and measures the signal using a light detector such as a photomultiplier tube (PMT). The advantages of fluorescence detection over absorbance detection are sensitivity, as well as application range, given the wide selection of fluorescent labels available today. For example, a technique known as calcium imaging measures the fluorescence intensity of calcium-sensitive dyes to assess intracellular calcium levels. Luminescence Luminescence is the result of a chemical or biochemical reaction. Luminescence detection is simpler optically than fluorescence detection because luminescence does not require a light source for excitation or optics for selecting discrete excitation wavelengths. A typical luminescence optical system consists of a light-tight reading chamber and a PMT detector. Some plate readers use an Analog PMT detector while others have a photon counting PMT detector. Photon Counting is widely accepted as the most sensitive means of detecting luminescence. Some plate readers offer filter wheel or tunable wavelength monochromator optical systems for selecting specific luminescent wavelengths. The ability to select multiple wavelengths, or even wavelength ranges, allows for detection of assays that contain multiple luminescent reporter enzymes, the development of new luminescence assays, as well as a means to optimize the signal to noise ratio. Common applications include luciferase -based gene expression assays, as well as cell viability, cytotoxicity, and biorhythm assays based on the luminescent detection of ATP. Time-resolved fluorescence (TRF) Time-resolved fluorescence (TRF) measurement is very similar to fluorescence intensity (FI) measurement. The only difference is the timing of the excitation/measurement process. When measuring FI, the excitation and emission processes are simultaneous: the light emitted by the sample is measured while excitation is taking place. Even though emission systems are very efficient at removing excitation light before it reaches the detector, the amount of excitation light compared to emission light is such that FI measurements always exhibit fairly elevated background signals. TRF offers a solution to this issue. It relies on the use of very specific fluorescent molecules, called lanthanides, that have the unusual property of emitting over long periods of time (measured in milliseconds) after excitation, when most standard fluorescent dyes (e.g. fluorescein) emit within a few nanoseconds of being excited. As a result, it is possible to excite lanthanides using a pulsed light source (Xenon flash lamp or pulsed laser for example) and measure after the excitation pulse. This results in lower measurement backgrounds than in standard FI assays. The drawbacks are that the instrumentation and reagents