The optical quality of an arc flash face shield has many variables that need to be considered. A few of the most important terms and definitions are listed below.
Prismatic Power and Imbalance
Prismatic power is defined as any the deviation of a ray of light through a specified point on a lens. Prism in a face shield can be produced when the front surface is not parallel to the back surface.  When light passes through the prism, the object viewed appears to be displaced. Any imbalance in these surfaces can cause the wearer to see differing prismatic effects that can cause discomfort for the wearer.
Refractive Power and Astigmatism
Irregularly shaped visors with a lack of optical symmetry can cause light to bend improperly creating astigmatism (i.e., refractive error that causes objects to appear blurry).  Refractive power (measured in diopters) is the ability of the lens to focus light properly thus keeping images crisp.
To better understand what an astigmatism is, a cosmetic raytraced image of a lens with target lines crossing the eye centers is listed below. By observing you can see the following.
Horizontally the image is compressed.  This is due to a strong curvature of the face shield.  This creates negative horizontal power.  The shifting of the image is what we call prism.
Vertically the image is less compressed. There is very little power and prism. This is due to the lower curvature in this direction.
The discrepancy between horizontal and vertical power is what causes astigmatism.