Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1990-10-09
1993-04-06
Turner, Samuel A.
Optics: measuring and testing
By particle light scattering
With photocell detection
356345, 356351, 356367, 25022711, 374161, G01J 345
Patent
active
052007960
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method and to apparatus for detecting and measuring a physical magnitude by means of sensors each comprising an element sensitive to the magnitude to be measured, and having a spectral transfer function which varies as a function of said magnitude, such sensors being illuminated by a beam of incoherent light and transmitting light signals to spectrum analysis means whereby the value of the physical magnitude is determined.
Methods and apparatuses of this nature are already described, for example in the Applicant's U.S. Pat. Nos. 4,814,604 and 4,867,565, in which the sensors used are of the interferometric type performing periodic modulation of the light spectrum that they transmit, with the modulation being at a characteristic modulation frequency which varies as a function of the physical magnitude to be measured. The analysis means are also of the interferometric type and serve to demodulate the light flux from the sensors by a Fourier transform. This optical Fourier transform of the light flux modulated by the sensor enables the characteristic modulation frequency in the sensor to be detected, thus detecting the value of the physical magnitude to be measured.
The object of the present invention is to provide a method and apparatus of the same type but making use of different means for analyzing the light flux from the sensors.
Another object of the invention is to provide a method and an apparatus of this type making it possible to achieve multiplexing and demultiplexing of the signals emitted by various sensors subjected to a physical magnitude to be measured.
To this end, the invention provides a method of detecting and measuring a physical magnitude, the method consisting in using a source of incoherent light to illuminate a sensor of the interferometric type and sensitive to the magnitude to be measured, in modulating in periodic manner the spectrum of the light flux passing through the sensor at a frequency which is a function of the magnitude to be measured, then in statically demodulating the light flux leaving the sensor by means of a demodulator interferometer, and in analyzing the demodulated light flux spectrum to deduce the value of the physical magnitude therefrom, the method being characterized in that it consists in using demodulated flux spectrum analysis apparatus having optical resolution such as to be insensitive to modulation frequencies generated by the sensor and by the demodulator, and in analyzing solely that modulation in the demodulated signal which is characteristic of a beat frequency between the spectrum modulation due to the sensor and that due to the demodulator.
The method of the invention thus makes is possible to use a spectrum analysis apparatus of relatively low resolution to obtain the value of the physical magnitude to be measured by combining the characteristic modulation in the sensor with a fixed demodulation frequency, rather than analyzing said characteristic modulation directly.
Depending on circumstances, providing the beat frequency is relatively large and greater than the reciprocal of the spectrum bandwidth of the light source, the method may consist in measuring the period and the phase of said characteristic beat modulation in the demodulated signal, otherwise, when said beat frequency is relatively low and less than the reciprocal of the spectrum bandwidth of the light source, the method may consist in measuring the apparent color of the demodulated light flux.
The value of the physical magnitude to be measured may thus be obtained either by conventional means for measuring phase and period, or else by spectrocolorimeter means as described in the Applicant's French patent number 2 588 656.
Advantageously, the method also consists, prior to measuring the intensity of the non-modulated portion of the flux from the demodulator, in adjusting the demodulator or the sensor so that the spectrum analysis becomes insensitive to the above-mentioned beat frequency, and then, with the sensor subjected to the physical magnitude to be measu
REFERENCES:
patent: 4814604 (1989-03-01), Lequime
patent: 4867565 (1989-09-01), Lequime
Bertin & Cie
Keesee LaCharles
Turner Samuel A.
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