device also has advantages of the holographic formation method. That is, a grating can be formed on a curved surface, the grating itself can have focusing characteristics, and the like. Fig. 6 shows a head-up display system for a vehicle using the described diffraction device 100. In the diffraction device 100 a light beam 32 from a display element 31 is reflected and diffracted by a volume-phase type diffraction grating 12 on the substrate 10, the diffraction grating 12 having been formed by exposing and developing the photosensitive material 11 according to the method described with regard to Fig. 5. Since the diffraction grating 12 has diffraction characteristic shown in Fig. 4, light components within a wavelength range of 520 nm to 590 nm are effectively diffracted in diffraction directions according to their wavelengths. A primary diffracted light beam 33 diffracted by the diffraction grating 12 is reflected and diffracted by a reflection type volume-phase type diffraction grating (hologram) 35 on a transparent substrate 34 to be converted to a primary diffracted light beam 36. The light beam 36 is observed by a pupil 51 of an observer. The diffraction grating 11 has predetermined color dispersion characteristics, and reflects and diffracts a light beam incident from an about 20° direction in a (0°) direction substantially perpendicular thereto, i.e., has a function of canceling color wavelength dispersion caused by the reflection type diffraction grating 35. As a result, a display image by the display element 31 is clearly displayed in air. In this case, an optical path length between the display element 31 and the diffraction grating 11 is set to be smaller than that between the diffraction gratings 11 and 35, so that the system is made compact and a viewing zone of the pupil is widened. When the viewing position 51 of the pupil is changed to a viewing position 52 or 53, the center wavelength of the light beam diffracted by the diffraction grating 35 and incident on the pupil is also changed. In this case, when the described diffraction device 100 is used, a clear image can be obtained. Fig. 7 shows the relationship of wavelength characteristics of diffraction efficiencies of the primary diffracted light beams from the diffraction gratings 35 and 12 of the diffraction device 100 in the head-up display system shown in Fig. 6. Assume that the viewing position of the observer is largely changed by ±90 mm in the vertical direction according to the setting height and position of the observer (driver), and the center wavelength (555 nm) of a light beam diffracted by the diffraction grating 35 and incident on the pupil is varied between 545 nm and 565 nm. The wavelength characteristics of the diffraction efficiency of the diffraction grating 35 in this case are indicated by dotted curves in Fig. 7. In this case, since the diffraction efficiency of the diffraction grating 12 has flat characteristics and the full-width of wavelength at half maximum of the diffraction efficiency is larger than that of the diffraction grating 35, a uniform and bright display can be attained regardless of the pupil position. Another method of preparing the diffraction device 100 will be described below with reference to Fig. 8. A light beam having a wavelength of 457.9 nm from an argon laser 1 is diffused by a microscope objective lens 6, and is collimated to a parallel beam 7 by a collimator lens 120. A hologram photosensitive layer 11 formed on the transparent substrate 10 is placed in a vessel 14 storing an index matching solution 15. A window 13 is formed at an incident position of the parallel beam. The parallel beam 7 is incident on the hologram photosensitive material 11 through the transparent substrate 10, and is reflected by a reflecting mirror 16 arranged behind the substrate 10 to be re-incident on the hologram photosensitive material 11. The two light beams, i.e., incident and re-incident light beams interfere with each other to form interference fringes. In the optical system shown in Fig. 8, an inclination angle of a grating formed on the photosensitive material 11 by the interference fringes can be adjusted by adjusting an inclination angle of the hologram substrate 10. When the type of the laser 1 is changed, a wavelength used can be variously changed. Fig. 9 shows wavelength characteristics of the diffraction efficiency of the diffraction device which is obtained such that a hologram recording material 11 containing poly(N-vinyl carbazole) as a major component is coated on a transparent substrate 10 to have a thickness of 5 µm, is exposed through the optical system shown in Fig. 8, and is developed at a developing temperature of 32°C. In this embodiment, since almost no polarization characteristics are caused and a high diffraction efficiency can be exhibited for light in a wide wavelength range, a diffraction device comprising a reflection type diffraction grating suitable for an apparatus using natural light as a light source, e.g., the above-mentioned head-up display can be obtained.

Claims: A head-up display system, comprising a display element (31) emitting a light beam (32) and a relaying element relaying said light beam from said display element (31) to a diffraction grating (35) of a beam combiner which diffracts the relayed light beam (33) and directs it to a viewing position wherein said relaying element is a diffraction device (100) comprising a diffraction grating (12) characterized in that the full-width of wavelengths at half maximum of the diffraction efficiency of said relaying diffraction grating (12) is substantially larger than that of said diffraction grating (35) of the beam combiner and is not less than 8% of the value of the center wave length of said light beam (32) from said display element (31). A head-up display system according to claim 1, wherein the material of said relaying diffraction grating (12) includes polyvinyl carbazole. A head-up display system according to claim 1 or 2, wherein said relaying diffraction grating (12) comprises a volume-phase type diffraction grating.

Inventors: KUSHIBIKI NOBUO, KUWAYAMA TETSURO, TANIGUCHI NAOSATO, YOSHINAGA YOKO, KUSHIBIKI, NOBUO, KUWAYAMA, TETSURO, TANIGUCHI, NAOSATO, YOSHINAGA, YOKO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: G02B5
IPC codes: G03H1, G02B27, G02B5, B60K35