Abstract: A portable photometer for measuring the transmission properties of a fluid contained in a container having a transparent bottom is disclosed which is characterized in that there is provided: a monochromatic light source and a collimator assembly, both of which are contained within the base of a moveable member; a locating member arranged to contact the top of the container to thereby locate the end of the tubular member at a predetermined position with respect to the bottom of the container; and a photodetector device positioned beneath the container to receive the light from the collimator.