# Displays Based on Dynamic Phase-Only Holography
## 5 System Application  
The two methods of projection presented in Figure 6 are fundamentally different in terms of the device, optics, and projection image quality. Conventional displays employ imaging where a large size of micro-display is preferable in order to increase the etendue of the system. The NA of the optics adjacent to the micro-display is large for the same reason. In diffractive projection, the light throughput is, in principle, not limited by etendue considerations. Diffractive spread from the SLM to the screen creates the projected image. The complex projection optics of the imaging system is reduced to a Fourier lens. Since large diffraction angles allow the volume of the entire display engine to be reduced, a small pixel size is highly desirable. The quality of the projected image in an imaging system can be markedly influenced by pixel defects and inter-pixel gaps. In contrast, the projected image in a diffractive projection is fault tolerant with regard to the SLM and the inter-pixel gaps do not influence the image. Since coherent light is used in the diffractive system, there is a degree of speckle in the image. Figure 6: High performance, phase-only SLM. The product of the image size and the field of view (FOV) in a Fourier transform system is given by the product of the number of pixels in the SLM and the wavelength of the coherent light illumination [8]. For 3D direct view, two distinct approaches have emerged. The first is to increase the number of pixels in the SLM by using Active Tiling from a fast electrically addressed SLM onto an optically addressed SLM [8]. The second is to settle for the number of pixels in current SLMs to restrict the view presented to the eye using eye tracking and increased optics associated with the display [25]. For 2D display, based on the SLMs discussed in Section 3, the product of the image size and the field of view is still far smaller than the field of view that is commercially acceptable. Therefore, a two-stage design is adopted where the first stage results in the projected image on a diffuser and the second stage is an imaging system. The FOV of the diffuser is engineered to give the correct aspect ratio for the FOV of the final image. The image size is magnified in the imaging system so that the FOV*image size product is an invariant. The brightness of the magnified image is consequently equal to the brightness at the diffuser if there is no attenuation in the imaging system. This is the brightness given in our previous study [23]. In order to reduce the speckle in the final projected image, the diffuser is rotated. The design of the diffraction system is relatively simple in concept. A coherent light source is collimated and illuminates an LCOS device, which displays the phase-only hologram and the reflected beam is brought to a focus by a lens. No polarizers are required if the light source is polarized, e.g., a diode laser. Laser-based light sources offer an improved color gamut in comparison with white LED light sources. In order to provide a display that is competitive in the market place, a large effort is made in the system detail. This will be outlined in this section. The beam-shaping optics for the diode laser should be designed for high efficiency with minimum wave front error at the LCOS. It is often convenient to tilt the LCOS plane with respect to the collimated beam in order to avoid the use of either a beam splitter or an X-cube. The former attenuates the throughput and the latter is costly. In a conventional amplitude-modulating display, this would lead to a tilted image plane and additional complications for the optics downstream of this plane. The many-to-one correspondence again allows flexibility in the design of the hologram so that the image plane can be perpendicular to the optical axis. The major benefit of a phase only holographic head up display in comparison with an LED/TFT solution for a typical HUD specification is in the throughput and this was detailed in Reference [23].