Microscope-based photometry permits detection of cellular fluorescence within a microscope field and displays and stores this as the number of photons per unit of time. Sample fluorescence is directed through the microscope, passing through a filter cube which contains the dichroic and blocking filters. Fluorescence is collected by a photomultiplier tube (PMT) attached to the microscope by a side arm which contains the iris diaphragm, the adjustment of which determines the size of the field analyzed. The PTI is not an imaging system. As such the system affords no spatial resolution, i.e., images of the cell are never displayed and, consequently, detection of possible differences in sub-cellular distribution of fluorescence is not possible. However, the system does offer enhanced temporal resolution when compared with conventional video-based systems such as the ACAS 570. Another unique feature of the PTI, as compared to the ACAS, is the potential for using dual-excitation, single emission probes such as fura-2. In fact, the PTI monochromator permits selection of a wide range of excitation wavelengths, from UV to red, and if required, the ratioing of any two.