Dear, friends
In our experience as optical engineers specializing in augmented reality design, many times we found ourselves explaining optical terms to engineers of different disciplines, colleagues, managers and customers. This encouraged me and my partners in JOYA Team to build our optical terms library. We 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. 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 FOV (Field of View)
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:
30° Horizontal x 20° Vertical
In systems with finite image distance (this distance is called AID: Apparent Image Distance – this term is explained in separate posts), FOV may be specified as actual size in millimeters, stating the image distance, usually in meters.
FOV Shape
In most displays &camera systems including augmented reality systems, FOV shape is neither round, nor rectangular, but a combination of them, and why is that?
In general, optical systems are built from optics, i.e. relay lens and an image source, i.e. a display (or sensor in case of camera). The display has, of cause, rectangular shape; lenses shape is basically round because of the manufacturing processes. Lenses can be cut to have any shape, but still, the fundamental issue is that image quality reduces with FOV due to aberrations. So, when designing a system with rectangular FOV so that the whole display is used fully, this means that the optics will be more complicated, heavy and expensive, but the gain in larger FOV will only be at the displayed image diagonal corners, which are in most cases not important.
Due to this, the FOV shape in AR systems is often as shown in the sketch below. This is not the case in VR systems or when displaying a full video image.
FOV Specification
Our addition to the FOV specification (rarely found in general specification documents):
FOV sketch shall be presented to see the FOV shape
FOV data shall include tolerances
FOV is not only a specification performances parameter, but also a definition for other specification performances parameter, such as CTF (or MTF), Field Curvature and others, so we define FOV areas for specifying other parameters within different FOV areas.
Things to Consider in FOV Requirement
The FOV Requirement values depend on the system’s intended use, when the projected image type and characteristics have a significant impact on the specific requirements. Here are several different cases when system FOV should be tailored to the use-case scenarios in order to create an optimal design:
In case of an augmented reality system, when the projected image is combined with direct scene view, wide FOV does not have an added value.
In case of a mixed reality system, when the projected image is combined with a direct viewing camera / night vision system, the projected FOV shall match the direct viewing and extending the FOV further does not have an added value.
In case of a virtual reality system, when the projected image has to create a sense of immersion, wide system FOV is required, but the image resolution in peripheral FOV can be dramatically reduced.
FOV Requirement Example
Our definition of FOV (example):
Horizontal: 30° +0.3°/-0; Vertical: 20°+0.2°/-0; Diagonal: 30° +0.3°/-0
FOV Zone A: Diameter 10°; Zone B: Diameter 20°; Zone C: Diameter 30°