In my days in Royal Mail we used different frequencies of UV light to detect phosphor markings on stamps to sort 1st and 2nd letters and to sort letters to destinations from phosphor dots we imprinted on letters on automatic sorting machines.
In my days in Royal Mail we used different frequencies of UV light to detect phosphor markings on stamps to sort 1st and 2nd letters and to sort letters to destinations from phosphor dots we imprinted on letters on automatic sorting machines.
