People sensing system

Each of one or more sensor units comprises a respective image sensor and a respective local processing module, with each local processing module being configured to detect locally at the respective sensor unit whether a person appears in the images captured by the respective image sensor. Further, at a device external to the sensor units, there is received a respective test image captured by each image sensor under conditions of being illuminated by a predetermined illumination from one or more already-commissioned luminaires at predetermined spatial positions, the test images being reduced to a lower resolution than used for the people detection. For each of the sensor units, the respective reduced resolution test image is compared with a respective expected response, the expected response being based on the predetermined positions of the respective one or more luminaires. Based on this, a problem with one of the sensor units can be detected.