# Development of a Holographic Waveguide with Thermal Compensation for Augmented Reality Devices
## 4 Experimental Results and Discussion
However, because the green color spectrum initially contains a large set of wavelengths with maximum radiation intensity, then thermal compensation will be provided for the entire wide spectrum of radiation from the green radiation source, i.e., radiation at all wavelengths of green color will come out perpendicular to the surface of the holographic waveguide. In the case of using a radiation source with a narrower radiation spectrum, the thermal compensation condition will also be fulfilled, as in the case of blue and red wavelengths. As a result, the following dependences of the angular displacement of the output image on the wavelength were obtained, as shown in Figure 16a. For the red and blue wavelengths, a slight angular displacement of the output image is observed--about \(\pm\)0.5 angular minutes with a shift of the dominant wavelength by \(\pm\)10 nm. For the spectrum of the green source, several transmission spectral filters were used at the input of radiation into holographic waveguides in order to obtain pronounced maxima within a wide spectrum of the radiation source. In this case, the angular displacement of the test line image was \(\pm\)1 angular minute. This change is extremely small, so we can talk about the effect of thermal compensation of this scheme. In Figure 16b shows the dependence of the angular displacement on the change in wavelength for the scheme indicated in Figure 1. As can be seen from the graphs, the angular displacement of the image in the output region is more than one degree for all wavelengths, which indicates the absence of thermal compensation. In practice, an angular displacement of 1 degree at an image output distance of 0.6 m from the user corresponds to a linear displacement of 10 mm, and for a distance of 1.5 m-about 25 mm. Of course, this angular displacement is not very large, but, as we noted, it may be significant for some applications. For example, for the field of medicine, when in augmented reality mode, three-dimensional models of his organs, arteries or veins can be superimposed on the patient's body during surgical operations to remove tumors or when administering medications (injections). The signature really did not sound quite right and was corrected. As can be seen from Figure 16, the above scheme for implementing a holographic waveguide based on relief-phase diffraction gratings (DOEs) of the same period, two of which are located at an angle of 60 degrees relative to each other, provides almost complete compensation for the angular displacement of the output image when the wavelength of the radiation source changes, which can occur, for example, when the temperature changes environment. It is important to note that a slight angular displacement of the output image was observed only in the vertical plane, i.e., in the plane perpendicular to the strokes of the output grid DOE4, while no image displacement was detected along the horizontal axis. Indeed, angular displacement in the vertical plane, i.e., in the plane perpendicular to the grating strokes on which diffraction occurs, is present. Most likely, this is due to inaccuracies in the manufacture of DOE (angular mismatch of DOE1 and DOE2) on the recording stand, but the permissible value of these inaccuracies was not evaluated in this study, because the obtained values of angular displacement are quite small. In the horizontal plane, i.e., in the plane of the grating strokes, this effect is not observed. Resolution, the size of field of view, brightness and unevenness of the image brightness within the eyebox, there is no noticeable deterioration in image quality (it only shifts). This is due to the fact that the diffraction gratings that we manufacture are relief-phase [30], i.e., they do not have spectral selectivity and will work equally with all incident beams of rays (deflect them, remove them from the substrate, etc. ), regardless of the displacement of the dominant wavelength.