2.1.4.2　Confocal microscope

The principle of confocal laser scanning microscope was first described in the 1950s.The first commercially available system was realized by Rajadhyaksha et al.at the Harvard Medical School in 1995 based on an 830 nm diode laser that is used as a point light source in a scanning mode providing horizontal tomographic images.The visualization of tissues is based on the different refraction indices of the distinct tissue chromophores and cell structures，especially melanin，collagen，and keratin.Melanin has the strongest refractive index of 1.7，and keratin's refractive index is 1.5.Collagen and inflammatory cells are other highly refractive structures.These structures appear bright white when reflected light reaches the detector whereas non-reflective structures appear dark.The interplay between highly，moderately，and non-reflective structures generates a black and white image representing the area at the cellular level.Generated images by reflectance confocal microscope（RCM）are horizontal and parallel to the skin surface，which contrasts with the perpendicular images viewed on histopathology.It has a lateral resolution of 1μm and magnification equivalent to a×30 microscope objective.The limit of this technique is that its penetration depth is about 200 μm.It allows the visualization of the dermal-epidermal junction and the superficial dermis，but not the deeper layers.Moreover，it requires extensive training to operate the device and analyze images.The main indications for RCM include superficial fungal infections，hyperpigmentation and hypopigmentation，skin tumors，inflammation and viral diseases of the skin.(https://www.daowen.com)