Aerodynamic density meter using photosensitive schlieren optical system

Claim 1

AN INSTRUMENT FOR THE MEASUREMENT OF DENSITY DIFFERENCES IN A MEDIUM EXPOSED TO AN AERODYNAMIC FLOW COMPRISING, IN COMBINATION: SCHLIEREN OPTICAL MEANS FOR PROJECTING A COLLIMATED BEAM OF LIGHT THROUGH SAID MEDIIUM; MEANS FOR DIVIDING SAID LIGHT BEAM INTO TWO PARTS AND FOR INTERRUPTING EACH PART ALTERNATELY; MEANS FOR PRODUCING AN ELECTRICAL OUTPUT SIGNAL PROPORTIONAL TO LIGHT INTENSITY, SAID LIGHT BEAM BEING DIRECTED AT SAID SIGNALPRODUCING MEANS TO PROVIDE AN ELECTRICAL OUTPUT COMPRISING A PULSATING D.C. SIGNAL EVERY SECOND PULSE OF WHICH IS PROPORTIONAL TO THE INTENSITY OF THE SAME PART OF SAID LIGHT BEAM, ALTERNATE PULSES BEING DERIVED FROM DIFFERENT PARTS OF SAID LIGHT BEAM; MEANS FOR OBTAINING AN A.C. SIGNAL HAVING A CHARACTERISTIC PROPORTIONAL TO A CHARACTERISTIC OF SAID PULSATING D.C. SIGNAL; NARROW-BANDWIDTH MEANS FOR PASSING SAID A.C. SIGNAL AND ELIMINATING FREQUENCIES OTHER THAN A NARROW BAND INCLUDING SAID A.C. SIGNAL, THEREBY ELIMINATING THE SPECTRUM OF NOISE SIGNALS OUTSIDE SAID NARROW BAND; MEANS FOR SEPARATING SAID A.C. SIGNAL INTO INDIVIDUAL COMPONENTS, EACH CORRESPONDING TO THE OUTPUT ASSOCIATED WITH A DIFFERENT ONE OF THE PARTS OF SAID LIGHT BEAM, SAID COMPONENTS COMPRISING STEADY D.C. SIGNALS; AND MEANS FOR OBTAINING AND INDICATING THE DIFFERENCE BETWEEN SAID STEADY D.C. SIGNALS.