## 4 Experimental Results and Discussion
First of all, let us analyze the transmission of a red image through a curved concentric waveguide combiner. The original image of the \"grid\" template is shown in Figure 16a. Let us transmit the original image through a waveguide, having DOEs with a constant period and without using the proposed correction method (Figure 16b). We will also transmit the same image through a waveguide with variable period DOEs obtained using the proposed method (Figure 16c). Then we compare the sharpness of the obtained out-coupling images in the meridional plane. To do this, we calculate the brightness distribution over the horizontal FOV in the meridional plane for all field components in the sagittal plane (Figure 16d). It can be seen from a comparison of the obtained profiles that when using the proposed method for compensating image aberrations in a curved waveguide, the vertical lines of the \"grid\" template have much higher resolution than for non-compensated case. Other examples of red, green and blue images transferred through waveguides optimized for corresponding colors are shown in Figure 17.  
As can be seen from Figures 16 and 17, the experiment demonstrated the achievement of equally high output image sharpness in the meridional and sagittal planes. The param  
Figure 16: Image of red test template (**a**), output images from prototypes of a curved concentric waveguides combiner without (**b**) and with (**c**) optimization, and their brightness distribution over the horizontal FOV in the meridional plane (**d**). eters of the in-coupling and out-coupling DOEs, the calculation of which was the main task of the present paper, as shown above, give the greatest impact to the sharpness of the transmitted images. So, it can be concluded that the correction method proposed allows achieving reliable virtual image quality when using cylindrical concentric meniscus as a waveguide combiner. The measured horizontal FOV of the out-coupled images was 28 \(\pm\) 1\({}^{\circ}\). The brightness across the FOV of the transmitted images is not uniform--their central part is brighter than the areas near the edges. This effect is due to the angular selectivity of the recorded HOEs. As was shown above, the angle of incidence of the reference beam was chosen close to the angle of propagation of the central component of the FOV in the waveguide. Thus, the maximum of the angular selectivity contour of diffraction efficiency was located in the center of the transmitted FOV. As a result, the in-coupling/out-coupling of the image to/from the waveguide occurred with greater efficiency for the central part than for the edge parts of the angular FOV. At the same time, due to not very high peak diffraction efficiency (DE) of the gratings (10-15%), their intensities at the maxima of the angular selectivity contours and outside them differ slightly, which makes it possible to observe the waveguide-limited FOV both with the camera and with the eye. Thus, the selectivity of the gratings does not restrict the transmitted FOV; however, it introduces nonuniformity of image brightness over the field of view. The problem of Bragg selectivity can be solved either by using multiple exposure recording (i.e., recording several superimposed holograms with Bragg angles distributed over the FOV) or by switching to relief-phase gratings, whose  
Figure 17: Images of red, green and blue test templates (**a**,**c**,**e**,**g**,**g**,**i**) and output images from a prototype of a curved concentric waveguide combiner with a curvature radius of 150 mm (**b**,**d**,**f**,**h**,**j**). selectivity is much lower. In addition, it was noticed in the experiment that brightness along the EMB is also non-uniform--the out-coupled image has a higher brightness near the incoupling region of the waveguide and a lower one at a distance from it. This nonuniformity is due to several factors. First, the DE of the out-coupling grating is set to be constant in the meridional plane, so less energy remains for out-coupling in the region far from the in-coupling.