Log ratio transmittance signal processor for photometric apparatus

Claim 2

Apparatus for generating a signal which varies as a function of the logarithm of the ratio of two input signals, said apparatus comprising: a capacitor; voltage control means for setting the voltage on said capacitor under feedback control to an initial value which is proportional to the amplitude of one of said input signals and then releasing control of the capacitor charging; means, including an amplifier and a resistance interconnected with said amplifier to provide positive feedback, for varying the charge on said capacitor after release by said voltage control means to generate a voltage waveform which, starting from said initial value, varies as an exponential function of time, the exponent being positive; a comparator circuit for detecting when the capacitor waveform reaches a threshold level which is proportional to the amplitude of the other of said input signals; means for enabling said voltage control means and said waveform generating means alternately for repetitive alternate operation at a substantially constant frequency; means controlled by said comparator circuit for generating a train of squarewave pulses of substantially constant amplitude, each pulse being initiated when control of capacitor charging is released by said initial voltage setting means and being terminated when the capacitor waveform reaches said threshold level, the average d.c. voltage of said squarewave pulse train being a function of the logarithm of the ratio of said input signals.

Claim 3

Photometric apparatus for measuring the absorption of a sample material, said apparatus comprising: a sample cell for receiving said sample material; a rEference cell essentially similar to said sample cell; means for transmitting radiation from a common source through each of said cells; means for chopping the radiation passing through said sample cell at a first frequency and for chopping the radiation passing through said reference cell at a second frequency; a radiation detector; means for combining the chopped radiation components from both cells and applying the combined radiation to said radiation detector; means for filtering the signal obtained from said radiation detector to separate the a.c. components corresponding to the chopped radiation components obtained from the respective cells; means for detecting said a.c. components to obtain a pair of transmittance signals; a capacitor; voltage control means for setting the voltage on said capacitor to an initial value which is proportional to the level of one of said transmittance signals and then releasing control of the capacitor charging; means for varying the charge on said capacitor after release by said voltage control means to generate a voltage waveform which, starting from said initial value, varies as an exponential function of time, the exponent being positive; a comparator circuit for detecting when the capacitor wave-form reaches a threshold level proportional to the level of the other of said transmittance signals; timing means for enabling said initial voltage control means and said waveform generating means alternately to provide repetitive alternate operation at a substantially constant frequency; means controlled by said comparator circuit and said timing means for generating a train of squarewave pulses of substantially constant amplitude, one edge of each pulse occurring at a fixed phase in the controlled timing cycle and the other edge occurring when the capacitor waveform reaches the level of said other transmittance signal, the average d.c. voltage of said squarewave pulse train being a function of the logarithm of the ratio of said transmittance signals.

Claim 4

Photometric apparatus for measuring the absorption of a sample material, said apparatus comprising: a sample cell for receiving said sample material; a rEference cell essentially similar to said sample cell; means for transmitting radiation from a common source through each of said cells; means for chopping the radiation passing through said sample cell at a first frequency and for chopping the radiation passing through said reference cell at a second frequency; a radiation detector; means for combining the chopped radiation components from both cells and applying the combined radiation to said radiation detector; means for filtering the signal obtained from said radiation detector to separate the a.c. components corresponding to the chopped radiation components obtained from the respective cells; means for detecting said a.c. components to obtain a pair of transmittance signals; a capacitor; voltage control means for setting the voltage on said capacitor to an initial value which is proportional to the level of one of said transmittance signals and then releasing control of the capacitor charging; means for varying the charge on said capacitor after release by said voltage control means to generate a voltage waveform which, starting from said initial value, varies as an exponential function of time, the exponent being positive; a comparator circuit for detecting when the capacitor wave-form reaches a threshold level proportional to the level of the other of said transmittance signals; timing means for enabling said initial voltage control means and said waveform generating means alternately to provide repetitive alternate operation at a substantially constant frequency; means controlled by said comparator circuit and said timing means for generating a train of squarewave pulses of substantially constant amplitude, one edge of each pulse occurring at a fixed phase in the controlled timing cycle and the other edge occurring when the capacitor waveform reaches the level of said other transmittance signal, the average d.c. voltage of said squarewave pulse train being a function of the logarithm of the ratio of said transmittance signals.

Claim 5

Apparatus as set forth in claim 2 wherein said charge controlling means includes an amplifier and a resistance interconnected with said amplifier for providing positive feedback thereby to generate across said capacitor, an exponential waveform having a positive exponent.

Claim 6

Photometric apparatus for measuring the absorption of a sample material, said apparatus comprising: a sample cell for receiving said sample material; a reference cell essentially similar to said sample cell; means for transmitting radiation from a common source through each of said cells; means for chopping the radiation passing through said sample cell at a first frequency and for chopping the radiation passing through said reference cell at a second frequency; a radiation detector; means for combining the chopped radiation components from both cell and applying the combined radiation to said radiation detector; means for filtering the signal obtained from said radiation detector to separate the a.c. components corresponding to the chopped radiation components obtained from the respective cells; means for detecting said a.c. components to obtain, respectively, a sample transmittance signal and a reference transmittance signal; means for attenuating said reference transmittance signal level to a value below the range of expected sample signal levels; a capacitor; voltage control means for setting the voltage on said capacitor to an initial value which is equal to the level of said attenuated reference signal and then releasing control of the capacitor charging; means, including an amplifier and a resistance interconnected with said amplifier for providing regenerative feedback, for varying the charge on said capacitor after release by said voltage control means to generate a voltage waveform which, starting from said initial value, varies as an exponential function of time, the exponent being positive; a comparator circuit for detecting when the capacitor waveform reaches the level of said sample signal; timing means for enabling said initial voltage setting means and said waveform generating means alternately to provide repetitive alternate operation at a substantially constant frequency; means controlled by said comparator circuit and said timing means for generating a train of squarewave pulses of substantially constant amplitude, each pulse being initiated when control of capacitor charging is released by said initial voltage setting means and being terminated when the capacitor waveform reaches the level of said sample signal, the average d.c. voltage of said squarewave pulse train being a function of the logarithm of the ratio of said transmittance signals.

Claim 7

Photometric apparatus for measuring the absorption of a sample material, said apparatus comprising: a sample cell for receiving said sample material; a reference cell essentially similar to said sample cell; means for transmitting radiation from a common source through each of said cells; means for chopping the radiation passing through said sample cell at a first frequency and for chopping the radiation passing through said reference cell at a second frequency; a radiation detector; means for combining the chopped radiation components from both cell and applying the combined radiation to said radiation detector; means for filtering the signal obtained from said radiation detector to separate the a.c. components corresponding to the chopped radiation components obtained from the respective cells; means for detecting said a.c. components to obtain, respectively, a sample transmittance signal and a reference transmittance signal; means for attenuating said reference transmittance signal level to a value below the range of expected sample signal levels; a capacitor; voltage control means for setting the voltage on said capacitor to an initial value which is equal to the level of said attenuated reference signal and then releasing control of the capacitor charging; means, including an amplifier and a resistance interconnected with said amplifier for providing regenerative feedback, for varying the charge on said capacitor after release by said voltage control means to generate a voltage waveform which, starting from said initial value, varies as an exponential function of time, the exponent being positive; a comparator circuit for detecting when the capacitor waveform reaches the level of said sample signal; timing means for enabling said initial voltage setting means and said waveform generating means alternately to provide repetitive alternate operation at a substantially constant frequency; means controlled by said comparator circuit and said timing means for generating a train of squarewave pulses of substantially constant amplitude, each pulse being initiated when control of capacitor charging is released by said initial voltage setting means and being terminated when the capacitor waveform reaches the level of said sample signal, the average d.c. voltage of said squarewave pulse train being a function of the logarithm of the ratio of said transmittance signals.