Photomultiplier protector for a fluorometer

In a photometer which is particularly adapted for sensing radiation emitted by a chemical sample held in a cuvette in a first compartment and which includes a highly sensitive photomultiplier in a second compartment adjacent the first compartment for detecting fluorescent radiation emitted by the chemical sample and passing through an opening between the compartments, the improvement comprising:

• means for protecting the photomultiplier from ambient light, said protecting means including a movable cylindrical light shield movable between a first position blocking the opening and a second position not blocking the opening and means for moving said light shield to said first position to protect said photomultiplier from light entering from the first compartment when the first compartment is opened for insertion or removal of a cuvette, and for moving said light shield to said second position not blocking the opening to permit radiation emitted by said chemical sample to impinge upon the photomultiplier in the second compartment, said light shield being made of opaque material and having a single opening therein which is in registry with the opening between said compartments when said light shield is in said second position so that radiation may pass from the chemical sample to the photomultiplier, said moving means including a shaft fixed at one end to the top of said cylindrical shield and coaxial therewith, an upper end of said shaft extending through the top of the second compartment, a knob fixed to the upper end of said shaft, a larger diameter apron beneath and integral with said knob, said apron being constructed with an undercut section opening onto the bottom of said apron and onto the side periphery of said apron through a predetermined arcuate angle so that said undercut section also is defined between a first end wall and a second end wall arcuately spaced from said first end wall in said apron, said apron further having a radial slot therethrough opening onto a top surface of said apron and into said undercut section adjacent said first end wall, a spring biased pin extending from the top of the second compartment, said slot in said apron being aligned with, and receiving therein, said pin when said shield is in said first position, the engagement of said pin in said slot locking said shield in said first position, and the top of the first compartment having a pivoted cover with an elongate tongue extending laterally from a side edge thereof and being received in said slot when the cover is moved from a cover raised position to a cover lowered position, said tongue then depressing said pin below said apron until said tongue is beneath said undercut section, and means for rotationally biasing said apron in one arcuate direction so that when said tongue is beneath said undercut section, said biasing means can move said apron to move said second wall against said tongue where said shield is locked in said second position and the cover is locked in the lowered position, and opening of said cover being enabled by rotating said knob in the opposite arcuate direction against said biasing means until said tongue is received in said slot.