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 IPD (Inter-Pupillary Distance)
In Augmented / Virtual / Mixed Reality Systems, an image source is projected using an optical system to infinity (or a finite distance) to be viewed by the user’s eye. So, user’s eye shall be positioned correctly relatively to the system, so that the image is fully visible, and the image quality is high, as designed (more about this term will be detailed in a separate post on System Pupil).
This is easier said than done, since the positioning of the device on the user’s head is done usually based on the head features, such as a head strap, earpieces or temples, nose pads. So, the device is fixed on the head, while the position of the eye is the important thing. Even more so, when the system is binocular – so the position of each eye is important and naturally it varies a lot between different people.
So, how do we know where the eyes are relative to the head? There are extended studies of head features measurements and their statistical distributions, mostly performed for military applications, such as Helmet Mounted Displays.
One of the parameters that is widely used in these studies, as well as in prescription glasses field is IPD (Inter-Pupillary Distance), which is the distance between the two eyes’ pupils' centers. A person’s IPD can be measured using simple dedicated ruler or using a mobile phone APP (such as Eye Measure, for example), as shown below.
IPD Statistics
An IPD term is commonly used in AR / VR systems specifications, mainly supporting the ability of a product to fit majority of the population of potential users.
The variation of IPD in the population is very large, as can be seen in the table below.
Source: Wikipedia (Representative data from the 1988 Anthropometric Survey based on Military Handbook 743A and the 2012 Anthropometric Survey of US Army Personnel)
IPD Specifications
IPD is specified in [mm], as a range. The general requirement would be, for example, a range that covers 98% of the population of male users, based on the above presented data:
IPD range: 56 - 72mm
So, how do we cope with this extensive IPD range, and what can be done in particular in Augmented / Virtual Reality system design so the product can fit all the users with different IPD values? Is there a basis to define a different IPD range in different cases or products?
IPD Specifications Insights
The IPD requirement is defined based on the user profile specifics, such as gender, origin, age group. But more fundamentally, it is important to set the system approach: is the device intended for a single user or multiple users? This decision is very critical in the early requirements stage, as an input to the optical design, since the entire design depends on it.
Our addition to the IPD specification:
The specific user's profiles have to be set, based on the systems intended use. The IPD range requirement shall cover the relevant statistical data.
The IPD also varies for different working distances (Apparent Image Distance), due to the eye conversion for close distance. Thus, there is Near IPD, which differs from Far IPD.
The system approach shall be decided: single user or multiple users.
For multiple users' device, it should be decided: whether several device sizes will be proposed, or there shall be an adjustment mechanism to cover different IPD values.
IPD range coverage is essentially provided by the System Pupil size in a combination with different product sizes or adjustment mechanisms.
Things to Consider in IPD Requirements
So, we set the IPD requirement based on the system’s user profile and system engineering approach. How can we cover the required IPD range is the design? The IPD coverage strategy is very important for the optical design, so it must be set as early as possible, since changing it in the process means that the whole design would start over.
Here are several different design techniques that can be considered for the IPD range coverage, each creating a different working point:
The most straight-forward strategy, that is used when the device is intended for multiple users, is to design a system with a System Pupil large enough to cover the whole IPD range (for each eye). For example, in order to cover the IPD range of 56 - 72mm, the System Pupil size for each eye shall be at least 14mm. This doesn’t take into account other parameters. In order to create a System Pupil this large, the system size, complexity and cost will be very high.
Another common approach is to design systems of several different sizes (Small/Medium/Large) to cover a partial IPD range and reduce the System Pupil size to ~1/3. This can be done by different products, or by mechanical adjustment with discrete steps. This approach allows to reduce the system size, complexity and cost, but is still fairly nonoptimal, justified only when the device has multiple users.
A more optimal approach, that allows to produce a system with minimal size and cost, is to use personal fitting mechanisms. These allow to bring the system pupil exactly to the user's eye pupil location. This approach is used in single user products. The downside of this approach is that adjustment mechanisms are needed, or many types of product variations, when each device is adapted and manufactured exactly for the single user. Some of the optical design concepts, such as visor projection do not allow to shift the pupil position easily.
A more novel approach can provide the benefits of personal fitting using a combination of adjusting optical elements and eye tracking technology, which is used to shift the system pupil to where the users eye is.
Uniformity Requirement Example
Our definition of IPD (example):
IPD range: 56mm - 72mm
Coverage Strategy: 3 Different sizes:
Small: covering IPD 56mm – 62mm
Medium: covering IPD 61mm – 67mm
Large: covering IPD 66mm – 72mm
Personal Fitting Strategy: no personal fitting; fits multiple users within the size groups.