A simple high optical efficiency reflective optical sensor for bill validator uses an inexpensive bulb having a case which is transparent to efficient luminous radiation. This case is used as a wave guide to return reflected radiation to at least one photo detector situated directly under the transparent bottom of the case. The bulb emits a narrow beam of light and is positioned in close proximity and perpendicular to a bill surface and illuminates it. The light reflected or fluoresced by the bill is collected widely with a convex lens end of the bulb case and this collected radiation is transmitted through the bulb case to the at least one photo detector. Preferably, the bulb is a light emitting diode.