A document photosensor is provided which comprises upper and lower sensor assemblies (1 and 2) disposed on opposite sides of a passageway (55) along which a document (50) is transported. The upper sensor assembly (1) comprises an upper substrate (11), an upper LED chip (21) and an upper PD chip (37) surface-mounted on the upper substrate (11). The lower sensor assembly (2) comprises a lower substrate (12), a lower LED chip (31) and a lower PD chip (38) surface-mounted on the lower substrate (12). These chips (21, 37, 31 and 38) are precisely located on the upper and lower substrates (11 and 12) so that the PD chips (37 and 38) exactly detects a light that is irradiated by the LED chips (21 and 31) and penetrates through a particular point on the document (50), and that improves performance of bill validation.