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A Clear View

The original 50’s rally computer had a bubble screen to reduce glare thus ensuring that the navigator or driver could read the mechanical pilot hands.

In creating GaugePilot, every detail had to be correct. Trials were completed using flat screens, but they made the GaugePilot look too modern - more like a modern sat nav than a period 50’s accessory.

The screen had to be a bubble to give that right look, but also had to have suitable optical performance for the LED backlit graphical display driving it from behind.

There are a number of methods available for producing a bubble screen. The first is blow forming. This is the traditional method that was used to form aero cockpit canopies by using heated air to stretch a heated plastic sheet to form a bubble shape. The problem with this method is that although the clarity is well maintained the shape is not consistent, and the forming of complex shapes to allow, for example, seals around the buttons is simply not possible.

We experimented with the second method, vacuum forming. This produced a more regular and uniform shape but the materials available created optical distortion.

Our third and final option was to injection mould the screen. This would produce a repeatable product and the wide choice of available plastic material combinations made our challenging performance requirements achievable. Injection moulding it was to be.

With this choice of manufacturing came a quandary. To produce a high quality injected optically clear item you need a very high quality mould tool and as every engineer knows “measure twice and cut once”. The design had to be spot on otherwise the expensive tooling would be fit only to produce scrap.

The services of John Blackwell Msc CPhy of Ely Optics Limited, Cambridge, UK (http://www.elyoptics.co.uk) were engaged. They specialise in imaging and illumination engineering and were engaged to carry out a full optical design study and simulation to identify the perfect form and thickness of a screen that would deliver the best clarity and ease of viewing for the GaugePilot’s display.

John’s analysis was conducted using the Zemax optical design system and software that enabled the direct import of our CAD design data, and facilitated both sequential and non-sequential ray tracing. A sequential ray trace enables a direct measurement of optical quality and non-sequential is a photometric analysis - the analysis of visible light in terms of its brightness to human vision.

Image quality was assessed using the Modulation Transfer Function (MTF) which is a measure of contrast against spatial frequency. Essentially this measures the resolving capability of the system using contrast against fineness of detail. With non-sequential ray tracing an image simulation can be conducted that gives a visual representation of any degradation or variation between different bubble designs.

Through completing a series of tests and simulations, John was able to confirm the optimum curvature window for the lens and also, interestingly, the best thickness for the screen. It was found that the thickness of material could have interesting negative effects on the clarity at different points across the curvature.

The field of vision was also simulated thus allowing investigation and measurement of the different views that the driver and navigator would have of the GaugePilot dials in the cockpit.

We would like to thank John for his guidance and expert support in the design of the GaugePilot bubble screen.