Burroughs SD-11 Sphericular Display

Burroughs SD-11 Sphericular Display
Written by AnubisTTP on 2017-08-19

Description

At first glance, the Burroughs SD-11 may appear to be a standard projection display, but it is in fact an unusual device known as a sphericular display. The front panel of the SD-11 is a complex two-dimensional array of faceted refractors which generate the displayed digit; there are no lenses or slides as in a standard projection display. The angle of the light striking the refractor controls which digit is displayed, through use of an ingenious pixel mask that blocks light to each pixel based on the positions of the bulbs in the back of the device. The pixel mask installed in this example can display the digits zero through nine with or without a decimal point. Eliminating the bulky lenses used in projection displays allows for the manufacture of a display that is a fraction of the weight of competing devices. The displayed characters on the SD-11 are not as clear as those displayed by most projection displays, but the visual appearance of the sphericular characters is quite distinctive. Unfortunately the SD-11 can not be easily modified to display different characters... the pixel mask is fused to the inside of the display with epoxy and is extremely difficult to remove.

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Burroughs SD-11 Incandescent Display
Burroughs SD-11-W-1000 sphericular optical display.

Burroughs SD-11-W-1000 disassembly
SD-11 incandescent display with back removed. The display body is a perplexing device that is mostly empty space and glued together, making further disassembly problematic.

Burroughs SD-12 Sphericular Display
Cutting through the front of the display reveals that all of the action happens in a single wafer approximately 2mm thick. This wafer can be further separated into three structures that do all the work of bending light into the numerical shapes.

Burroughs SD-12 Incandescent Display
The front-most layer is a pixel mask that contains dots representing each digit superimposed on each other.

Burroughs SD-12 digit lens
The middle layer is a lens made up of a 30x30 matrix of sub-lenses. Each sublens projects a miniature reproduction of the bulb pattern at the back of the device onto the pixel mask.

Burroughs SD-12 digit mask
Here is the pixel mask at 20x magnification. Each pixel of the pixel mask contains a transparent reproduction of the projected bulb pattern from the back of the device. Pixels that are not used in a given digit are blacked out in the bulb pattern on the mask. In this manner the mask 'programs' which pixels are lit for each bulb.

Burroughs SD-12 shadow mask
The back layer of the image stack is a metal shadow mask, which is fused to the lens layer with melted plastic. The shadow mask prevents light from bleeding between adjacent pixels for a crisp display.

Burroughs SD-12 display
This composite image shows the principle of operation in effect. Two digits are lit at the same time, with the second image showing what the bulb pattern in the back of the display looks like. The third image is a 20x close up of the pixel mask, showing the bulb pattern reduced in miniature and selectively blocked.

Burroughs SD11 display
A comparison of sphericular digit reproduction with other period technology. From top left; Burroughs SD-11, Burroughs 4021 Nixie tube, IEE 340 projection display, IEE BA-0000-P31 Nimo tube, KGM Electronics IND-1803 edge lit display, Hewlett Packard 5082-7000 LED.

Burroughs Incandescent Display
Since the SD-11 requires no lens stacks or slides, is much lighter than the average projection display.

Burroughs Display with LEDs
Due to the way the SD-11 works, its appearance is noticeably improved by replacing the bulbs with LEDs. Narrow beam LEDs produce clearer numbers because they produce less backscatter that can strike unwanted pixels.

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