LEDs are not just used for illuminating a sample in time-lapse microscopy. They are also used in most focus drift correction systems (from manufacturers such as Leica, Nikon and Zeiss), which are feedback devices used to maintain focus. At high magnifications, the in-focus plane is a few microns, making such small focal planes susceptible to focal drift, which is typically caused by thermal expansion and contraction.
To overcome the challenge of thermal drift, there are two common approaches: software-based algorithms and hardware-based z-correction. Software routines can adequately maintain focus as long as cells are restricted to a similar image plane, but can be confused by free floating non-adherent cells. In hardware drift correction systems, light is back-reflected down the objective to a detector, monitored in real time and relayed into the microscope Z-drive. This will maintain the desired focal position irrespective of the cellular environment and is crucial for many research projects such as focal adhesion formation or new superresolution techniques (i.e. PALM, STORM and GSD).
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