On 25 November we carried Nimbus down from the drawing room and set it on the long bench in our Beijing studio, and we left the door open until the street lights came on. Anyone could lift the lamp, walk it across the room, stand it on a shelf and see what it did to the wall behind. Nimbus began as a sketch on tracing paper. That evening it went home in people's hands.

A cloud held by one line

The first drawing took under a minute: a soft round shape with a single stroke beneath it, the way a low cloud rests on a horizon. We pinned it above the bench and lived with it for months. Tracing paper is honest. It keeps every line you were unsure about, and there were many. What survived was the roundness, the low centre of gravity and the rule that the light source should never show itself. The rest we took away.

Arguing with physics

A portable lamp has to be light enough for a child to carry and heavy enough to stay put when someone brushes past the table. We cast the base, bored it out and cast it again. The diffuser took longest: too thin and the light turns hard and flat, too thick and the lamp glows like a bottle of milk. We settled on a hand-finished shell that softens the beam without robbing it, and a brass ring exactly where a hand wants to land. Four things changed between the paper and the object.

• The handle disappeared; the body became the grip.
• The flat disc turned into a shallow dome, so light falls on the table rather than into the eyes.
• Weight moved down into the base, and the lamp holds its ground on a linen cloth.
• The switch became a touch surface you can find in the dark.

Visitors stayed longer than we had planned for. A woman from the street held Nimbus for ten minutes and asked, twice, how the shade was made. Two architects set theirs side by side on the windowsill and compared the shadows. By eight the bench was crowded and we kept moving the lamps so everyone could see them lit at once. We sold nothing that evening, because nothing was for sale; we wanted to know how the lamp behaved among strangers.

Nimbus goes into production in the spring. The tracing paper is still pinned above the bench, curling at the corners, with the first cloud on it. We leave it there on purpose. A drawing takes a minute. A lamp you can carry takes a year of taking things away.