Optical Terms Library for AR/VR/MR Systems – System Pupil

Dear, friends

Sharing our experience in optical design and engineering specializing in augmented reality design, me and my partners at 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. This post is about System Pupil. 

The XR challenge

In Augmented / Virtual / Mixed Reality Systems, an image source is projected using an optical system to a certain working distance to be viewed by the user’s eye.

The optical system is wrapped in some kind of head-worn device which can be divided into different types of form factor:

• HMD (Helmet Mounted Display) which is used in Defense for pilots for example.

• A smaller form factor headset or harness, such as used in Industrial or Medical fields.

• Smart Glasses form factor, looking very similar to regular prescription or sunglasses form factor in Consumer, Sports fields of applications.

In any case, the whole system lays on the user’s head. So, this fact creates an engineering challenge, since every head shape is different, the head features used as interface, such as forehead, ears, temples, nose bridge, have large variation statistics. This challenge is amplified in binocular systems, where the position of each eye relatively to the selected interface is important and naturally it varies a lot between different people.

There are extended studies of head features measurements and their statistical distributions, mostly performed in military field for various applications.

One of the parameters that is widely used in these studies, as well as in prescription glasses field is IPD (Inter-Pupillary Distance), you can read about this term in one of our past posts here: Ultimate Optical Terms Library for AR/VR/MR Systems – IPD

So, how do we make sure that any user will be able to see the projected image and enjoy the same high image quality and user experience, with all the variations?

System Pupil Definition

System Pupil

This challenge is the basis of the most important and widely discussed term in XR systems: System Pupil.

System Pupil, in one sentence, is the 2D space where the user's eye can be located and where the full systems performances are controlled and valid. This means that the eye can be anywhere within the System Pupil, and the user will enjoy the full system performance and quality.

Eye Box

There are other terms that are used in the XR space, one of them is Eye Box. The Eye Box is the 3D alternative of System Pupil, when also the Eye Relief range is added to define the 3D space where the users eye receives the controlled system performance and quality.

We define Eye Relief as a separate parameter; you can read about it in our past article in this blog: Understanding Eye Relief: Optical Terms for AR/VR/MR Systems

Eye Glow

Another term people are using that is connected to the System Pupil term is a visual effect known as Eye Glow (or White / Green Glow). This term refers to an effect of highly illuminated area on the eye and around it, where the projected image is seen. So, when the image intensity is very high or when the surroundings are dark you can clearly see the highlighted illuminated area looking at a person wearing the XR system, which suggests the System Pupil area.

The Source of System Pupil Size Requirements

We begin our consideration of System Pupil requirement with the definition of another term that contributes to this process, the Measurement Pupil. The Measurement Pupil defines the aperture size used in the definition, optical design, and later on the measurement of any other system performances. The human pupil aperture size changes with different factors such as ambient lighting (day q night) or physiological factors, but we need a fixed single definition of the aperture that is used through all the engineering stages to define performances.

So, if Measurement Pupil represents a certain single eye aperture size, the System Pupil represents the integral area where the Measurement Pupil can be contained.

The reason we need a larger System Pupil is the users’ eye pupil position variation due to many factors. Some of them are listed below:

• The variation of IPD

• Variation of pupil vertical position

• Eye movement through FOV range

• Pupil dilation in dark environments

• System’s instability during head and body movements

• Elastic or soft materials used on the interfaces

• Optical and mechanical tolerances

• Other effects...

Covering the IPD range is the most significant influence on the System Pupil Size specification, since the variation range within the population is very extensive. The following illustration visualizes this factor, also highlighting the System Pupil and the Measurement Pupil.

System Pupil: IPD Range Coverage

System Pupil Specification

Basic System Pupil Requirement

System Pupil is specified as area size, with value for horizontal and vertical axes in [mm], or as area diameter for a circular area shape.

Measurement Pupil is specified as aperture diameter in [mm].

It’s almost never specified in detail, since there is a lot of vagueness in these terms’ definition.

The general requirement would be, for example, for a circular area shape:

• System Pupil: Ø12mm 

• Measurement Pupil: Ø3mm 

So, how can we cover this extensive area, so with all these variations the user will still get the image performance as designed to provide the desired user experience?

IPD Coverage Architecture

As said above, covering the IPD range is the most significant part of the System Pupil Size specification, since the variation range within the population is very extensive. So, first, we need to establish the IPD coverage architecture. This is done in several steps:

1. Define the user profile specifics, such as gender, origin, age group.

2. Gather the relevant IPD statistical data to define the IPD range requirement.

3. Based on the intended use case and marketing strategy, define whether the device is intended for a single user or multiple users.

4. Define the IPD coverage architecture approach. In large, there are 3 different architecture options:

a.     One size fit all (large System Pupil);

b.     Several sizes (medium System Pupil);

c.     Personal fit (smaller System Pupil).

This topic is covered in detail in the article about the IPD Term: Ultimate Optical Terms Library for AR/VR/MR Systems – IPD

Things to Consider in System Pupil Requirement

IPD Coverage is the major part in System Pupil requirement formation, but it’s not the only factor.

Here are some additional factors that need to be considered when defining the System Pupil requirement:

• IPD covers the user’s eye pupil position variation in the horizontal axis only, but there is also eye pupil position variation in the vertical axis. Often this variation is covered by using adjustable nose pads or a set of different nose pads that can be provided with the product. This solution can be applied when the user’s nose bridge is used as an interface. Other mechanical adjustments can also be efficient.

• System FOV also has an impact on the System Pupil requirement, since the eyeball rotation to scan the FOV also creates an eye pupil position shift. Roughly this can be calculated using eyeball center as the reference rotation point. So, for example, while scanning the horizontal FOV of ±30°, the horizontal pupil shift would be:

Pupil position shift calculation vs FOV

As we see, for large FOV the eye shifts are significant and need to be taken into consideration.

• Eye pupil changing aperture size: Eye pupil aperture changes in dark environments, so for systems used at night (Night Vision, for example) we should consider this factor when defining the Measurement Pupil diameter.

• Head &body movement: The use case specifics that have an impact on system’s instability relatively to the user’s eye, during head and body movements, need to be taken into consideration in order to ensure the image stability for the user. This factor can be covered by increasing the System Pupil or by providing some mechanical stability measures, such as headband or head strap, or this could be a combination of both.

• Product design &materials: Product design often calls for elastic or soft materials to be used on the interfaces with the user’s head to create better user experience, especially for long use. On the other hand, these materials can cause the headset to sag with time, especially in active use cases. The System Pupil requirements need to consider this shift, again, in terms of image stability throughout the longer use, the activity and movements.

• Opto-mechanical System tolerances: In every optical system there are optical and mechanical tolerances, including elements placements tolerances that create a variation of actual System Pupil position versus designed nominal.

All these factors and effects need to be carefully analyzed and taken into consideration during the design and engineering stages to make sure that each actual product provides the required optical and image quality performances.

System Pupil Requirement Example

Our definition of System Pupil(example):

• System Pupil:

  • Vertical: 12mm;

  • Horizontal: 6mm;

  • Rounded rectangular shape (see image)

• Measurement Pupil: Ø3mm

XR System Pupil shape example