It is worth to optimize yours systems to receive a fine and reliable signal. In many cases only a week signal, such as a small transient absorption change or ultra-weak phosphorescence emission, can display photophysical and photochemical processes in the system.

First, spectroscopic sensitivity is just a possibility to get a high signal-to-noise ratio and,
correspondingly, to detect a small signal.
Second, it is also an ability to see an interesting signal and do not mess it with all around.
S.M.Bachilo (unpublished)

• High dynamic range in emission. It some our measurements the ratio of the signal amplitude to the noise was determined as 10⁶ with microsecond temporal resolution. This makes possible time-resolved detection of a very weak emission after an intense main signal and their comparison.

Example: time-resolved study of weak delayed fluorescence after intense fast fluorescence.
 
• Low noise in transient absorption detection by flash - photolysis. In an optimized experimental system the noise level should only be caused by the quantum noise of the probe light. In our experience, this noise level can correspond to as low as 10^-5 O.D. for a single laser shot with a filament probe lamp, and to 10^-6 O.D. with a flash probe lamp. Application of averaging can decrease the noise in reverse proportion to the square root of the number of pulses. With excitation rates of 50 Hz the noise level will drop below 10^-6 O.D. in a few seconds of data collecting.

Example: detection of symmetry-forbidden absorption between the first and the second excited states in molecular oxygen in solutions.
 
• High dynamic range and low noise in optically-gated transient absorption technique. Conventional femtosecond pump-probe systems can have noise as low as 5x10^-5 O.D. for 1000 pulses (0.2 - 1.0 s averaging per point). Application of special methods can make the noise lower for a factor of ~3. A system with high repetition rate (as 5 kHz) can show noise equivalent to 1x to 2x 10^-5 with just one second averaging per point.

Example: using a very low excitation power to study transient absorption in objects with high non-linearity, such as photosynthetic systems.

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