# Image Quality for Near-Eye Display Based on Holographic Waveguides
## 4 Discussion  
Interesting results were obtained during the experiments concerning the arrangement of individual waveguides (for RGB colors) in the stack. There are two options for incoupling conditions: the photoresist layer with DOEs is located in the direction of light incidence or the light is coming from the substrate. At the same time, inside the stack, the waveguides can be arranged in different ways to protect the photoresist layer from mechanical influences, as, for example, shown in Figure 10a. The quality of the reproduced image will be affected by the combined structure appearing while a recording of superimposed input DOEs, as shown in Figures 6-9. The light in-coupling in two possible arrangements in relation to the input light is illustrated in Figure 10a,c. When the light diffracts on the DOE in the reflection mode (structure glass-resist) it is associated with a greater change in the optical path difference (Figure 10a). In the other case when the light diffracts in transmission mode (structure air-resist) it has a smaller optical path difference. Figure 9: Fourth configuration of the holographic waveguide: (**a**) DOE configuration scheme; (**b**) photo of a single holographic waveguide; (**c**) resulting AR image. The stack configuration of Figure 10a provides higher diffraction efficiency and, accordingly, increased brightness of the image, as illustrated by the photo in Figure 10b. Thus, the angular field of view is high and the chromatism is low, but the bright stripe is a significant artifact. In this connection, it is preferable to choose the arrangement of all the RGB waveguides as in Figure 10c when the side of the photoresist layer faces the input light. An example of the resulting image is shown in Figure 10d. However. this arrangement requires an additional cover substrate to protect the photoresist layer. The most promising implementation for analog holographic waveguides in terms of best uniformity of image brightness is the configurations shown in Figure 9. Its optical efficiency is higher than that of other configurations considered. The holographic waveguide is divided into five DOE areas: two for the in-coupling of the light, two for horizontal exit pupil expansion, and one for the out-coupling of the light towards the operator's eye and vertical exit pupil expansion. In the first area, two separate gratings (DOE1a and DOE1b) are recorded at an angle of plus and minus 60 degrees relative to each other, i.e., the input area is physically divided into two parts by the front, each responsible in-coupling of the linear part of input pupil. In the relay areas, one-dimensional diffraction gratings are recorded, for the right and left parts with orientation angles of \(-60^{\circ}\) (DOE2a) and \(+60^{\circ}\) (DOE2b), so that the radiation diffracted on it is directed downwards toward the output diffraction grating (DOE3). For one spectral channel the period of all three DOEs on the waveguide plate is the same, i.e., only the orientation of the lines differs. The range of 100-150 nm of relief depth for DOEs is experimentally confirmed for better resulting non-uniformity conditions. One cannot strive for maximum diffraction efficiency due to several factors that can affect image quality along with it:  
Figure 10: Options of DOE arrangements in relation to the input light representing the axial vertical section of the waveguide: (**a**) on the substrate side; (**c**) on the photoresist side. Examples of resulting images based on the holographic waveguide: (**b**) for scheme (**a**); (**d**) for scheme (**c**). 1. Different field angles diffract with different intensities and travel different paths in the waveguide, and therefore acquire different losses until they are outputted from the waveguide. The lower diffraction efficiency in this case is designed to average the discrepancy between the brightness unevenness of the image at the output. 2. As long as the light is unpolarized, the intensities mismatch between perpendicular polarizations should be considered. 3.