Dear, friends
Sharing our experience as optical engineers specializing in augmented reality design, 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.
What is Image Source Size & Resolution
In Augmented / Virtual Reality Systems, a Micro-display Image Source is projected using optical system to infinity or a finite distance, when the optical system EFL (Effective Focal Length) will determine the resulting FOV. The Image Source size is translated into the resulting FOV, and the Image Source resolution is translated into the smallest possible detail that can be projected to the user’s eye.
In general, optical systems are built from optics, that can be divided in 2 main blocks: a relay lens and an image source, i.e. a display (or sensor in case of camera). During the Optical Design process, we design the relay lens and integrate an image source into this design, while the design requirements correspond to the system requirements on one hand and to the image source parameters on the other. Here we concentrate on the image source specifications and their impact on the resulting image.
Micro-display Image Source in XR Systems
In Augmented / Virtual Reality Systems usually micro-displays are used, because of the incentive to minimize the system size / volume / weight etc. Micro-display is usually specified by its diagonal size and resolution abbreviation, for example:
0.6inch VGA display
Since the Image Source is a key component of the AR / VR system and its impact on the resulting system performances is very significant, we feel that the emphasis on the micro-display integration as part of the system design is crucial and the specifications shall be in accordance.
Micro-display role in Pixel Resolution
In particular, the combination of Image Source size &resolution determine the smallest possible detail that can be projected to the user’s eye, which is connected to the Pixel Resolution term. Pixel Resolution is defined in [mRad /pixel] – which is simply put by the angular size of the smallest image detail (when the image is projected to infinity).
Image Source Resolution limits the highest resolution that can be displayed (this term is also known as the Nyquist Frequency, which is specified in cycles/mRad or lp/mRad).
System Resolution
The overall system image quality (or CTF) will determine whether the Nyquist Frequency will be resolvable by the user, so if the CTF for the Nyquist Frequency is sufficiently high - then System Resolution will be the same as the Nyquist Resolution. In general, there are more factors that have impact on System Resolution, such as projected image quality, display contrast, system contrast, even electronic noise... We concentrate here only on the micro-display structure.
What the Image Resolution Looks Like?
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.
Image Source Size &Resolution Specifications Insights
Our addition to the Image Source Size & Resolution specification (rarely found in general specification documents):
Image source size shall be specified in [mm], in both Horizontal and Vertical axes. This adds the information about the image Aspect Ratio which can be significant for adjusting the projected image format.
Image source resolution shall be specified in both Horizontal and Vertical axes as well. This adds the information about the image Aspect Ratio which can be significant for adjusting the projected image format.
Pixel Resolution shall be specified in mRad/pixel.
In addition, image source pixel size shall be specified in [mm], with detailed sub-pixel's structure and arrangement information, preferably with illustration.
Things to Consider in Image Source Size &Resolution Requirements
Here are several different cases when system Image Source 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 contains symbols or text which is combined with direct scene view, and accurate symbol position on real world details is necessary, higher image resolution brings the required added value.
In case of a mixed reality system, when the projected image is combined with a direct viewing camera / night vision system, the projected image resolution shall match the direct viewing image resolution so that there is no resonance that can cause the user some uncomfortable effects.
In case of a virtual reality system, when a full video image is projected, a high Primary FOV resolution creates a crispy and sharp image (provided the system CTF is sufficient), but the Peripheral FOV resolution can be dramatically reduced, since the human eye can only resolve in a very narrow central FOV.
Image Source Size &Resolution Requirement Example
Our definition of Image Source Size &Resolution (example):
Image Source Size: 12mm Horizontal X 9mm Vertical; 15mm Diagonal
Image Source Resolution: 800x600 pxls
Pixel Resolution: 0.45 mRad/pixel
Pixel Size: 15µm x 15µm, sub-pixels structure: RGB 5m x 15m
Image Source Sub-Pixel Structure - RGB