With multiphoton microscopy we can image several hundreds of micrometers deep into biological samples as scattering of red-shifted light in tissues is highly reduced compared to visible light. For multiphoton excitation, pulsed infrared lasers with wavelengths of up to 1300 nm in case of the OPO (optical-parametric oscillator, What is an OPO?) are used.
Deep tissue imaging of thick specimens or in whole animals allows to observe cellular processes in their natural context. Especially intravital imaging with multiphoton excitation plays a growing role in many biomedical research areas.