How does a confocal microscope work?

Focused beams of laser light are scanned across the sample (Laser Scanning Microscopy, LSM) and light only from the desired focal plane is allowed to enter the detector. Depending on the fluorophores in your sample, and the number of detectors available, multiple fluorophores can be excited and detected simultaneously. Traditional detectors are photo-multiplier tubes, however more recent Gallium Arsenide Phosphide (GaAsP) detectors have been developed to increase sensitivity (Confocal 2 and 4, Airyscan; Confocal 5, BiG detectors). Leica have also developed their own higher sensitivity detector, known as a HyD - a hybrid detector combining the best properties of traditional PMTs and the highly sensitive avalanch photo-diode detectors (Confocal 1, HyD). 

These microscopes provide excellent axial resolution, and very good signal to noise sampling, however this often comes with a sacrifice in temporal resolution due to the slow nature of scanning pixel-by-pixel across the image during capture. Some systems overcome this speed issue with either faster scanners (Leica SP8  - Confocal 1 uses an 8 kHz resonant scanner) or with multiple pinholes (Andor Dragonfly Spinning Disc Confocal and Yokogawa CSU-X1) solving this speed limitation in unique ways.