Ambi's different because it has an external light source - either a lamp mounted on its stand, or a nearby dedicated light source matching the key light.  The illuminant is directed at the adjustable mirror on Ambi's rear side, and light is then transferred through the illuminator's color-corrected diffuser panel to light the test target.  Ambi's illumination is inherently even because its light source is remote from the diffuser.  The test transparency slips easily in front of the diffuser panel, and is adjusted with a built-in leveling device.  Ambi has a removable front hood.
 

Because its light source is external, Ambi lets you alternate between  quartz, HMI, fluorescent, daylight and other illuminants.  Used with different lamp types, Ambi provides test transparency illumination that is precisely color-matched to the types of lighting a camera will encounter.  Setting up a camera's color matrix to several different illuminants, and then using the appropriate stored matrix setting during a shoot, can dramatically improve visual quality and consistency.