The TC20 cell counter automatically detects the slide and checks whether the sample contains trypan blue. The
auto-focus then identifies the best focal plane to discriminate cells from background.

If trypan blue is detected in the sample, the TC20 cell counter uses multifocal plane analysis in which each cell is scored on multiple focal planes to determine if it is live. This prevents undercounting of live cells, which can happen when a single focal plane is used for analysis.

Effect of multifocal plane analysis on live/dead count accuracy for HeLa cells.
Viability Assessment	Live Cells, %	Dead Cells, %
Single best focal plane*	35	65
Multiple focal planes*	65	35
* On single focal plane, 30% of live cells were misidentified as dead cells.