Optical Terms Library for AR/VR/MR Systems – Spectrum &Polarization
Sharing our experience as optical engineers specializing in #augmentedreality, me and my partners in JOYA Team want to create a common language, a database that can be shared and used by anyone who wishes to learn and understand the specifics of augmented &virtual reality systems – our optical terms library. If there is a term you want to learn about - leave a comment and we promise to give our interpretation of this term.
The next term is Spectrum &Polarization
In Augmented / Mixed Reality Systems, a Micro-display Image Source is projected using an optical system to infinity or a finite distance and super-imposed on an outside scene either directly viewing of through camera / night vision system. The Image Source Spectrum in combination with optical materials and coatings determine the projected color properties, when the Image Source Polarization in combination with optical materials and coatings affect the projected image contrast ratio as perceived by the user’s eye.
Commonly, in Augmented / Virtual Reality Systems micro-displays are used because of the incentive to minimize the system size / volume / weight etc. Micro-display spectrum is usually specified in a vague way, the details are rarely provided. So, it’s common to see only the general display type:
White LED LCD Display / OLED Display / RGB Laser Diode LCoS Display
The Image Source Spectrum plays a major role in the output AR / VR / MR system image color. Various micro-display technologies have significantly different output color space, so much more attention should be paid to the micro-display technology trade-off and integration in the optical design, non-imaging and illumination design as part of the system design. This is translated directly to the user experience and expectations of real-world objects color perception.
To learn more about the different micro-display technologies, read our article at: https://www.oled-info.com/microdisplay-technologies-ar-and-huds
Our addition to the Spectrum &Polarization specification:
Image source spectral curve shall be specified as a normalized weight versus wavelength curve in [µm] or [nm]. For a Gaussian spectral curve, it’s sufficient to provide only the peak wavelength and the spectral width (FWHM: Full Width at Half Maximum).
If the image source is built of sub-spectrums, as in LCD sub-pixels, the spectral curve for each sub-spectrum shall be provided.
Image source polarization shall be specified, when both polarization type and orientation vector shall be detailed, preferably with illustration.
The system resulting image spectrum, as seen by the user, shall be generally defined as the u',v' (or x,y) prime color coordinates (RGB) on the CIE Color Chart. This provides the information about the system resulting color space.
We can illustrate the typical resulting color space triangles of different micro-display technologies on the CIE Color Chart. Basically, the display image can contain only the colors within the triangle, with prime RGB colors at the triangle vertices. For those familiar with the CIE Chromaticity Diagram, this provides a good qualitative comparison between the different micro-display technologies.
For those less familiar with CIE Chromaticity Diagram, we present a more visual qualitative comparison between the different micro-display technologies prime colors:
The Image Source Requirements strongly depend on the system’s intended use, when the projected image type and characteristics have a significant impact on the specific requirements. Here are several different cases when system Image Source Spectrum &Polarization should be tailored to the use-case scenarios in order to create an optimal design:
In case of an augmented reality system, when the projected image is combined with bright direct scene view, the scene illumination causes the projected image colors to appear washed out and the color space is significantly reduced. Each prime color brightness of the projected image must be high enough to maintain discernable color. Some of the systems use local or global outside scene dimming in order to enhance both the image contrast and color.
Another aspect of augmented reality systems is image polarization, which shall be defined, as well as the polarizing effects of see-through scene view. The system shall not interfere with user’s ability to see other displays or information sources properly. Some of the use cases involve user’s wearing sunglasses or other potentially polarizing elements.
In case of a mixed reality system, when the projected image is combined with a direct viewing camera / night vision system, the projected image color space shall match the direct viewing image colors so that there is no significant difference between the two channels in terms of real-world familiar objects color perception that can impact the user experience.
In case of a virtual reality system, when a full video image is projected, the impact of image color and polarization is much lower, when there is no comparison to the real-world objects. The human vision is highly comparative and sensitive to contrast, so the color space image characteristics can be compromised without or with low impact on user experience.
In all system types, the image source polarization has a significant impact on the projected image Contrast Ratio, hence the ability to discern different image grey levels by the user’s eye as well as on the See-Through Contrast Ratio and the, so called, “curtain” effect. We will discuss in detail the Contrast Ratio term in our future posts.
Our definition of Image Source Spectrum &Polarization (monochrome display example):
Image Source: monochrome Green OLED Display
Image Source: Peak Wavelength: 0.525μm; FWHM: 0.040μm
Image Source Polarization: un-polarized
Provide full spectrum curve or each sub-pixel spectrum
Provide polarization axis orientation (where relevant)