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 Ghost & Stray Light
In Augmented / Virtual / Mixed Reality Systems, an image with some information is projected using an optical system to the user’s eye. The optical system is designed in a way to produce the required image quality, but in reality, it also produces unwanted artifacts that degrade the image quality. These can be caused by reflections from mechanical parts, coatings, diffusive surfaces, scattering, unwanted TIR effects, even bonding material scattering and just rays going where we didn’t intend them to go!
There are several terms that are used for these artifacts, we mainly use 2 of them to classify 2 primer types of artifacts:
Ghost: this term is used for artifacts that hold some connection or resemblance to the projected information but appear to the user at a different Field Angle / Position / Size / Orientation / Intensity / Color / Apparent Image Distance (any of these or more typically a combination of several) than the actual image. Ghost is sometimes also referred to as “Double Image” – when the unwanted image is close to the actual image. Ghost is a type of artifact that can cause misleading information: like when the user may see a target at the wrong position, this can mislead him to make a wrong decision.
Stray Light: this term is used for artifacts that don’t have any connection to the projected information and appear to the user as blurred spots or shapes that reduce image contrast, quality or just create distraction. Stray Light is a type of artifact that cannot create misleading information, because they don’t give the user any information, but they just interfere with the projected image, reduce its quality, make it less visible or obscured.
Ghost &Stray Light: Non-Imaging Design Parameters
Optical design software tools (such as Zemax, CodeV, OSLO etc.) are made to design the optical systems, explicitly only the optical surfaces of the system are designed. These tools have very limited simulations of all the peripheral parts of the system, such as coatings, filters, scattering or diffusive elements, elements’ edges and rims, mechanical parts, bonding layers and many more. So, basically, best practice techniques are used (low curvature surfaces, anti-reflection coatings, rims blackening, baffles on mechanical parts etc.). This results in a painful discovery of all the artifacts only when the prototypes are ready. Even then, there are no analytical tools to find the sources of the artifacts and eliminate them other than rigorous testing and lab experiments. This extensively increases the time and effort required to produce mature and ready-for-production products without artifacts. Many products just go to the market with the artifacts and the engineers work to reduce the artifacts on the next product version release.
Non-Imaging Design Tools
The recently made available optical design software tools for non-imaging and illumination design, such as LightTools (from Synopsys) provide the ability to model, analyze and solve the Ghost & Stray Light artifacts in any system at early design stage. This tool is also used for evaluating other image parameters, such as Brightness, Uniformity, Color (more on this will be detailed in a separate post). Even if some of the artifacts are not revealed until the prototypes are evaluated, the system model allows to find the sources of Ghost & Stray Light in the system, see the exact path of the unwanted rays and eliminate all the artifacts.
Ghost & Stray Light Specification
Ghost & Stray Light are defined in percent, relatively to the main image intensity (or Brightness). The standard requirement originated from the Military specifications is:
Ghost : <2%
Stray Light : <2%
Are these requirements sufficient to assure the artifacts will not be seen by the user? Based on extensive experience with AR systems, we know they aren’t…
Our addition to the Ghost & Stray Light specification:
Ghost &Stray Light have different effect so the requirements shall be separate. Typically, Ghost intensity requirements shall be lower than the Stray Light.
Projected image content has significant influence on the visibility of the artifacts, especially the type of image: symbols on dark background vs full video image. Thus, the Ghost &Stray Light requirements shall be set as ratio between the artifact to background as well as ratio between the artifact and main image.
The Ghost &Stray Light requirements should not be limited to the system FOV only, since the artifacts outside the FOV can also be seen and bother the user.
Things to Consider in Ghost &Stray Light Requirements
The Ghost &Stray Light Requirements depend on the system’s projected image content, specifically on the image background or see-through background. Here are several different cases when system Ghost &Stray Light Requirements should be tailored in order to create an optimal design:
Generally, in all XR systems, the projected image content has significant influence on the visibility of the artifacts, especially the type of image: symbols on dark background vs full video image. Thus, the Ghost &Stray Light requirements shall be tailored to the image content.
In case of an augmented reality system, when the projected image contains symbols or text which is combined with direct scene view, the outside scene background brightness is high, especially in outdoor applications. In this case, the Ghost & Stray Light requirements can be very relaxed.
In case of a mixed reality system, when the projected image is combined with a direct viewing camera / night vision system, the direct viewing system background brightness will also dictate the Ghost &Stray Light requirements.
In case of a virtual reality system, when a full video image is projected, the image background is very low, and even low artifacts are highly visible on a dark background. In this case the Ghost &Stray Light requirements shall be very strict.
Ghost &Stray Light Requirements Example
Our definition of Ghost &Stray Light (example):
Define outside scene brightness
Ghost: <0.5% (from nearest image symbol) and <5% (from nearest background)
Stray Light: <1% (from nearest image symbol) and <10% (from nearest background)