How to measure AID (Apparent Image Distance)
- Olga Resnik
- Jun 15
- 4 min read
Optical Terms Library for AR/VR/MR Systems
Measurements of AR/VR/MR Systems parameters
The incentive behind this and other articles that discuss the measurement of different parameters of AR/VR/MR systems is essentially assuring the user experience that we want to achieve. This is a significant part of system engineering process, the basis of optical system development. The center of AR/VR/MR systems is a user, and a specific task this system aids the user perform, or a use-case. In order to cover this purpose, the user experience has to drive the development process. This also includes the measurement of the parameters that are part of the user experience.
One of the most important user experience parameters, especially if AR Systems Design that also depends on the use-case is the AID.
What is AID (Apparent Image Distance)
Let’s start with the definition.
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 design is made in a way that it 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.
For detailed explanation and requirements considerations see our post about AID.
AID Spec Example
AID, like many other terms, depends on many factors, here are just a few:
regulations requirements
specific applications / use-case requirements
sometimes it’s a goal parameter and the precise value is not important
Here is an example of AID and Field Curvature requirements.
AID and Field Curvature Requirements Example
AID and Field Curvature Requirements Table:
How to measure AID
The traditional instrument that’s dedicated to measure AID is called Dioptometer (or Diopter Telescope). This equipment is used by a trained operator, and eventually the test is visual, meaning that the result is subjective and depends on the operator proficiency.
So, let’s cover all the advantages and drawbacks of using Dioptometer.
AID Measurement with Dioptometer: Advantages
The theoretical measurement accuracy (not taking in the account the operator) can be as low as 0.1 Diopter, which covers most of the application requirements.
The Dioptometer is very compact, can be used in any setup, doesn’t require any additional equipment, such as camera, PC, software, etc.
The Dioptometer is low cost, simple piece of equipment. It doesn’t require maintenance.
AID Measurement with Dioptometer: Drawbacks
The measurement using Dioptometer is manual and subjective, performed visually by a highly skilled operator. So, the results are always questionable.
The measurement pupil size cannot be fully controlled because of the eye pupil being a part of the “measurement setup”.
The manual measurement nature has an impact on the measurement accuracy which may vary from one operator to the other. It may also vary from one test to another, may also depend on temporary factors, such as illumination level, operator fatigue, weather, air dryness etc.
The manual measurement requires calibration before each measurement, because of the subjective nature.
Hybrid Setup: Dioptometer with Eyepiece Camera
There is a flexibility to use a Dioptometer with Camera instead of manual operation. In this case, all the problems of manual measurement are overcome, and the measurement can be very repeatable and objective.
The initial calibration / mapping must be done, but no calibration is needed before each measurement.
To get even better stability and even use automation – a dedicated software can be used basing on image processing and algorithms. In this case the measurement accuracy is optimized.
Measurement using Autocollimator with Adjustable Focus
An Autocollimator with adjustable focus may be used to measure AID. This is tricky though, because Autocollimator is not a measurement equipment and in order to turn it into one a deep understanding is required to choose the right components and build the measurement setup. Using an Autocollimator with adjustable focus has also a list of advantages and drawbacks, let’s consider some of them here.
AID Measurement with Autocollimator with adjustable focus: Advantages
When using an Autocollimator with adjustable focus in a combination with eyepiece camera the measurement is objective, accurate and repeatable.
The measurement accuracy depends on the selected equipment (mainly on the Autocollimator focal length), so an optimal setup can be chosen for a specific system requirement.
Measurement aperture can be controlled using a diaphragm.
The calibration / mapping is not needed before each measurement.
AID Measurement with Autocollimator with adjustable focus: Drawbacks
This equipment is expensive and it’s not really usable for other measurements apart from AID and See-through Diopter.
Setup initial measurements and mappings are required in order to “qualify” the setup for the AID measurements.
This equipment is large and is often not usable due to the extensive size, especially for Field Curvature measurements.
Low-end testing using Camera Focusing
Another option worth mentioning is just using a camera focusing range to measure AID. This can provide a Go-No go testing option with lower accuracy, but sometimes this could be enough. Here also an initial measurement and mapping can improve
accuracy, but it would never match the accuracy that can be achieved using Dioptometer or the Autocollimator with adjustable focus.
Field Curvature Measurement
Field Curvature is essentially an AID difference at different FOV points. So, to measure Field Curvature, only means to measure AID at different FOV points, that means that the measurement equipment (i.e. Dioptometer) has to be rotated with reference to the System Pupil plane. This often requires dedicated mechanical jigs enabling the equipment rotation in 2 axes with the specific rotation reference plane.
Testing Strategy
The measurement strategy always depends on the requirements tolerances and the level of automation that’s required. Sometimes a combination of high precision manual tests for smaller sampling with the lower precision in-line testing can cover the measurement and sampling requirements optimally.
PASS/FAIL Criteria
The Pass/Fail criteria commonly is based on the requirements tolerances, that’s why it’s very important to establish the requirements from the beginning. They will drive everything: the measurement strategy, the equipment, sampling and the Pass/Fail criteria and can also impact the system calibration strategy and process.
