Abstract: An optical arrangement in photometrical measuring devices for use in fluorescence and absorption measurements, extinction, nephelometrical and turbidimetrical measurements comprises a light source for emitting a monochromatic radiation, a field aperture for said radiation, and lenses which focus said monochromatic radiation onto a sample material for being analyzed. The sample, in response to said monochromatic radiation emits a first radiation portion incorporating the measuring informations from said sample, and a second radiation portion not containing measuring information. An optical imaging system is provided for directing said first radiation portion onto a detector. A removable optical member for retaining the second radiation portion is provided adjacent the optical imaging system and has an aperture greater than the field aperture.