A variable candela strobe unit couples a substantially constant trigger voltage to a trigger electrode of a gas discharge tube irrespective of the voltage provided across the tube which is specified by a selected candela output value. Un-color matched pixels that form a window include a first region that includes un-color matched pixels and a second region that includes un-color matched pixels. The rail assembly comprises an optics mount, a pivot mount, an inline mechanism mount. The transconductor includes a control voltage generator that generates a certain level of control voltage using the tuning current. This is particularly important in systems that employ fast imaging techniques such as fast spin echo, where the invention can speed imaging substantially. The control unit sets a first area and a second area on a screen and senses a position on the screen touched by the user.