The Sheet — Gel Library, Bristol

From wineto polyester

What has sat in front of the lamp, in order.

Shakespeare's time
Red wine in a glass
Theatres coloured light by shining it through wine held in a glass container.
Later
Coloured water, silk
Then tinted water, and silk stretched in front of the light.
Until at least 1979
Gelatin
The colour was cast right through the sheet, and it had no melting point. It charred at high heat and went brittle once used. It also gave the filter its name.
By 1945
Acetate
Heat-tolerant, self-extinguishing and dyed through: Strand Electric's Chromoid, then Cinemoid; Roscolene in the US.
Late 1960s
Halogen arrives
Tungsten-halogen lamps came into wide use and ran too hot for acetate.
1969
Deep-dyed polyester
Berkey Colortran's Gelatran. The process still makes all of GAMColor and about 30% of Roscolux.
Today
Polycarbonate, polyester
Polyester melts at 249 °C, the highest of the modern bases. Lee Filters and Apollo coat the dye onto the surface instead of dyeing through.

The standard theatre sheet

20 × 24in

The size comes from the gelatin days. A standard baker's sheet was used to cast the gel, so the sheet was the size of the tray, and it never changed. In theatre the sheets are cut down to the frame before use.

Film sets

Film crews cut from rolls 24 or 48 in (600 or 1,200 mm) wide and 50 ft (15 m) long, because the piece might cover a practical lamp or a whole window.

Burn-out

Filters wear out in the lamp, and they go from the centre first. Saturated colours and blues go soonest: they let the least light through and absorb the most energy. When the middle of the cut looks paler than the edge, replace it.