Police Effect

Overview

The Police Effect (also known as "Cop Light Effect", "Police Car Light Gag", or in US slang "Whirligig"/"Whirly-Gag") is not a single device, but a lighting effect: it mimics the pulsating, rotating lights of emergency vehicles – the red-blue flashing of police, fire, or ambulance vehicles. It is used to flood a scene with the impression of an approaching or stationary emergency vehicle, without the vehicle itself needing to be in the frame.

In Europe, pure blue lights dominate, while North American productions typically use the classic red-blue combination. The effect characteristically creates moving cones of light across walls, ceilings, and faces, and is preferably generated practically on set because the reflections and movement on real surfaces are difficult to achieve credibly in post-production.

On-Set Implementation

Several methods have become established for the Police Effect:

• Whirligig / Spinning-Mirror-Rig: The classic version. A spotlight (often a 1K PAR or 2K Fresnel) is directed into a rotating mirror setup. The mirrors are mounted in a triangle or as a cube and carry red and blue gels; the rotation creates rotating light reflections. Such ready-made rigs can be rented from rental houses (in the US, e.g., Hollywood Rentals, Cinelease, Acey-Decy).
• Spinner / Cherry-Light: A rotating light, modeled after a real emergency beacon, which is colored with gels.
• Programmed LED Effect: Modern LED fixtures offer the effect as a built-in macro. Astera includes a pre-programmed "Police Car" effect in the AsteraApp, allowing speed and colors to be adjusted. Alternatively, the effect can be programmed via the console as a step-based color effect (chase).
• Practical Emergency Light Bars: Real LED police/visor lights with built-in flashing patterns, often battery-powered, as a compact source directly in or on the frame.

Control and Camera

To make the effect appear credible, the pulse frequency (typically in the single-digit Hertz range, often approx. 1–4 Hz), color, and intensity are adjusted to the scene. Programmable dimmer consoles or the fixture's own app allow for smooth panning and pulsing without motor noise – an advantage over purely mechanically spun rigs when recording live sound.

When using the camera, attention must be paid to the interplay between the flashing frequency and the shutter: rolling shutter sensors can show banding or partial exposures with harsh strobe patterns. Softer pulsing (instead of hard on/off) and synchronizing frequency and exposure time reduce such artifacts.