To　avoid　mal-defrost　phenomenon,　an　innovative　photoelectric　sensor　is　developed　and　presented　in　this　paper.　It　is　referred　to　as　＂Tube　Encircled　Photoelectric　Sensor＂　(TEPS).　Experiments　are　carried　out　in　a　controlled　environmental　chamber　under　standard　frosting　conditions.　Ten　TEPSs　in　4　different　models　are　tested　on　a　commercial　size　air　source　heat　pump　with　the　nominal　heating　capacity　of　60　kW.　The　characteristics　of　the　air　source　heat　pump,　together　with　the　performance　of　the　TEPSs　are　investigated　during　9　periodic　frost-defrost　cycles.　Compared　with　the　original　defrosting　control　strategy　equipped　by　the　manufacturer,　the　proposed　TEPS　sensor　reveals　its　potential　ability　to　accurately　control　the　defrosting　process.　Experimental　results　demonstrate　that　TEPSs　can　substantially　prolong　defrost　intervals　from　28.8　min　to　52　min　under　the　experimental　conditions,　and　the　number　of　defrost　cycles　can　be　reduced　from　9　to　5.　The　performance　improvement　is　found　to　be　6%　to　the　heating　efficiency,　and　5%　to　the　COP.　(C)　2012　Elsevier　Ltd.　All　rights　reserved.