IMAGE 2013 Conference

HIGH PERFORMANCE MIRRORS FOR HIGH DEFINITION SYSTEMS

Harry Streid – The Boeing Company – St Louis, MO USA

Monty MaGill – Displays and Optical Technologies (DOTI) – Georgetown, TX USA

ABSTRACT

Flat mirrors are a ubiquitous, yet underappreciated component of projected display systems. Ideally they do nothing except bend the light according to the laws of reflection. As projectors have advanced toward ever higher resolutions from smaller and smaller imaging devices we have learned how far mirrors can depart from this ideal. At the same time, demands for smaller footprint, motion compatible and go‑anywhere‑in‑the‑world containerized training devices have driven requirements toward larger, yet lighter and more durable mirrors. This paper examines trade‑offs in meeting these requirements and discusses advances in specifying and testing mirrors for advanced display systems.

INTRODUCTION

The simple flat folding mirror may be one of the least understood and poorly appreciated components in a display system. Selecting the right mirror requires understanding mirror technologies and their trade‑offs. Requirements shift as projector technology changes; approaches suitable for fixed‑base simulators may be unsuited for motion‑based or deployable devices.

COMMON MIRROR TYPES

First‑surface coated glass mirrors have long been preferred where size and weight permit due to high reflectivity, robust coatings, and availability of large sheets.

Stretched Mylar® (polyester film) became popular in the 1990s–2000s as a lightweight fold‑mirror solution for CRT‑based full‑field‑of‑view mission training systems.

Acrylic mirrors are widely available but suffer from flatness issues, coating adhesion problems, softness, and cleaning difficulty, preventing widespread use in simulation.

Characteristics of Common Mirror Types

Commercial Grade, Unpolished First Surface Mirrors

Float‑glass manufacturing produces near‑optical‑quality glass, but the surface exhibits periodic micro‑structures known as “draw lines” or “float lines.” These arise either from the drawing process or turbulence in the molten tin bath. Interference testing reveals periodic variations of several wavelengths per inch, which can degrade projected display performance.

Polished Glass First Surface Mirrors

More demanding applications require polished glass. Historically, weight was a drawback. Studies showed that ½‑inch float and BoroFloat glass could be polished to eliminate draw lines. DOTI achieved 1/10 wave per inch flatness and now controls flatness to sub‑wave per inch on materials as thin as 6 mm, with R&D ongoing down to 0.5 mm.

Mylar® Mirrors

Mylar mirrors are less reflective, cannot be cleaned without damage, and exhibit a woven‑cloth‑like pattern visible with modern projectors. They can be flat enough for some applications but are prone to twist or saddle deformations that vary with environmental conditions. These issues were masked in CRT systems but became obvious with digital projectors.

Acrylic Mirrors

Acrylic may show manufacturing patterns, is difficult to clean, is compliant (shape changes with temperature/humidity), has low reflectivity, and cannot accept enhanced aluminum coatings. Protected aluminum coatings adhere poorly due to moisture absorption, causing flaking and failure.

CRT VS. DIGITAL PROJECTORS

Characteristics of CRT Projectors

CRT projectors used large, low‑f/# lenses and short throw distances, requiring large mirrors. Large pixels and large apertures made CRT systems less sensitive to localized mirror defects, though susceptible to low‑frequency sag‑induced aberrations. Mylar mirrors helped reduce sag.

Characteristics of Digital Projectors

Digital projectors (LCOS, DLP) use smaller imaging devices, higher f/# lenses, and smaller apertures. Higher pixel density means each pixel covers a smaller mirror area, making localized defects more visible. Increased coherence of RGB light highlights distortions. Convexities and concavities create visible bright/dark bands on the screen. CRT systems would have averaged these out.

Susceptibility to Low‑Amplitude, Intermediate‑Frequency Defects

Digital projectors reveal subtle flatness irregularities as intensity disturbances in flat‑field scenes (e.g., clouds). These can be mistaken for horizon lines. Draw lines are one example of such defects.

DRAW LINES: APERTURE EFFECT

Draw‑line waviness ranges from sub‑millimeter to ~20 mm period, with amplitudes of fractions of a micron. Wavefront disturbances can modulate contrast, similar to Schlieren or shadowgraph effects. Visibility depends on illumination source size relative to defect size.

An experiment varied aperture size (½”, 2″, 5″) and photographed the resulting draw‑line visibility. Smaller apertures made draw lines more visible.

RESULTS

Contrast‑enhanced images showed draw‑line patterns consistent with those seen under digital projection.

OPTICAL REQUIREMENTS AND TESTING

Traditional mirror specifications (optical figure, scratch‑and‑dig) come from telescope optics and may not suit projection systems, which are illumination systems. Large flats are difficult to test with traditional methods. It may be better to test mirrors under simulated use conditions to detect common defects.

CONTEMPORARY APPLICATIONS

Air Mobility

Deployable simulators require lightweight, durable mirrors that withstand heat, humidity, vibration, and g‑forces during transport.

Mission Training

Multiple trainees and devices require compact systems. Large glass mirrors help minimize footprint.

Night Vision Training

Mirror coatings must support extended spectral response for night‑vision stimulation.

CONCLUSION

There is a need for large, lightweight, high‑quality flat mirrors for simulation and training. Draw lines are a newly significant phenomenon due to modern projection technology. Engineers now seek large mirrors that are rigid, free of zonal breaks, and free of local defects. Simulation mirrors are high quality but not as precise as telescope mirrors. As projectors get brighter and pixels shrink, flat mirrors become increasingly critical.

ACKNOWLEDGMENTS

Thanks to Ted Willis, Boeing optical engineer, for reviewing and editing.

REFERENCES

[1] L. A. B. Pilkington, 1969, “Review Lecture. The Float Glass Process”, Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, The Royal Society, Vol. 314, No. 1516 pp. 1-25

[2] G.S. Settles, 2001, “Schlieren and Shadowgraph Techniques”, Springer Verlag., Berlin

[3] Edmond Optics, 2004, “Optical Flat Manual”, www.edmondoptics.com