How can an intensified camera give fluorescence lifetime contrast?
There are several types of image intensifier. Early intensifiers (‘Gen-I’) used a photosensitive cathode and a phosphor screen in a vacuum envelope equipped with focusing electrodes. A very high voltage (several tens of thousands of volts) accelerates emitted photoelectrons which collide with the phosphor screen causing light to be emitted. Intensifiers of this type typically have low ‘gain’ (i.e. the light from the phosphor screen is not very bright at low incident light level). For this reason such intensifiers were commonly connected in series, so that the screen of one intensifier is optically coupled to the photocathode of another intensifier. For optimum sensitivity three intensifiers were often connected in this way. The disadvantage of this approach is the considerable bulk of the package and the need to supply very high voltages. Each of the intensifiers has an associated fluctuation or ‘noise’ in the output and the combination of intensifiers amplifies this ‘noise’. It is dif
