Compact and lightweight optical designs achieving visually acceptable image quality, field of view, eye clearance, eye
box diameter and operating across the visible spectrum, are the key to the success of next generation head-worn displays.
There have been several approaches in the design of head-worn displays including holographic optical elements and
laser scanner systems. For example, Minolta has pursued a monochromatic display (green) with a 3 mm exit pupil
realized by a 3.4 mm thick light guide with a holographic optical element to achieve an eyeglass form-factor head-worn
display [1]. Our approach in this paper is to investigate the field of view, eyebox diameter, and the performance limit of
a single element magnifier comprised of freeform surfaces. The surface shape is a major variable in such a constrained
system with respect to the optimization degrees of freedom.
Typical optical surfaces are functions mapping vectors in R2 to real numbers representing the sag of the surface. A
majority of optical designs to-date have relied on conic sections to which are added polynomials as the functions of
choice. The choice of conic sections is easily justified, since conic sections are stigmatic surfaces under certain imaging
geometries. The choice of polynomials from an image quality analysis point of view is understood since the wavefront
aberration function is typically expanded in terms of polynomials. Therefore, a polynomial surface description may link
a designer's understanding of the wavefront aberrations and the surface shape. However, from the point of view of shape
optimization and representation, polynomial shape descriptions can be challenged. In Section 2, we briefly describe the
radial basis function approach to represent freeform optical surfaces. In Section 3, we apply the RBF to design a single
element see-through compatible head-worn display.
An infra-red interferonieter for testing optical elements and assemblies in their operational wavelength band is described.
The interferoineter is a phaseshifting, LUPI-style Twyman-Green interferometer operating at a wavelength of 3.392pm.
The system is designed for optical testing in a vacuum chamber. For that purpose the interferometer is contained in a
hermetic, vacuum-compatible enclosure, and all internal alignment functions are remotely controlled. The light source is
an infrared HeNe-laser at 3392 p m. The camera sensing the interferograms consists of a PtSi CCD-array with 256x256
pixels. For operating the system in a vibration environment, short exposure pulses are generated from the laser light. In
addition, a specialized phase.shifting data acquisition and reduction algorithm is employed which senses the random phase
shifts and adapts the phase algorithm accordingly. A vacuum compatible diverger (f/2.5) and beam expander (250mm beam
diameter) allow for a wide variety of optical testing configurations. Measurement results and system performance
parameters are presented.
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