Two scale adjustment controls, one each for horizontal and vertical, calibrate the meter reading for a given distance between the cursors. Calibrations must be made any time the optics or distance between the image sensor and the object under scrutiny change. These controls require a small screw driver for adjustment in order to avoid inadvertent disturbance of the calibration.
The cursors can be set to white, black, or off.
Power input requirement is 9 VAC at about 150 mA. An AC adapter is included. The adapter is UL listed for safety and isolates the model 307 from ground in order to avoid interference inducing ground-loops.