Synchronous optical measurement and inspection method and means

An optical measurement and inspection arrangement, comprising:

• at least two light emitters,
• at least one light receiver,
• at least one signal generator connected to at least one of the light emitters and at least one of the light receivers, and
• means for converting the received light to electrical current, wherein
• a sheet of material (410, 510, 610, 710) is arranged between and/or in front of the at least two light emitters (400, 500, 600, 700) and the at least one light receiver (420, 520, 620, 720),
• the at least two light emitters (400, 500, 600, 700) are arranged to emit at least two rays of light incident on at least one sheet,
• at least two rays of light grazing the sheet, transparent to the sheet or reflected from the sheet are arranged to be detected by the same at least one light receiver (420, 520, 620, 720),
• at least one ray of light is arranged to be converted to photocurrent by at least one photoelectric device (530, 630, 730)
• the at least one signal generator is arranged to control at least one of the light emitters (400, 500, 600, 700) and at least one of the light receivers (420, 520, 620, 720) by sending them an electronic synchronisation signal and thereby synchronises the emission and detection of rays,
• a common carrier waveform AC voltage signal is fixed 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,
• the at least one signal generator is arranged to drive 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,
• an intensity of at least one said emitted ray of light is arranged to follow at least one of the carrier waveform signals and at least one received light ray is arranged to be demodulated from the carrier waveform signal using the electronic synchronisation signal,
• the symmetrical wave signal, processed from the common carrier waveform signal, is configured for rectifying photocurrent signal,
• at least one DC component is removed from the photocurrent signal, and
• the photocurrent and/or changes in photocurrent are arranged to be diagnosed and observed to find defects and/or determine characteristics of the said sheet of material (310), wherein at least one of said carrier waveform signal is a sine wave, cosine wave, or a square wave signal.