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  • Writer's pictureOlga Resnik

Optical Terms Library for AR/VR/MR – See-Through FOV & Obscuration

Updated: Feb 25

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 See-Through FOV and Obscuration

So, what is See-Through FOV and how it is different from System FOV?

While System FOV specifies the angular range of the projected image to the user’s eye (read our post on System FOV), See-Through FOV specifies the unobscured outside FOV with the presence of the projection system. The obvious goal is to minimize the Obscuration, but any visual system creates real world obscuration, even our prescription glasses.

See-Through FOV Requirements

See-Through FOV is defined in degrees, stating the direction of obscuration, for Example:

  • 50° Left &Right x 20° Up &Down

Design Tools to Optimize and Control See-Through FOV &Obscuration

In most AR systems, the see-through FOV is not stated, and the efforts to minimize the obscuration of the see-through FOV is not considered as a key requirement. In general, it is assumed that the user will cope with the obscuration as he does with prescription glasses or sunglasses.

In fact, any system that is put in front of your eye creates obscuration, the question is what obscuration is acceptable, and this strongly depends on the use-case, the mission, how much you rely on the information of the outside scene.

There can also be partial interruptions with See-Through FOV, such as partial shading which can be produced by abrupt coating edges, elements edges or other discontinuities in the direct FOV since the human eye is very sensitive to local contrast.

In our experience, AR systems with critical See-Through FOV requirements, such as HMDs for pilots (Helmet Mounted Displays), are typically based on Visor Projection system architecture, since the Visor creates a high unobscured See-Through FOV due to its conformity with the face structure, as can be seen in the image below.

See-Through FOV in HMD
See-Through FOV in HMD

Figure Source:

In a binocular system, partial obscuration can be compensated by the other eye, so the information is seen by one eye, at least.

Traditionally, optical designers do not deal with the See-through FOV as part of the projection system design, and the only effort to minimize the obscuration is done by opto-mechanical and product designers. This results in highly non-optimal products and poor user experience in the aspect of See-through FOV and obscurations. Sometimes, opto-mechanical solutions such as “stow” positions are used when the projection system obscuration is significant and extensively intervenes with the real-world viewing.

The recently made available optical design software tools for non-imaging and illumination design, such as LightTools (from Synopsys) provide the ability to design and optimize the See-through FOV parameter at early design stage and evaluate the product with “virtual prototyping” done using LightTools Photorealistic Rendering tool. This tool is also used for evaluating other parameters (read more about this in our post on Virtual Prototyping)

See-through FOV Specifications Insights

Our addition to the See-through FOV specification (rarely found in general specification documents):

  • See-through FOV sketch shall be presented to see the See-through FOV shape and the obscurations details.

  • See-through FOV data shall include tolerances.

  • See-through FOV is not only a specification performances parameter, but also a definition for other specification performances parameters, such as See-through Power, Prismatic Deviation, Symbol Accuracy, See-through Color Shift and others (more on these parameters will be detailed in separate posts), so we define See-through FOV areas for specifying other see-through parameters within different FOV areas.

Things to Consider in See-through FOV Requirements

The See-through FOV Requirement values depend on the system’s intended use or the use-case, when the outside scene field of regard have a significant impact on the specific obscuration requirements. Here are several different cases when system See-through FOV should be tailored to the use-case scenarios in order to create an optimal design and user experience:

  • In case of an augmented reality system, when the projected image is combined with direct scene view, wide see-through FOV has to be large enough with minimal obscuration in order to create a sense of immersion and not a feeling of “looking through binoculars”.

  • In case of a mixed reality system, when the projected image is combined with a direct viewing camera / night vision system, the direct viewing FOV depends on the direct camera FOV, which has to be large enough to create a sense of immersion.

  • In case of a virtual reality system, the user only sees the projected image, so the system FOV is all the FOV that is seen, there is no “see-through” FOV per say. Here, again, it’s important to create as large FOV as possible to create a sense of close to natural human visual filed as shown in the Fig. below.

See-Through FOV vs Human Vision
See-Through FOV vs Human Vision

Base Figure Source:

See-through FOV Requirement Example

Our definition of See-through FOV (example):

  • Horizontal: ±50° ±1°; Vertical: ±20° ±1° (elliptical shape)

  • Zone A: Diameter 10°; Zone B: Diameter 20°; Zone C: Diameter > 20°

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