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Sharing our experience as optical engineers specializing in augmented reality, 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.
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What is Eye Relief (Exit Pupil Distance)
In Augmented / Virtual / Mixed Reality Systems, a Micro-display Image Source is projected using optical system. This is done using optical software, such as CodeV, Zemax OSLO etc.
One of the design input parameters is Eye Relief, which is defined as a distance from the center of the system pupil along the optical axis to the first optical element. Another name for this term is Exit Pupil Distance.
From the user’s perspective, Eye Relief is the distance from the eye, the cornea vertex, to the plane where the first optical element shall be positioned. This distance is linked to the Eye Clearance, in case the optical element is flat, and is constant through the entire System Pupil, that can be fairly large. However, when the first element is curved and /or tilted, the Eye Clearance distance depends on the size and shape of the first optical element, as shown in the image below.
An explanation for different terms, their source and meaning are due to clear miss-understandings and create a common language.
Eye Relief is a term used in optical design, it is a definition, an input to the design, hence it is defined on the optical axis at the center of the System Pupil. This definition is also equivalent to Exit Pupil Distance.
Eye Clearance is a term from the user’s point of view, which defines the clear space where other elements – optical or mechanical elements - can be placed, for instance, prescription glasses.
Eye Relief Specification
Eye Relief is specified in [mm]. Usually, the Eye Relief is defined as nominal distance:
Eye Relief: 25mm
The Eye Relief distance is defined based on the other system elements, constrains and user’s need, correlating with the Eye Clearance requirements. Many peripheral objects, that are not strictly a part of the system, will drive this requirement or at least have an impact on it: for instance, adding an element between the user’s eye and the first optical element, such as: prescription glasses, protective visors, clip-on elements, mechanical fixtures, head mounting fixtures etc. or just taking into account the user’s physical structure: nose, eyebrows, cheeks etc.
Our addition to the Eye Relief specification:
Eye Relief shall be defined as a range, and not a single value, when this range is a part of the System Eye Box or Eye Motion Box or System Pupil Space (more on this will be detailed in a separate post dedicated to System Pupil).
Eye Relief data shall include a tolerance.
Eye Relief is not only a specification performances parameter, but also a definition for other specification performances parameter, such as CTF (or MTF), Field Curvature and others, so we define the Eye Relief range for specifying other parameters requirements.
Things to Consider in Eye Relief Requirement
So, Eye Relief requirement especially in the AR system design, shall be derived from the Eye Clearance requirement which depends on the system’s intended use, the use-case and other peripheral constrains. 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 means that the whole design would start over. Here are several different cases and considerations for the system Eye Relief requirements that should be tailored to fit the use-case scenarios in order to create an optimal design:
In general, the smaller the Eye Relief, the smaller would be the system size and form factor, so the requirement shall be minimal fit for the systems’ comfortable use.
In Augmented Reality Systems (Monocular or Binocular) based on small elements in front of each eye, where prescription glasses are required – a rule of thumb for the Eye Clearance requirement is ~25mm for comfortable use.
In case clip-on prescription lenses can be used instead of the regular glasses, the Eye Relief can be reduced to ~20mm and still provide a comfortable experience for the user.
The opto-mechanical design and the headset design shall support placing the system in the correct Eye Relief, so all the system performances are as designed. This is easier to achieve when the headset is firmly attached to the user’s face, as done in most Virtual Reality products.
Sometimes, particularly in Military applications, an eye shield / protective visor has to be used for protection reasons and this creates a constraint for the Eye Relief requirement to be significantly larger, for instance ~50mm thus enlarging the first element and the whole system size.
Another type of constraint is mounting the system on a helmet which created opto-mechanical interfaces, mounting features and also has an impact of Eye Relief requirements.
Eye Relief Requirement Example
Our definition of Eye Relief:
Eye Relief nominal: 25mm ±1mm
Eye Relief range: 20mm - 30mm
Eye Clearance: ≥23mm
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