Polarized dermoscopes (PD), like non-polarized dermoscopes, contain light-emitting diodes to provide illumination and are equipped with a magnification lens. However, PDs use two polarized filters to achieve cross-polarization (see below). Hence, they do not require direct contact with the skin, and do not require the use of immersion liquids.
PD allow visualization of subsurface structures located at the dermal-epidermal junction (DEJ) or superficial dermis, and they are nearly "blind" to the skin's surface and to structures in the superficial epidermis (e.g., comedo-like openings).
Some PD devices allow the user to opt between non-contact PD and contact PD. Under contact PD, the use of an immersion fluid (e.g. 70% alcohol) can enhance image quality, probably by allowing more source light to enter through the stratum corneum.
Optical properties of light during the use of PD
- Light emitted from the dermoscopy unit (source) passes through a polarizer, resulting in the generation of polarized (unidirectional) light.
- Light reflecting back toward our eye (detector) must first pass through a cross-polarized filter whose direction is perpendicular (orthogonal) to that of the source polarizer.
- Polarized light cannot pass through the cross-polarizing filter unless the light changes its direction by 90°, which occurs if the original polarized light undergoes sufficient scattering events in the skin that change its direction ("randomization of polarization").
- Surface glare: maintains its original polarization, and thus cannot pass through the cross-polarized filter. Therefore, PD is "blind" to surface glare.
- Superficial light: does not undergo enough scattering events to result in randomization of polarization. Therefore, PD is also "blind" to back-reflected light from the superficial layers of the epidermis.
- Deep penetrating light: reaches the level of the DEJ and superficial dermis, undergoing multiple scattering events that result in randomization of polarization. Thus, back-reflected "deep penetrating light" can pass through the cross-polarization filter and enter our eye, allowing the visualization of dermoscopic structures from the DEJ and superficial dermis.
Shiny white streaks, a feature only visible on PD:
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