Time-gating

Fig. 2. Three time gates set to measure backseattered pulse from cylinder interface echo (gate 1), echo from discontinuity (gate 2) and back echo (gate 3).

The Evaluation system is a Windows based open system through DLL, Dynamic Link Library, which provides great flexibility in evaluation and presentation of data. It also makes it possible to customise evaluation and images for special applications The time gates can be set after testing and there is a 256 colour range for amplitude. The software include FFT -facilities which enables measurements on used probes for parameters such as center frequenzy and bandwidth. 

The PE data was obtained by repeating the scanning of the object, now measuring the received echo at Transducer 1. For every position, (x, y), an A-scan was obtained from which we extracted the back wall echo by means of a time gate. This back wall echo is denoted s(x, y). Note that s x, y) is a time signal that can be written s(f, x, y) where t is the time index. One example of such a back wall echo is shown in Figure 2. 

In conclusion, GD-OE S is a very versatile analytical technique which is still in a state of rapid technical development. In particular, the introduction of rf sources for non-conductive materials has opened up new areas of application. Further development of more advanced techniques, e. g. pulsed glow discharge operation combined with time-gated detection [4.217], is likely to improve the analytical capabilities of GD-OE S in the near future. 

The key to the good resolution of ZEKE-PFI is in its discrimination against electrons with >1 cm-1 kinetic energy. This is due to the delayed extraction and time-gated detection. A bubble of 1 cm"1 electrons expands to a radius of 1.6 cm during the 2.0 ps delay. Such kinetic electrons either miss the detector or arrive... 

Figure 2 Time-gating as a way of separating short-lived luminescence from long-lived luminescence.

Lifetime imaging can be implemented both in wide field and in scanning microscopes such as confocal microscopes and two-photon excitation microscopes. The most common implementations in time-domain fluorescence lifetime imaging microscopy (FLIM) are based on TCSPC [8, 9] and time-gating (TG) [2, 10],... 

In TG methods, the fluorescence emission is detected in two or more time-gates each delayed by a different time relative to the excitation pulse (see Fig. 3.3). In the case of a detection scheme equipped with two time-gates, the ratio of the signals acquired in the two time-gates is a measure of the fluorescence lifetime. For a decay that exhibits only a single exponent, the fluorescence lifetime is given by ... 

Fig. 3.3. Principle of time gating (TG). After exciting the specimen with a short light pulse, the fluorescence is detected in a number of time gates that open after a specific delay with respect to the excitation pulse.

Fig. 3.4. Schematic diagram of a TG setup. The TG electronics need careful synchronization with the excitation pulse. Here, time-gated single photon... 

Time-gated detection offers the possibility to suppress background signals correlated with the excitation pulse. Direct and multiple scattered excitation light as well as Raman scattering reaches the detector at t 0, and can be effectively suppressed by opening the first gate a few hundred picoseconds after t = 0. 

In Fig. 3.5A a comparison between time-gated detection and TCSPC is shown. The time-gated detection system was based on four 2 ns wide gates. The first gate opened about 0.5 ns after the peak of the excitation pulse from a pulsed diode laser. The TCSPC trace was recorded using 1024 channels of 34.5 ps width. The specimen consisted of a piece of fluorescent plastic with a lifetime of about 3.8 ns. In order to compare the results, approximately 1700-1800 counts were recorded in both experiments. The lifetimes obtained with TG and TCSPC amounted to 3.85 0.2 ns and 3.80 0.2 ns respectively, see Fig. 3.5B. Both techniques yield comparable lifetime estimations and statistical errors. 

For the simple TG scheme with only two time-gates, the optimum gate-width amounts to 2.5 t. Consequently, the total integration time per pulse amounts to 5 t and approximately 99% of all photons in the decay are detected. The detected fraction decreases when an offset is applied between the laser pulse and the opening of... 

In time-gated photon counting, comparatively high photon count rates can be employed count rates as high as 10 MHz are often used. TG has the advantage of virtually no dead-time of the detection electronics ( 1 ns), whereas the dead-time of the TCSPC electronics is usually on the order of 125-350 ns. This causes loss of detected photons, and a reduced actual photon economy of TCSPC at high count rates. 

Not only PMTs and other detectors such as avalanche photodiodes suffer from dead-time effects also the detection electronics may have significant dead-times. Typical dead-times of TCSPC electronics are in the range 125-350 ns. This may seriously impair the efficiency of detection at high count rates. The dead-time effects of the electronics in time-gated single photon detection are usually negligible. 

Therefore, the throughput of current systems based on time-gated SPC is somewhat higher than in TCSPC-based systems. 

A more common approach to time domain wide field FLIM is based on a time-gated image intensifier MCP in combination with a CCD camera (see Fig. 3.8). After every excitation pulse the gated... 

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