PHOTOCELL

PHOTOCELL

A PHOTOCELL IS A TECHNOLOGICAL APPLICATION OF THE PHOTOELECTRIC EFFECT. IT IS A DEVICE WHOSE ELECTRICAL PROPERTIES ARE AFFECTED BY LIGHT. IT IS ALSO SOMETIMES CALLED AN ELECTRIC EYE. A PHOTOCELL CONSISTS OF A SEMI-CYLINDRICAL PHOTO-SENSITIVE METAL PLATE C (EMITTER) AND A WIRE LOOP A (COLLECTOR) SUPPORTED IN AN EVACUATED GLASS OR QUARTZ BULB. IT IS CONNECTED TO THE EXTERNAL CIRCUIT HAVING A HIGH-TENSION BATTERY B AND MICROAMMETER.

                                           SOMETIMES, INSTEAD OF THE PLATE C, A THIN LAYER OF PHOTOSENSITIVE MATERIAL IS PASTED ON THE INSIDE OF THE BULB. A PART OF THE BULB IS LEFT CLEAN FOR THE LIGHT TO ENTER IT.

                                                                       A PHOTOCELL

WHEN LIGHT OF A SUITABLE WAVELENGTH FALLS ON THE EMITTERC, PHOTOELECTRONS ARE EMITTED. THESE PHOTOELECTRONS ARE DRAWN TO THE COLLECTOR A. PHOTOCURRENT OF THE ORDER OF A FEW MICROAMPERE CAN BE NORMALLY OBTAINED FROM A PHOTOCELL. A PHOTOCELL CONVERTS A CHANGE IN INTENSITY OF ILLUMINATION INTO A CHANGE IN PHOTOCURRENT. THIS CURRENT CAN BE USED TO OPERATE CONTROL SYSTEMS AND IN LIGHT MEASURING DEVICES. A PHOTOCELL OF LEAD SULPHIDE SENSITIVE TO INFRARED RADIATION IS USED IN ELECTRONIC IGNITION CIRCUITS.

IN SCIENTIFIC WORK, PHOTOCELLS ARE USED WHENEVER IT IS NECESSARY TO MEASURE THE INTENSITY OF LIGHT. LIGHT METERS IN PHOTOGRAPHIC CAMERAS MAKE USE OF PHOTOCELLS TO MEASURE THE INTENSITY OF INCIDENT LIGHT. THE PHOTOCELLS, INSERTED IN THE DOOR LIGHT ELECTRIC CIRCUIT, ARE USED AS AUTOMATIC DOOR OPENERS. A PERSON APPROACHING A DOORWAY MAY INTERRUPT A LIGHT BEAM THAT IS INCIDENT ON A PHOTOCELL. THE ABRUPT CHANGE IN PHOTOCURRENT MAY BE USED TO START A MOTOR THAT OPENS THE DOOR OR RINGS AN ALARM. THEY ARE USED IN THE CONTROL OF COUNTING DEVICE WHICH RECORDS EVERY INTERRUPTION OF THE LIGHT BEAM CAUSED BY A PERSON OR OBJECT PASSING ACROSS THE BEAM. SO PHOTOCELLS HELP COUNT THE PERSONS ENTERING AN AUDITORIUM, PROVIDED THEY ENTER THE HALL ONE BY ONE. THEY ARE USED FOR THE DETECTION OF TRAFFIC LAW DEFAULTERS: AN ALARM MAY BE SOUNDED WHENEVER A BEAM OF INVISIBLE RADIATION IS INTERCEPTED.

                                      IN A BURGLAR ALARM (INVISIBLE) ULTRAVIOLET LIGHT IS CONTINUOUSLY MADE TO FALL ON A PHOTOCELL INSTALLED AT THE DOORWAY. A PERSON ENTERING THE DOOR INTERRUPTS THE BEAM FALLING ON THE PHOTOCELL. THE ABRUPT CHANGE IN PHOTOCURRENT IS USED TO START AN ELECTRIC BELL RINGING. IN FIRE ALARM, A NUMBER OF PHOTOCELLS ARE INSTALLED AT SUITABLE PLACES IN A BUILDING. IN THE EVENT OF BREAKING OUT OF FIRE, LIGHT RADIATIONS FALL UPON THE PHOTOCELL. THIS COMPLETES THE ELECTRIC CIRCUIT THROUGH AN ELECTRIC BELL OR A SIREN WHICH STARTS OPERATING AS A WARNING SIGNAL. PHOTOCELLS ARE USED IN THE REPRODUCTION OF SOUND IN MOTION PICTURES AND IN THE TELEVISION CAMERA FOR SCANNING AND TELECASTING SENES. THEY ARE USED IN INDUSTRIES FOR DETECTING MINOR FLAWS OR HOLES IN METAL SHEETS.