contains an illuminating lamp 21 which emits light of suitable frequency and intensity light from lamp 21 is reflected by a cold mirror 25 through an illuminator lens 27 and a transmission aperture 31 onto a transmission portion of sample 9 the lamps used in transmission densitometers must emit high intensity light and they unavoidably produce a great deal of infrared light which undesirably heats the sample in the present invention infrared heating of the sample is minimized by placing a cold mirror in the light path the cold mirror reflects desired visible light toward the sample but transmits the infrared radiation that would cause sample heating illuminator lens 27 focuses the light coming from the lamp and increases its intensity in the preferred embodiment as depicted provisions are made to vary the transmission aperture 31 such as by providing a removable aperture plate 35 which fits into a socket in the base an assortment of plates having different sized apertures allows the user to choose the proper size light beam in accordance with ansi iso 5 2 1985 ansi ph2 19 1986 light from the illuminating subassembly 19 passes through aperture 31 to reach the sample 9 placed on the measurement table light transmitted through the sample 9 passes through a pot opal diffuser 5 mounted in the measuring head 3 and enters the transmittance fiber optic lead 6.