FIGS. 1A and 1B schematically show light 5 entering from the left into a diffractive optical element 10 comprising a diffractive layer 11 having a repeated diffraction spacing, d. The light 5 is transmitted through the interior of the diffractive optical element 10 by total internal reflection and is diffracted by the diffractive layer 11. When diffracted, some of the light 5 is out-coupled from the diffractive optical element 10, as represented by arrows 15. The angles θn of the out-coupled light for each diffracted order n are determined by the wavelength λ of the light source and the repeated diffraction spacing d of the diffractive layer 11 according to the well-known equation:d(sin θn+sin θi)=nλ  [Eqn. 1]where θi is the angle of incident light, n is an integer, θn is the angle of the diffracted light and λ is the wavelength of the light. As shown in FIG. 1A, incoming light 5 entering the diffractive optical element 10 at smaller angles generally travels a lesser distance by each total internal reflection before being out-coupled by diffraction, so that the out-coupled light 15 is mostly concentrated near to where the light has entered the diffractive optical element 10, fading out rapidly from left to right. On the other hand, as shown in FIG. 1B, incoming light 5 entering the diffractive optical element 10 at larger angles can travel a greater distance by each total internal reflection before being out-coupled by diffraction, so that the out-coupled light 15 is mostly concentrated far from where the light has entered the diffractive optical element 10. Similar considerations would apply in a left-right mirror image if the incoming light 5 were instead to enter the diffractive optical element 10 from the right in FIGS. 1A and 1B, rather than from the left.
FIG. 1C schematically shows a pair of such out-coupling diffractive optical elements 10L, 10R, each of which comprises a respective diffractive layer 11L, 11R. The out-coupling diffractive optical elements 10L, 10R are arranged in optical communication with an in-coupling diffractive optical element 30 comprising a similar diffractive layer 31 to that of the out-coupling diffractive optical elements 10L, 10R and having the same repeated diffraction spacing d. The in-coupling diffractive optical element 30 receives incoming light 5, which is diffracted by diffractive layer 31 and transmitted by total internal reflection to the out-coupling diffractive optical elements 10L, 10R.
FIG. 1C also schematically shows eyeballs 81, 82 of a viewer gazing at the out-coupling diffractive optical elements 10L, 10R. As may be understood from FIG. 1A, for a viewer looking to the right, as shown in FIG. 1C, the out-coupled light 15 will therefore have lowered brightness in the region A, whereas light in the region B will fall outside the gaze of the viewer and be wasted. On the other hand, as may also be understood from FIG. 1B, for a viewer looking to the left, the out-coupled light 15 will similarly have lowered brightness in the region B and will fall outside the gaze of the viewer in region A and be wasted.
FIG. 2A schematically shows a pair of components 1, 2 of an apparatus for out-coupling polychromatic light, for use, for example, in a binocular near-eye display. By polychromatic light is meant light of at least two different wavelengths. The components 1, 2 each have left and right halves which are mirror images of each other, configured to out-couple light to a pair of eyes. Component 1 therefore comprises a pair of out-coupling diffractive optical elements 10L, 10R, a pair of corresponding in-coupling diffractive optical elements 30L, 30R, and a pair of intermediate optical elements 51, 52, which respectively direct light from the in-coupling diffractive optical element 30L to the out-coupling diffractive optical element 10L and from the in-coupling diffractive optical element 30R to the out-coupling diffractive optical element 10R. Component 2 similarly comprises a pair of out-coupling diffractive optical elements 20L, 20R, a pair of corresponding in-coupling diffractive optical elements 40L, 40R, and a pair of intermediate optical elements 53, 54, which respectively direct light from the in-coupling diffractive optical element 40L to the out-coupling diffractive optical element 20L and from the in-coupling diffractive optical element 40R to the out-coupling diffractive optical element 20R.
Components 1 and 2 differ from each other in that the in- and out-coupling diffractive optical elements 30L, 30R, 10L, 10R of component 1 have a first repeated diffraction spacing, d1, whereas the in- and out-coupling diffractive optical elements 40L, 40R, 20L, 20R of component 2 have a second repeated diffraction spacing, d2, which is different from the first spacing, d1. Components 1 and 2 can therefore provide respective channels of optimized efficiency for diffracting light in two different wavelength bands. For example, component 1 may provide a channel for red light and component 2 may provide a channel for green-blue light. Thus if components 1 and 2 are superposed one on top of the other, as is schematically represented in FIG. 2B, and if the optical elements of each component are carefully aligned, polychromatic light from a single display can be projected into the in-coupling diffractive optical elements 30L, 30R, 40L, 40R. Light in two different wavelength bands will then be out-coupled from elements 10L and 20L and will re-combine to provide polychromatic light to a viewer's left eye, whilst light in two different wavelength bands will also be out-coupled from elements 10R and 20R and re-combine to provide polychromatic light to the viewer's right eye. A similar arrangement can be used in a monocular near-eye display to out-couple polychromatic light to a single eye by using only a left or right half of each of the components 1 and 2.
The components 1, 2 may each be understood as being similar in construction and function to the in- and out-coupling diffractive optical elements described above in relation to FIG. 1C. Therefore, they suffer from the same problems as were explained above in relation to FIG. 1C. Typically, for example, if the components 1, 2 are incorporated into a binocular near-eye display, they will provide a field of view of less than about 40 degrees. The “eye box” can be increased by scaling the size of all of the optical elements, starting with the display, but this is undesirable, from the point of view not only of cost, but also of wearability. On the other hand, a much wider field of view would be highly desirable, considering that the natural field of view of a healthy human can extend beyond 180 degrees in the horizontal direction.