text: A major improvement to the product imaging technique was achieved by Eppink and Parker.  A difficulty that limits the resolution in the position-sensing version is that the spot on the detector is no smaller than the cross-sectional area of the ions excited.  For example, if the volume of interaction of the molecular beam, photolysis laser, and ionization laser is, say 1 mm x 1 mm x 1 mm, then the spot for an ion moving with a single velocity would still span 1mm x 1mm at the detector.  This dimension is much larger than the limit of a channel width (10 μm) and is substantial compared to the radius of a typical detector (25 mm).  Without some further improvement, the velocity resolution for a position-sensing apparatus would be limited to about one part in twenty-five. Eppink and Parker found a way around this limit.  Their version of the product imaging technique is called velocity map imaging.
  Velocity map imaging is based on the use of an electrostatic lens to accelerate the ions toward the detector.  When the voltages are properly adjusted, this lens has the advantage that it focuses ions with the same velocity to a single spot on the detector regardless where the ion was created.  This technique thus overcomes the blurring caused by the finite overlap of the laser and molecular beams.
  In addition to ion imaging, velocity map imaging is also used for electron kinetic energy analysis in photoelectron photoion coincidence spectroscopy.