Non-polarising dermatoscopy (NPD) requires direct contact of the dermatoscope with the skin. A liquid medium (such as alcohol gel, oil or aquagel) is used between the glass plate of the dermatoscope and the skin. This reduces the reflection of light therefore reducing glare allowing visualisation through the stratum corneum. Most of the structures seen in NPD are generated from light scattering from more superficial structures. Air bubbles should be avoided as they will reduce the quality of the image produced.

 

Depolarising dermatoscopy (PD) can either be in direct contact with the skin or not in contact. It uses two filters (polarisers) to produce cross-polarisation.  This allows the dermatoscope to capture more light scattered from deeper structures in the skin. The fact that PD does not need direct contact with the skin gives the added advantage of being able to scan quickly over lesions.

 

So why does this matter? What is the difference in the images produced? For most skin lesions the images produced are similar. 

 

In general more superficial structures like milia-like cysts or blue/white veil are more prominent with NPD whereas deeper stuctures like chrysalis structures and vascular structures are better appreciated with PD.

 

Another important factor that can affect the quality of the image produced is the pressure applied by the dermatoscope. PD has the advantage of being non-contact and can lead to less compression of vessels. When using NPD just by releasing some of the pressure, vascular patterns can be appreciated (see below).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A nodular BCC visualised with NPD. Altering the pressure applied allows visualisation of the arborising pattern.