Each time an airplane touches down on a runway, it leaves a little bit of itself behind. A single 747, for example, deposits 7.5 lb of rubber whenever it lands. This rubber accumulates over time and can produce a film that coats landing lights, decreasing the efficiency of illumination intended to give pilots critical visual cues from the ground.A mobile system for monitoring airfield lighting calculates light output from runway lamps, enabling cost-effective, targeted maintenance of critical illumination.TMS Photometrics Ltd. has developed a mobile monitoring system for airfield lighting that can test the intensity and alignment of 800 light sources in just a few minutes and identify which ones are underperforming. The system consists of an array of photodiode sensors and a multi-range lux meter that is calibrated against a standard photometer provided by the UK National Physical Laboratory.The device is towed down the runway at speeds up to 40 mph. Light from the tungsten-halogen lamps is collected and processed to calculate candela values at each azimuth and elevation. This provides an isocandela diagram that is available to the vehicle data logger or to an office computer.Hugh Dawson, a 26-year veteran of airfield management, has been using the system continuously for four years at Birmingham International Airport. The airport handles 7.9 million passengers every year and has a 2600-m runway. Dawson has five two-man rotating shifts that test the 5000- to 10,000-cd AGL lamps to ensure that they are performing at a minimum of 70 percent. Before the monitoring system was developed, the airports either changed fittings when they were out or made regular "block changes" of a whole section. Neither method was cost-effective. The new system has achieved good photometric performance that enables the targeted maintenance that is crucial for safety in low-visibility conditions. Dawson calls it a "block change in philosophy.""It has given me confidence in the lighting standard that we produce," he said. "We use it religiously every week on the centerline -- and [on] edge lighting and touchdown-zone lighting every fortnight."He can download information from the monitor onto his PC and produce a spreadsheet that provides him with a lamp evaluation and maintenance record. Based on this report, he can decide which lamps to change.Dawson said that his maintenance team doesn't do block changes anymore and that it hasn't increased costs in five years, despite inflation. In fact, it saved 10 percent this year, which has given him the opportunity to reinvest the money.He said that when the Civil Aviation Authority carried out its annual audit before, it would complain about the low output of the lamps, assuming that it was a result of the undulation of the runway. Now Birmingham's lamps are the standard for the country. In fact, some of the pilots have asked the airfield to turn down the brilliance. In terms of improving performance, Dawson said, "We can't do any more than what we're doing."