5WG1255-2DB21 KNX Brightness Sensor with Constant Light Controller 5-CH UP 255D21

The 5WG1255-2DB21 (UP 255D21) is a KNX brightness sensor with integrated constant light level controller, designed for ceiling installation. Unlike presence detectors, its primary function is precise illuminance measurement via reflected light from the working plane below, with a range of 0–2,000 lux.

It supports mixed-light measurement to distinguish between daylight and artificial light contributions. The built-in constant light controller manages up to 5 lighting groups (1 main + 4 additional), maintaining a configured lux setpoint by automatically dimming or brightening artificial lighting. An integrated IR receiver supports 6-pair remote control, and the sensor can also detect motion for supplementary occupancy functions.

The UP 255D21 is designed for spaces where maintaining a consistent illuminance level on the working plane is critical, regardless of changing daylight conditions.

Primary applications include offices with large windows, atriums, skylit corridors, retail showrooms, museums, and art galleries — any environment where daylight harvesting must be combined with artificial lighting to maintain visual comfort.

The constant light controller automatically compensates for daylight variations, dimming artificial lights when sufficient natural light is available and increasing output when it is not. The mixed-light measurement ensures accurate distinction between daylight and electric light contributions for precise control.