A head-up display (HUD) is a transparent display that shows the background view overlaid with other essential information. This helps a driver/pilot to keep their viewpoint at the front, without having to look at a separate display or dashboard. To achieve this, a HUD should be a transparent display and be able to reflect/project light from other directions.
This example demonstrates a multi-stack coating that provides high reflection at three specific wavelength ranges, which could match the RGB spectrum of a display, while transmitting other wavelengths. We start from micro-scale optics using STACK and then export data for Speos and Zemax (OpticStudio) ray-tracing tools.
The dielectric film is composed of repeated periods of two materials with a refractive index (n1, n2). We design appropriate phase differences such that all reflected lights at the interface could match the constructive interference condition. As a result of this, reflection is then enhanced.
With this approach, we could design a reflector for a particular wavelength band. However, it is transparent for wavelength outside this band. In order to reflect light at R,G,B three bands simultaneously, we would cascade three dielectric stacks. Each of them reflects a particular band. (R : 632nm, G : 535nm, B : 450nm) Additionally, the order of the sub-stacks (e.g. R,G,B or G,B, R…etc) can affect the overall response because there is some reflection outside the stopbands of the individual filters.
The script contains three parts. The first part defines the materials and simulation settings. For example, the target wavelength range, incident angle, etc. In the second part, we call the STACK simulator to calculate the reflectance and transmittance, including all coherent effects, for both S & P polarization over the specified wavelength and incident angle range. In the final part, we plot results and export the results to a Speos file format (with .coated extension) and a Zemax file format (with .txt extension).
- Open the script file Lumerical-STACK-Speos-Zemax_Reflector_R_filter.lsf and run it.
This plot illustrates the refractive index of different layers. As the above picture shows, the final structure contains three parts. Each part contains ~100 layers and it could respectively reflect specific wavelength. It should be mentioned that we use refractive index apodization in this step. Apodization is a well-known method to reduce transmission spectrum sideband ripples.
The following figure shows the result of STACK which illustrates the angular and the wavelength dependence of the reflectance. (left: RP, right: RS) It is obvious that we achieve high reflectance in three wavelengths at the angle 30 we set in the simulation.
We can also check the spectrum at this angle.
Export a file for Speos
The script generates a “.coated” file automatically. The data format is the following:
Export a file for Zemax
The script also generates a .txt file that can be read into Zemax OpticStudio. Documentation on this file format can be found in the Zemax OpticStudio help file at: Zemax OpticStudio Help File > The Libraries Tab > Coatings Group > Defining Coatings > The TABLE Data Section.