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 AID (Apparent Image Distance)
In Augmented / Virtual / Mixed Reality Systems, a Micro-display Image Source is projected using optical system to infinity or a finite distance. This means that a virtual image is produced, in other words, the optical system creates rays that our brain interprets as an image located at the rays intersection point. But where this image is located in space? Apparent Image Distance is the distance at which the virtual image is seen by user’s eye.
AID is a commonly used term in military applications (HMD or HUD systems) where the user’s objects of interest in the scene are located at infinity so the projected image must also appear at infinity. When the AID of the two images matches, both images are in focus and no eye accommodation is needed to switch between them. Another resulting effect of the 2 images non-matching AID is the parallax effect that has high impact on projected symbol position accuracy on the outside scene, which is very important in military applications (more on this will be detailed in a separate post dedicated to Parallax).
AID is specified in [meters] or in [Diopters], which is the reciprocal of meters. In most systems the definition you see is nominal AID:
The AID is a tricky term, because for human eye, the objects at a distance above 6-8 meters are perceived as located at infinity, or at 0 to -0.17Diopters, but if the AID in diopters is positive, the eye sees diverging rays and creates an image behind the eye instead of being in front of it. This causes nausea, headache and eye fatigue, and is highly problematic. So, the safe range for the AID is always negative and the nominal AID: infinity is also the end of the safe range. Thus, this design parameter should be treated with extra care.
This is the case of monocular system, but what happens when we have a binocular system?
In natural binocular vision, our eyes and brain act together and when we’re looking at an object at a certain distance, there are 2 effects that create our binocular vision with depth perception, or our 3D vision:
There is a conversion between the 2 eyes, and the angle between the eyes is interpreted in the brain to perceive the object distance.
Each eye accommodates to the object distance by slightly changing the optical power of our eye lens (using eye muscles), so the best focus vision is adjusted to the object’s distance.
When we are trying to create the 3D image and depth perception artificially, the first effect is easily created by projecting a slightly shifted image for each eye. Doing this we can create a conversion to any desired image distance. The thing is, that the second effect is much more complicated to create, since we have to introduce moving or accommodating optical elements. This is very costly in terms of system complexity, size and cost. So, in most cases depth perception is created only by conversion. This creates the desired 3D effect, but since this is not natural to our vision, the effect called VAC (Vergence - Accommodation Conflict) is caused. The user’s eyes and the brain will be at constant conflict between the position of best clarity image, or the system AID, which is fixed and the physical rays interception point due to conversion, which is changed for different image positions. This will cause eye fatigue and headache after long use.
Our addition to the AID specification:
AID shall be defined as a range and not one value, specified in meters and Diopters (for clarification).
AID shall be defined for a given spectrum and measurement pupil size (more on this will be detailed in a separate post dedicated to System Pupil).
AID requirements shall be defined for a Center FOV, nominal Exit Pupil Distance and at the Center of System Pupil.
AID requirements shall be defined separately for different FOV areas. This is also known as Field Curvature, which is the measure of the image non-flatness.
AID requirements for binocular system shall specify the difference between the eyes.
The AID Requirements strongly depend on the system’s intended use. This parameter is a design parameter for the optical design so it must be set as early as possible, since changing it in the design process may mean that the whole design would start over. Here are several different cases when system AID 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 direct scene view, the distance of the objects of interest in the scene will determine the projected image AID requirement. In case that the objects of interest are located at close distances, the projected image AID should match this distance. However, the image projected to close distance will introduce strong parallax effect (more on this will be detailed in a separate post dedicated to Parallax). If high range distances need to be covered, it can only be achieved by using moving or adjusting optical elements.
In case of a mixed reality system, when the projected image is combined with a direct viewing camera / night vision system, and the same optical system is used for both images projection, the AID can be set to a comfortable looking distance and the objects sizes can be adjusted accordingly.
In case of a virtual reality system, the same approach can be used, setting the AID to a comfortable looking distance and the objects sizes can be adjusted accordingly.
In all AR/ VR/ MR binocular systems, it’s very important to consider the human vision characteristics, thus each eye image AID shall be matched with the Conversion between the 2 eyes in order not to produce the effect called Vergence - Accommodation Conflict. (more on this will be detailed in a separate post dedicated to Prismatic Deviation)
Our definition of AID (example):
AID for Center FOV, Center System Pupil, Spectrum, Nominal Exit Pupil Distance @ Measurement Pupil: >10m (0 to -0.1D)
Provide Field Curvature Requirements Table (see example):