There are many reasons to check for stress in mounted lenses. Too much stress on a lens may not manifest itself until the day your patient exits a warm office building into sub-freezing temperatures and the lens implodes. Aside from viewing stress patterns you can determine axis in polarized lenses. This is a compact bench top model that you can hold in your hand. Operates using two AA batteries (included). 2-3/4" W x 2-3/4" H x 4" D.