OPERATIONAnnex LSM 5104-190 B 40-051 e 07/98Examples of application:a) Checking the laser powerThis function is not automized so far. To qualitatively measure the laser power the diode can be used insuch a way, that the graylevel obtained in the Line Scan mode at a certain setting of the whole systemis stored as a text overlay together with the image (manually done by user). As the diode setting (Ampl.Gain, Ampl. Offset, ND filter) is stored together with the image, the setting is automatically reloadedwhen using the button. If deviations can be observed it is easy to set the laser power to the oldvalue by means of the AOTF transmission.b) Noise Reduction by RatioIn contrary to the PMT signal, the signal of the monitor diode information. Thus it can be used to ratio the PMT signal to get rid of the laser noise (due to any laser as aphysical fact) and thereby improve the signal to noise ratio of the fluorescence or reflectance image. Themajor condition which has to be fullfilled to use the monitor diode for this purpose, is, that thedominating source of noise is laser noise. The signal of the monitor diode will always be dominated bylaser noise (independent of the power set at the laser, or the transmission set at the AOTF), whereas thedominating source of noise in the PMT signal can also be the shot noise of light (shot noise especiallyoccurs in low light fluorescence application; as rule of thumb it can be noted, that the shot noise islimiting the signal to noise ratio, if the PMT voltage has to be set to a value > 400V).Note: Any kind of noise which can not be observed in both channels at a time will be amplified and notreduced by the ratio process. Low or high frequency laser noise is the only source of noise which iscorrelated in the PMT signal and the signal of the monitor diode.Using the HeNe lasers, noise is extremly low, so that in all applications, if, the shot noise of light willreduce the signal to noise will reduce the signal to noise ratio of the image. Low or high frequency lasernoise is mainly introduced, if the Ar, ArKr lasers are used at a tube current lower than 8A (Ar-Vis, ArKr)or 20 A (Ar-UV) respectively.
