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Digital Subscriptions > Doctor Who Magazine > DWM Special 43 – Special Effects > The GOLDEN AGE of CSO


In the era of glam rock, hot pants and Dalek’s Death Ray ice lollies, Doctor Who created futuristic control centres, invisible aliens and giant robots with the aid of yellow backdrops and a clever gizmo.
Production assistant George Gallaccio in Studio 4 at Television Centre for the recording of Planet of the Daleks in 1973.

In the hostile jungle of the planet Spiridon, the Doctor and a trio of Thals come upon an invisible mechanical device, immobilised by what Thal scientist Codal calls “light-wave sickness”. A circular depression in the ground is the only indicator of its presence – until Codal passes the Doctor a can of liquid colour. As black paint coats the device, a familiar form takes shape. A Dalek!

The Thals’ ‘liquid colour spray’ and the BBC’s Colour Separation Overlay reveal the threat in Episode One of Planet of the Daleks.

This memorable scene from Episode One of the 1973 serial Planet of the Daleks dramatically demonstrates a visual effects process that has become synonymous with 1970s television. Colour Separation Overlay (CSO) was a technique that combined analogue colour television images, mixing shots from separate cameras to provide an instant composite picture recorded‘live’ in the studio. Also known outside the BBC as ‘Chromakey’, CSO has often been derided and dismissed as symptomatic of low-budget programming, yet in its time it was widely regarded as the industry’s most significant development in decades, offering an affordable method of realising imaginative concepts that would otherwise have been impossible to convey.

Director Christopher Barry retained this Telesnap of the inlay effect seen in The Dead Planet (1963), the first episode of The Mutants (aka The Daleks).

Prior to colour broadcasting, black-and white television combined separate images with a system called inlay where the picture from a studio camera was masked on one side and electronically overlaid onto a telecine film insert. This process was first used in Doctor Who in the opening episode of the 1963-64 serial The Mutants (aka The Daleks), for the scene in which the Doctor (William Hartnell) and his companions see the Dalek city for the first time. The actors appeared on the studio forest set on the right with pre-filmed footage of the Dalek city model on the left.

Recording a CSO scene for Episode Two of Planet of the Daleks at Television Centre on 23 January 1973. The Spiridon and the nearby Dalek are blending in with the yellow backdrop.
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About Doctor Who Magazine

Special effects can transport audiences to alien planets, render familiar surroundings unrecognisable and bring terrifying monsters to life. Doctor Who has been at the forefront of such television trickery for more than 50 years. This richly illustrated publication celebrates the series’ greatest effects and meets the people who created them. From the trailblazers of the 1960s to the digital artists of today, here is the story of Doctor Who’s journeys into the impossible.