Patent ID: 7605921

Claim:
An optical measurement and inspection method comprising: providing at least two light emitters, providing at least one light receiver, providing at least one signal generator connected to at least one of the light emitters and the at least one light receiver, and providing means for converting the received light to electrical current, wherein providing a sheet of material that lies or traverses between and/or in front of the at least two light emitters and the at least one light receiver ( 200 ), controlling, with the at least one signal generator, at least one of the light emitters and the at least one light receiver by sending them an electronic synchronisation signal and thereby synchronises emission and detection of light rays ( 205 , 215 , 245 ), fixing a common carrier waveform AC voltage signal in frequency, and a symmetrical 50% duty cycle wave signal is processed from the common carrier AC voltage waveform signal and the common carrier waveform AC voltage signal and the 50% duty cycle wave signal carry equal frequency and phase in terms of zero-crossings, driving, with the at least one signal generator, the at least two light emitters with different carrier frequencies waveforms and/or phases, and the at least one light receiver with both of these frequencies, waveforms and/or phases, emitting, with the at least two light emitters, at least two rays of light ( 220 ), the at least two rays of light being incident on the stationary or traversing sheet ( 225 ), detecting by the same light receiver ( 230 ) at least two rays of light grazing the sheet, transparent to the sheet, reflected from the sheet or directly from the light emitters, an intensity of at least one said emitted ray of light follows at least one of the carrier waveform signals and at least one said detected light ray is demodulated from the carrier waveform signal using the electronic synchronisation signal, converting at least two of the rays of light to photocurrent ( 240 ), and rectifying photocurrent signal with the symmetrical wave signal, processed from the common carrier waveform signal, removing at least one DC component from the photocurrent signal, and diagnosing and observing the processed photocurrent and/or changes in the processed photocurrent to find defects and/or determine characteristics of the said sheet of material ( 250 ), wherein at least one said carrier waveform signal is a sine wave, cosine wave, or a square wave signal.