Means you are afraid of having your picture taken.
Magnahelic gauges only measure static differential, a photohelic gauge has has alarm contacts for low and high set points.
PHOTOHELIC® Pressure ControlPHOTOHELIC® models combine the MAGNEHELIC®gauge with a precise pressure switch that lets you control low and high gas pressures. Knob controls adjust dual set points, which allow for variable deadband control. These set points contain photocells that actuate DPDT relays when room pressures reach a preset upper or lower limit, and they can be set up to control corrective damper action. As the room pressure changes in response to damper motion, and the indicator returns to the null band, damper motion is halted.