A drainage mat for use in building structures, particularly in exterior walls and roofing, to improve drainage and ventilation within such structures. A transmission beam detector detects a light intensity of a transmission beam which is a second portion of the reference beam, wherein the transmission beam is not diffracted to reproduce the holographic data and passes through the holographic medium. The rail caps have a vertical front surface and an inclined rear surface which are configured to be coupled to the end of the ladder.