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 Power
In Augmented Reality Systems, unlike in Virtual or Mixed Reality Systems, in addition to a virtual image that is projected to the user’s eye, there is also a see-through view of the real world. Actually, the essence of Augmented Reality is a combination of a projected image with the real world which is seen directly looking through the combining part of the optics.
One of the key goals of the system design is to keep the real-world view natural, uninterrupted, and undistorted as much as possible while creating the added value of the superimposed virtual image.
See-Through Aspects in AR Systems
So, See-Through requirements in general define how much the direct see-through vision quality can be de reduced so this does not disturb the user. See-Through requirements are a family of requirements for parameters that we classify as “See-Through parameters”, that are often omitted from the specifications because the system designers focus on the additional virtual image and not so much on keeping the see-through image quality under control. This family includes the different parameters, which we address in different separate posts:
See-Through Power (addressed in this post)
Prismatic Deviation
See-Through Transmission & Uniformity
See-Through Power Effect on User Experience
See-Through Power is one of the parameters that we classify as a “See-Through parameter”. To understand the meaning of this term, you can think of prescription glasses that are used for vision correction: a lens with a certain optical power is placed in front of the viewer's eye in order to take a real-world image and create a virtual image of it at a different distance. For example, if you’re short-sighted (which is the most common case for vision correction), then a lens with a negative power is used to create a virtual image of real-world objects located far away when their image is located where your vision is good enough and you can see it clearly.
Now, going back to Augmented Reality Systems, if your system has non-zero see-through power, it’s like putting someone else’s glasses on. If you tried this, you know that this gives very distorted and blurred image, and you immediately get a dizziness and headache. So, this effect needs to be minimized as much as possible, or below a discernable limit.
See-Through Power Specification
See-Through Power as a general optical power is specified in [Diopters], which is the reciprocal of meters. In most cases there is no specification for this requirement unless we’re dealing with military applications, that are covered by MIL-STD requirements (mostly driven by Night Vision Imaging Systems), where you can see the definition:
See-Through Power: 0D to ≤ 0.1D
Why can’t it just be ZERO? Why compromise and define the extent of natural vision quality reduction instead of just specifying no reduction at all?
The answer is simple: if you put something in front of your eyes, no matter how “perfect” and corrected it might be, it will decrease your natural vision. There are no “perfect” elements in real life, and even if you have a design that is see-though corrected, the correction cannot work for the whole FOV range, for all different eye positions within the System Pupil (we’ll define this term in a separate post) and on top of this there are also manufacturing tolerances to consider. In the end, the as-built optics have reduced performances compared to the nominal design, which in any case has reduces performances compared to no optics at all.
Is this so awful? Not, really… We’re all used to look outside a window and see a distorted image of the outside world, and we live with this quite not noticing, since our vision adjusts to this.
So what is the limit of not noticeable image quality reduction, specifically for See-Through Power? When you get prescription glasses for your vision correction, the glasses power comes in steps of 0.25D, and it’s considered that we don’t need more accurate correction, or smaller steps. So, it’s safe to say, that for any regular system that is used for human vision, the optical power error can be as large as ±0.25D.
See-Through Power Specifications Insights
Our addition to the See-Through Power specification:
See-Through Power, as any of the performances' parameters shall be defined for a defined See-Through FOV range. (detailed in a post dedicated to See-Through FOV).
In systems where projected image AID is a finite distance (detailed in a separate post dedicated to AID), See-Through Power correction shall be done for the same distance as the projected image distance.
The is relevant only for the Augmented Reality Systems, as in VR / MR system there is no see-through vision.
Things to Consider in See-Through Power Requirements
Here are several aspects to consider when forming the system See-Through Power requirements in order to create an optimal design and user experience:
See-Through Power requirement is one of the few requirements that don’t depend on the system’s use and the image content, but rather just minimizing the effect of the presence of optical elements in front of user’s eyes. Thus, the requirements considerations that will ultimately create good user experience are based totally on the human vision characteristics.
See-Through Power cannot be corrected electronically, i.e., by manipulating the projected image, as done for Distortion correction (see separate post on Distortion). The only way of controlling See-Through Power is with direct viewing of the optical element's design and tolerances. On the other hand, it is important not to require over-strict tolerances, which affect elements complexity and price. These parameter requirements trade-off is very important.
In case of a binocular system, the largest See-Through Power difference between the eyes is double the specified error for each monocular. So, this should also be taken in mind when defining the requirements.
See-Through Power Requirement Example
Our definition of See-Through Power (example):
See-Through Power, within see-through FOV: ≤ 0.25 Diopter