When making microfluorescence measurements using a single point detector, such as a photomultiplier, it is important to mechanically define the Region of Interest (ROI) that reaches the photodetector.

The Cairn rectangular diaphragm and holder assembly provides a convenient and flexible means of doing this, and is straightforward to interface with any detection (or illumination) system. The assembly features a C-Mount input thread (for accurately positioning the aperture in an image plane) and a 30mm (inner diameter) output tube for compatibility with Cairn photomultiplier connector optics. Although usually used in conjunction with a microscope it is also compatible with C mount camera lenses. The aperture can be positioned in the primary image plane or directly in front of a photomultiplier with an LED to project the aperture back on to a camera (for simultaneous photometry and imaging).

The device comprises of an aperture delimiter with independent X & Y adjusters to set an arbitrary rectangle of upto 12mm square. This framing component sits in a housing which allows the aperture to be moved (within the field of view) and rotated so as to orientate it with the required object.