# Development of a Holographic Waveguide with Thermal Compensation for Augmented Reality Devices
## 4 Experimental Results and Discussion  
To conduct experimental studies of the proposed structure of a holographic waveguide on the issue of providing thermal compensation, the following experimental scheme is proposed. As a test image, a grid image of blue, green and red on a black background is used, as shown in Figure 11. The image was generated using the optical engine, which is a miniature image projector based on a DPL matrix with LED backlight. The radiation from three RGB LEDs is combined into one branch to obtain white color using dichroic mirrors, and for the best mixing of the three colors, a microlens raster is additionally used. The test image is output using a holographic waveguide. The photo of the output image is shown in Figure 12.  
To analyze the change in the angles of radiation output from the substrate when the dominant wavelength of the source radiation changes, which accordingly leads to an angular displacement of the output image on a holographic waveguide (all three substrates were stacked), a collimator with a reference grid was used, as shown in Figure 13.  
The image output through the waveguide is recorded using a collimator, and its angular displacement is estimated in angular minutes by a reference grid located in the focus of the eyepiece, as shown in Figure 14. In this case, the initial position of a separate line of the test image is fixed on the collimator grid, and then the angular displacement of the position of this line is measured already when the wavelength of the radiation source (LED or laser diode/display) changes. Figure 14 shows examples of test images, namely: Figure 14a is an image of RGB grids reproduced using a holographic waveguide to evaluate its performance, and Figure 14b,c are images of the grid reproduced \"before\" and \"after\" the LED wavelength shift. In the case of a wavelength shift, a small deviation of the test image of the grid relative to the measuring scale is observed, i.e., angular displacement of the image output through the holographic waveguide. Thus, it is shown that the operation of a holographic waveguide, and, accordingly, the position of the displayed image, depends on the change in the wavelength of the radiation source, but can be partially compensated by a certain structure of the arrangement of diffraction gratings on its surface. The change in the wavelength of the radiation output from the holographic waveguide was recorded by the magnitude of the change in the dominant wavelength of the radiation spectrum using the Jeti Spectraval 1501 spectroradiometer, as shown in Figure 15.  
Figure 11: Test Images. Figure 12: An image observed using a holographic waveguide. Figure 14: (**a**) Test image observed against the background of the collimator grid. (**b**) Wavelength shift is 0.5 angular minutes. (**c**) There is no wavelength shift. Figure 13: The scheme for determining the angular displacement of the output image. 1–3—collimator (2—measuring grid); 4—waveguide; 5–18—optical engine (13, 16, 18—LEDs; 8, 12, 15, 17—lenses; 10, 11, 14–mirrors; 9—microlenses; 6—prism; 7—display; 19—filter only for green LED). Figure 15 shows the emission spectra of the RGB LED radiation source used as part of the optical engine. The dominant wavelengths correspond to 457 nm, 520 nm and 628 nm (for the red wavelength, an example of a shift of the dominant wavelength by 10 nm as a result of a temperature change of 15 \({}^{\circ}\)C is given). The normal ambient temperature was 20 degrees, and then it was changed by plus / minus 15 degrees Celsius. At the same time, it should be noted that for the radiation source used, the width of the red and blue spectra is quite small and reaches 12-15 nm, which really allows us to talk about a change in the dominant, i.e., the main wavelength in the spectrum. In the case of a green radiation source, the spectrum width reaches almost 100 nm, so a change in the dominant wavelength by an amount of about 10 nm will not significantly affect the angular displacement of the output image.