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In the monitoring of gas or vapor concentrations in a gas sample by comparing the transmission of light through the sample at different wavelengths corresponding to behaviorial maxima and minima of the gas, the different wavelengths are selected by permitting light to fall alternately on different s

In the monitoring of gas or vapor concentrations in a gas sample by comparing the transmission of light through the sample at different wavelengths corresponding to behaviorial maxima and minima of the gas, the different wavelengths are selected by permitting light to fall alternately on different sets of slits in an entrance mask of a spectroscope so that light diffracted by a grating in the spectroscope at the wavelengths represented by the slits in the two sets will fall on an exit slit in an exit mask of the spectroscope and thence pass to a photodetector. This arrangement enables the shift between the different sets of wavelengths to be achieved without introducing mechanical jitter, since the photodetector output may be electronically gated so as to be measured only for predetermined periods when light is passing first through one and then through the other set of slits.

대표청구항 ▼

Apparatus for monitoring a low concentration of a gas or vapour in a gas sample comprising: a source of light containing certain predetermined wavelengths in a desired spectral region, a collimator receiving said light and concentrating it upon a slitted area in an entrance mask, a slitted shutter m

Apparatus for monitoring a low concentration of a gas or vapour in a gas sample comprising: a source of light containing certain predetermined wavelengths in a desired spectral region, a collimator receiving said light and concentrating it upon a slitted area in an entrance mask, a slitted shutter movable adjacent said entrance mask into a first position such that slits in the shutter permit light to pass through a first set of parallel slits in the entrance mask and a second position such that first slits in the shutter permit light to pass through second slightly displaced set of parallel slits in the entrance mask, means to move the shutter cyclically into said positions, a dispersion grating disposed to receive and diffract light from said parallel slits in said entrance mask, an exit mask defining at least one exit slit disposed to receive light diffracted by said grating at specific wavelengths determined by the relative dispositions of said entrance mask slits, said grating and said exit slit, said wavelengths comprising a first set of wavelengths characteristic of the absorption maxima of the gas being monitored and determined by said first set of slits and a second set of wavelengths determined by said second set of slits which set is displaced from said first set of wavelengths and characterized by minimum absorption by the gas being monitored, means establishing an optical path through a gas sample being monitored, an optical relay system extending from said exit slit to the beginning of said optical path, a photo-electric sensor device, an optical relay system extending from the end of said optical path to said sensor device, means to generate signals in response to the entry of said shutter into said positions, a frequency stable electronic clock pulse generator generating pulses at a frequency which is very high compared to the cycling rate of the means moving said modulator, gate control means receiving said clock pulse signals from said clock pulse generator and operative to count said pulses and generate gating signals commencing a predetermined time after receipt of a response signal from said signal generating means and lasting a period corresponding to a predetermined count of said gate pulses related to the rotation rate of the shutter such that the gating signals commence after the shutter has entered its said positions and end before it leaves them, a transmission gate controlled by said gating signal and passing the signals from said sensor device in response thereto, comparator means receiving the signals from said gate and comparing the magnitude of the signals passed when the shutter is in said first and second positions, and wherein said shutter has a third position such that light can pass neither set of slits in the entrance mask, and further comparator means receives signals from said gate when the shutter is in said third position for comparison with the signals passed in one of said first and second positions.