Emitting nanosecond light pulses

Another micro-destructive technique is laser-induced breakdown spectroscopy (LIBS) which is used to analyse the paint layers. Nanosecond laser pulses vaporise a small amount of material from the surface of the painting and the amounts so lost are only 50 billionths of a gram and too small to be seen with the naked eye. The vapour passes between two high-voltage electrodes which excite the atoms and these then emit a pattern of light energy bands which identify the elements. LIBS together with Raman spectroscopy has been used to examine Russian icons which are multilayered. 

What all FLIM instruments have in common is that 1) the excitation light is intensity-modulated or pulsed, and 2) that the emitted fluorescence light is measured time-resolved. Since the lifetimes that have to be resolved are typically in the nanosecond range, this means that both the modulation of the excitation light and the detection need to be performed at extremely high speeds. Consequently, most of the conventional instrumentation used for steady-state fluorescence microscopy cannot be used. In the next section several modes of implementation of FLIM are discussed. 

For many lasers used in scientific work, the light is emitted in a short pulse lasting only a few nanoseconds, but the pulses can be repeated at very short intervals. Other lasers produce a continuous output of light. 

In the typical setup, excitation light is provided by a pulsed (e.g., nanosecond) laser (emitting in the visible range, e.g., at 532 nm, if Mb is investigated), while the probe is delivered by a continuous-wave (cw) laser. The two beams are spatially overlapped in the sample, and the temporal changes in the optical properties (such as optical absorption or frequency shift) that follow the passage of the pump pulse are registered by a detector with short response time (relative to time scale of the processes monitored), such as a fast photodiode. 

After the excitation pulse, the intensity of luminescence emission decays with the rate that is characteristic to the used label. The photons emitted during the delay time, the time between the pulse and activation of the detector, are lost and, thus, a short delay time is preferred in the detection. The background luminescence decays typically very rapidly (within tens of nanoseconds) and long delay times are not required. However, many used light sources exhibit a switch-off delays and afterglow that prevent the use of short delay times. This switch-off time is an important characteristic of a light source and may limit the performance of a time-resolved fluorometer significantly. 

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