Optical Terms Library for AR/VR/MR Systems
Starting from Spec Requirement
From the early optical system development stage, a significant part of system engineering process is to make sure that every Spec requirement should be addressed in every development stage. The spec has a purpose: each parameter we define is based on a user need, a use case or application, a function that is vital for the product use. Thus, each parameter is defined in order to be taken into account in the design stage. The requirements are set as a goal, leading the product development, so the development stage has to provide evidence that this parameter requirement is fulfilled by the design and engineering process.
Measurement Planning
So, the design shall support complying with the requirement for each parameter. The process of validation of this compliance is called POD – Proof of Design. This process includes design reports that show how the design complies with the Spec requirement. The final stage of POD is an actual measurement or test of the parameter on the product.
So, from the Spec stage, we need to have a clear understanding or a procedure: how this parameter can be measured?
What is FOV (Field of View)
Let’s start with the definition.
FOV is one of the most referred and discussed terms in optical design. Most system specs will include this term, and there is a "competition" between the different optical system developers - to reach higher FOV.
FOV specifies the angular range of the projected image to the user’s eye, when the image is projected to infinity. FOV is defined in degrees, usually specifying system’s Horizontal and Vertical FOV, sometimes Diagonal FOV is also specified, for Example:
Horizontal: ≥30°; Vertical: ≥20°; Diagonal: ≥30°
In systems with finite image distance, FOV may be specified as actual size in millimeters, stating the image distance, usually in meters.
For detailed explanation and requirements considerations see our post about FOV.
FOV Spec Example
FOV, like many other terms, can be defined in different levels of detail. It depends on many factors, here are just a few:
regulations requirements
the level of precision that’s based on the market or application
the importance of the specific parameter
sometimes it’s a goal parameter and the precise value is not important
Here are 2 examples of FOV requirements with different levels of requirement precision.
Example 1 – High Precision
Horizontal: 30° +0.3°/-0; Vertical: 20°+0.2°/-0; Diagonal: 30° +0.3°/-0
Example 2 – Low Precision
Horizontal: ≥30°; Vertical: ≥20°; Diagonal: ≥30°
How to measure FOV: High Precision
How to measure FOV? The general answer to the general question: “it depends”. Each example will require a different measurement approach, different strategy and equipment will be used, and also different measurement statistics will apply.
Several general points to keep in mind:
The FOV measurement is highly dependent on the measurement setup, the measurement pupil position, or Eye Relief (read more about Eye Relief in our dedicated post)
The FOV measurement is also highly dependent on the measurement pupil size. So, an external aperture should be used to define the measurement pupil size. In a case of measurement using camera, for instance, the aperture shall be placed on the camera lens and located at the Eye Relief distance from the measured system.
As the FOV edges might have vignetting, you'd need to specify the intensity drop at which the FOV criteria is defined: let's say for example, 50% intensity of the edge relative to the FOV center would be used to measure FOV size.
When there is intention to use a camera to measure the FOV, the selection of sensor and lens should be done after system engineering calculations in order to comply with the measurement accuracy and cover the FOV range. This task can be tricky, especially due to the use of external aperture on the camera lens, and sometimes more than one iteration is needed to create a measurement setup based on available equipment.
In any case when using a camera to measure FOV, the camera and lens mapping using external equipment is required in order to turn the camera into a viable measurement equipment.
When camera is used to measure the FOV, and camera FOV is not sufficient to cover the whole system FOV, then a rotation jig can be used to rotate the camera to see the full image. The rotation axis must be located on the aperture plane, so that the measurement pupil doesn't move during the rotation. Keep in mind that the rotational stage tolerance adds to the measurement accuracy calculation.
Example 1 – High Precision
In this case, when the requirement comes with very tight tolerances, the measurement process and equipment need to provide and cover the following points:
Measurement accuracy
Measurement accuracy and precision need to be ~5 times better than the value you’re trying to measure. So, if you need to measure the value with a precision of 0.05°, in order to distinguish between 0.3° and 0.4°. Then the measurement precision or error should be <0.01°.
Now, you understand what kind of equipment to look for, it should be something that can guarantee this measurement precision. In this case a Theodolite which is a professional angle measurement equipment is required.
Finite Image Distance Measurement
In general, as said, FOV is specified in degrees for an image distance of infinity. There are different cases, when image distance is close - can be 1 meter, for instance. In this case it's reasonable to talk about image size in millimeters. Then the whole setup is different. For example, a laptop placed at a predefined distance can be used and a ruler to measure the image distance, which is captured using a camera, placed at the Eye position (system pupil). The measurement precision is then can be based on the laptop pixel size.
Measurement process &automation
There is a huge different between needing to test one or several units and serial production testing. When you measure several units, an engineering setup will be enough, there is no need for automation, standardization, the testing time can be longer, the testing personnel can be high skilled engineer.
When you plan in-process serial production testing, all these have to change. Only very careful planning and testing process design can assure the serial production testing process is cost-effective and runs smoothly.
Hybrid Testing Strategy
In order to fill the points above, it’s often the combination of different processes that build the testing strategy:
Engineering testing of a few units as POD (proof of design),
Detail tolerance analysis that assures as-built system will also meet the tight requirements tolerances
Assembly and calibration process that documents the production process and minimizes errors
In-line testing of a go-no go type with lower precision and higher throughput that aims to track human errors
How to measure FOV: Low Precision
Example 2 – Low Precision
In this case, when the requirement comes with no tolerance, it’s a completely different approach. Then just a Go-No Go test is enough.
Go-No Go testing
We just need to ensure that a certain image size is seen. So, if a certain image with clear boundaries is projected and it can be seen, (within the defined intensity criteria) then we pass the test.
So, it’s sufficient to view the image without any special instruments or take a camera image and make sure the whole image is seen.
Measurement process automation
When the test is a simple go-no-go evaluation, this testing process can be highly automated. The image can be captured by a camera and then processed to assure the whole image is seen. All using algorithms and software, without any man-in-the loop.
This process is highly scalable, since it can be duplicated, more testing stations can be produced, and the throughput can be increased.
PASS/FAIL Criteria
When FOV requirement is clearly defined in the Spec, including tolerances and the intensity criteria, then also the measurement criteria are obvious. When the requirement is wage - it leaves more room for interpretation also for the measurement Pass/Fail criteria. That's why we start with requirements and definitions.
Sometimes, the actual performances are much better than the requirement, and this leaves a high margin for the testing strategy.
It’s not common for the FOV parameter to be designed with large margin, particularly in the AR/VR/MR systems, since many of the products’ main competition is in the FOV specs. In some cases, though, when there is a large margin, that means that the expected yield is 100%, then sampling could be very low, just to ensure nothing went wrong in the production process.
Another example for under sampling, is when there are no FAIL criteria. When any result is OK, and no quantitative measurement is required. This can be the case for consumer products, low-grade products or prototypes / POC units.
