This project started internally at DTU Fotonik when Henrik C. Pedersen and Henning E. Larsen came up with the idea that light depreciation of street lamps – either due to aging or due to dirt – could be compensated for by adjusting the driving current according to actually measured light output values. The system was developed and sold to Hesa Light A/S in Roskilde, Denmark.
The majority of street lamps are today being equipped with LEDs which offer life times of 15 – 25 years. In this life span the lamps will degrade gradually both due to aging and due to dirt accumulating on the cover glass. Some lamp manufacturers try to compensate for the degradation by gradually increasing the driving current in the lamps. However, the rate of degradation depends highly on ambient temperature, traffic load, amount of insects, etc. and is therefore hard to predict. As a result, we expect great variations in lamp performance to occur over time.
Our solution consists of a light detection system that can be mounted at a car roof and a wireless communication system by which the car can communicate with the individual lamps. When driving the car under a row of street lamps the individual light outputs from each lamp are measured and compared with the nominal value. The discrepancies from this nominal value are then communicated wirelessly to the lamps in which an intelligent receiver adjusts the drive current so that the lamps are reset to nominal lighting level and thereby comply with regulations. A blinking module was also developed and installed in each lamp. In this manner each lamp was wirelessly instructed to start blinking with its own unique frequency. Hence, even in road areas with overlapping light beams, the individual light contributions could be separated by the sensor module mounted on the car. Moreover, the blinking also made it possible to do the measurements in full day light, as the blinking light beams could easily be distinguished from the sunlight. This is illustrated on the videos below.