Ellipsoids are spheres that have been stretched about one or more axes.

We can construct useful ellipsoidal mirrors called “Prolate Spheroids” from two special points, called the “Foci”. 

ellipsoid = new Sphere(radius = 0.5);
ellipsoid.position = (focus1 + focus2) / 2;
ellipsoid.rotation = orientZFromTo(focus1, focus2);
majorAxis = sqrt(pow(interFociDistance / 2, 2) + 
                 pow(minorAxis         / 2, 2)) * 2;
ellipsoid.scale = Vec3(minorAxis, minorAxis, majorAxis );

Raytracing Ellipsoids

Ellipsoids have some very special optical properties. So it’s useful to be able to raytrace against them quickly to determine how light will interact with them.

Conceptually, the simplest method is to transform both the ray origin and ray direction into the stretched coordinate space of the sphere. This allows one to raytrace against the sphere directly. After that, simply transform that hit point (and normal) out of the stretched coordinate space to use it.

function raytraceEllipsoid (rayOrigin, rayDirection) {
  sphereSpaceRayOrigin    =  worldToSphereMatrix *  rayOrigin;
  sphereSpaceRayDirection = (worldToSphereMatrix * (rayOrigin + rayDirection)) - sphereSpaceRayOrigin).normalized;
  intersectionTime        = intersectRaySphere(sphereSpaceRayOrigin, sphereSpaceRayDirection, Vector3.zero, radius, insideSurface = true);
  if (intersectionTime > 0) {
    sphereSpaceHitPoint   = sphereSpaceRayOrigin + (sphereSpaceRayDirection * intersectionTime));
    hitPoint              = worldToSphereMatrix.inverse * sphereSpaceHitPoint;
    return hitPoint;
  } else {
    return null; // No Hit
  }
}

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Foci Mirrors

Now, the Foci of Ellipsoids possess two very useful properties:

  1. The sum of distances from the foci to any point on the ellipsoid will add to a constant number. (This is true, even for ellipsoids with more than two foci!)

  2. All rays that pass through one focus will always pass through the other focus when reflected from the internal surface of the ellipsoid.

Due to property 1., the path length of each of these rays will be the same.

Chaining Ellipsoidal Mirrors

One may even chain ellipsoids together by their their foci to reflect light through an arbitrary path.

It is also possible to switch some of the ellipsoidal reflectors to convex surfaces, as long as there is a concave reflector afterward to collect the rays again.

This configuration is a special case of an optical system called an “Offner Relay”. Convex mirrors inserted into the optical path tend to reverse the aberrations caused by the concave mirrors (and visa-versa).

Fresnel Reflectors

Simple ellipsoidal optics are rare due to their bulk. This trade-off is apparent in Project North Star’s “Bird Bath” combiners. When designing it, we chose to sacrifice form-factor for field-of-view and image quality.

Nearly all methods of reducing size (additional optical elements, folding the path, using holographic elements, etc.) tend increase both optical aberrations and cost. Aberrations tend to accumulate the more times light reflects or refracts through a surface.

However, there may be one powerful way to move light around while keeping the benefits of a single-bounce reflector…

By simply taking planar slices of many confocal ellipsoids to approximate a larger ellipsoid, one can preserve their optical properties while reducing their form-factor!

  • Note how these slices emulate a refracting lens when the two foci are on opposite sides of the plate.

Practical Fresnel Reflectors

This is a relatively complex shape to assemble. There are great guides for building large, simplified fresnel reflectors at home to focus sunlight.

However, for near-eye displays, embedding these mirrored surfaces within clear material may be the most viable near-term structure.

There are two primary paths towards constructing them:

  • Machining a Mold (High Volume, Slow Turnaround)
    1. Use Wire EDM to cut slices from many traditionally machined ellipsoidal mirrors.
    2. Assemble these pieces concentrically to produce a mold.
    3. This mold can be used to cast the first half of the part from a clear material like resin, epoxy, plastic, or glass.
    4. Sputter or coat this internal surface with a 50% mirror coating.
    5. Cast this part with the same material to produce a flat upper surface (to eliminate changes in refractive index at the internal mirror’s surface).
    6. Coat the smooth exterior of the part with an antireflective film to minimize secondary reflections.
  • 3D Printing (Low Volume, Quick Turnaround)
    1. Use an optical rapid-prototyping company like Luxexcel to simply print one half of the part.
    • One may print a larger cross-section than is necessary, and shave it down afterwards to accomodate the printing processes’ poor handling of discontinuities.

Afterwards, one may follow steps 4-6 from the first process.

  • Though the flat-plate structure of this technique undoes much of the field-curvature (or “Petzval”) aberration of traditional ellipsoidal reflectors, this technique can easily be extended to curved structures to suit even better form factors.

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