Robert Whitman had worked with the optics of spherical mirrors in his performances during the 1960s, with performers moving in front of curved mirrors in Two Holes of Water-3 at 9 Evenings in 1966; flexible Mylar mirrors in an installation titled Pond, at the Jewish Museum in 1968; and was collaborating with optical scientist John Forkner on the optics of corner reflectors for a work for the US Pavilion at Expo '70. He helped develop the idea of using mirrors as a part of the design of the main domed space of the pavilion.

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Billy Klüver's group at Bell Laboratories had put up the first communication satellite, called ECHO in 1959 : It consisted of a aluminum-coated Mylar balloon about a hundred feet in diameter, which circled the earth at a height of about a hundred miles and was inflated by the chemicals in mothballs. The satellite achieved its purpose by passively reflecting any radio signal directed towards its large shiny surface. For eight years it relayed radio and television signals, and made intercontinental telephone calls possible. But no one had ever looked inside the ECHO during its manufacture.

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The group knew theoretically that a spherical mirror has the effect of creating real images in space The optical effect in a spherical mirror of producing a real image would resemble that of a hologram. The difference was that, because of the size of the mirror for the Pavilion, a spectator looking at the image of a person standing in the center of the floor, could  walk around the image and see it from all sides.

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The idea for using a large mirror surface in the Pavilion began as an idea for a  cylindrical section of a sphere surrounding an elevated performance platform with a glass bottom, so images of the performers would float upside down above the visitors’ heads. But through intensive discussions, the artists agreed that they wanted the images of the visitors themselves to be a large part of the experience of the Pavilion, and only a full hemisphere mirror would provide the immersive image-filled environment they envisioned.  Scientist Elsa Garmire, a professor at California Institute of Technology, joined the group and calculated the properties of such a mirror and tolerances needed to produce the desired real images.

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To test these optical effects and demonstrate them to the Pepsi executives, the group decided to build a full-scale model of the mirror. E.A.T. - Los Angeles, under the direction of David McDermott and Ardison Phillips, had been investigating air structures and organized the installation of the 90-foot diameter mirror in a former dirigible hangar at the Marine Air Corps Station at Santa Ana, just south of Los Angeles. The mirror ripped as they were inflating it, but Phillips simply put it back in its box and sent it to Raven Industries in Sioux Falls, where it was repaired and successfully installed a few weeks later. E.A.T. held a press conference and party for the artists and Pepsi executives to experience the spectacular images in the immersive visual environment of the mirror.  E.A.T. then  reached out to  G.T. Schjeldahl, in Northfield, Minnesota,  to manufacture and install the final mirror because of its experience in building the ECHO and PAGEOS satellites and its ability to hold the optical tolerances needed to produce high quality real images.

 

The architect for the project, John Pearce, devised an ingenious way the mirror could be installed and inflated and then held up by negative pressure. An air-tight cage structure was built just inside the outer dome of the Pavilion, and the Mylar mirror was secured inside this structure. A slight vacuum of less than 1/1000 of an atmosphere, was created in the space between the mirror and the cage structure, so the atmosphere inside the dome inflated the Mylar mirror and held it fully inflated. The necessary vacuum sufficient to hold up the mirror could be created by two or three large sized fans By having a negative-pressure air structure, there was no need for cumbersome air locks, so visitors could freely enter and leave the Mirror Dome.​

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The visual space in the mirror was gentle and poetic, rich and always changing. It was complex in spite of its simplicity. The group discovered new and complicated optical effects every day. Once the visitors could see themselves as a real image in the mirror, the reaction was incredible, and created much more visual excitement and participation than the group ever could have expected.