Flicker-free dimming

CCR and PWM compared

LED lighting suitable for film recording

Cameras are integrated into almost all of today's mobile devices and record our environment in moving images: for example video conferences on laptops, augmented reality apps and smartphone clips for social media. Among other factors, lighting is decisive for the quality of the recording. Today's camera chips are powerful enough to generate good images even in buildings with dimmed lighting conditions. For videos with high quality, the dimming method of the lighting installation and the quality of the control gear therefore must be taken into account by the lighting design.

CCR and PWM: the difference in perception

For architectural illumination two dimming methods have prevailed – constant current dimming (CCR) and pulse width modulation (PWM). CCR continuously reduces the current and therefore brightness, whereas the signal is interrupted with PWM. A sequence of on-off conditions is created. Our eye is too slow to perceive this rapid on-off switching. With film recordings however, depending on the frequency, interferences in the image material are created. With a motionless object and still camera, modern camera systems are able to compensate for this after a short time. Commercially available devices however cannot implement this compensation. These image artefacts are displayed as horizontal lines on the display for example with smartphones and tablets.

LED lighting suitable for film recording

The relative luminous flux of LEDs and the output current of the control gear are almost proportional with both methods. As such, the representation of the operating current corresponds to the visual impact of the constant current dimming (CCR) and pulse width modulation (PWM) methods of dimming. With CCR, the current is reduced and therefore also the luminous flux of the LEDs. PWM on the other hand functions with an on-off signal and alternating pulse duration. The operating current with on-state of the LEDs remains constant. In a similar way to a sequence of single images with film formats, our eye does not perceive this on-off frequency as single pulses. As a consequence, the impression of lower brightness is created. Camera systems scan an image line-by-line though. Off-states of the LED are therefore displayed as dark lines on the image.

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