Pulse width modulation of continuum sources for determination of chemical composition
A light source and a method for its use in an optical sensor are provided, the light source including a resistively heated element. The light source includes a power circuit configured to provide a pulse width modulated voltage to the resistively heated element, the pulse width modulated voltage including: a duty cycle with a first voltage; and a pulse period including a period with a second voltage, wherein: the duty cycle, the first voltage, and the pulse period are selected so that the resistively heated element is heated to a first temperature; and the first temperature is selected to emit black body radiation in a continuum spectral range. Also provided is an optical sensor for determining a chemical composition including a light source as above.
1. An optical sensor for determining a chemical composition of a sample, the optical sensor comprising:
a light source comprising a resistively heated element;
a power circuit configured to provide a pulse width modulated voltage signal to the resistively heated element to heat the resistively heated element to a temperature selected to emit black body radiation in a continuum spectral range;
an optical delivery system to direct an illumination light to a sample;
a sample cavity containing the sample and configured to receive the illumination light;
an optical collection system to collect a sample light;
a detector synchronized to the pulse width modulated voltage signal, the detector configured to measure the sample light from the optical collection system; and
an optical modulator configured to modulate the illumination light at a second frequency.
2. A method of determining a chemical composition of a sample using an optical sensor, the method comprising:
generating an illumination light from a light source using a pulse width modulated voltage, wherein generating the illumination light comprises heating a resistively heated element of the light source to a temperature selected to emit black body radiation in a continuum spectral range;
collecting a sample light;
measuring the sample light with a detector;
the detector producing a signal that is representative of the measured sample light; and
processing circuitry determining, based on the signal, a chemical component in the sample; wherein the pulse width modulated voltage comprises a plurality of pulses having a first frequency, and a duty cycle,
wherein collecting a sample light further comprises directing the sample light to an Integrated Computational Element (ICE).
3. The method claim 2 , further comprising:
obtaining a first measurement at a first temperature and a second measurement at a second temperature; and
determining a contrast value from the first measurement and the second measurement, the contrast value indicative of an analyte concentration in the sample.
4. The method of claim 3 wherein the contrast value comprises a difference in a measurement at a first wavelength and a measurement at a second wavelength.
5. The method of claim 3 wherein the contrast value comprises a ratio of a measurement at a first wavelength and a measurement at a second wavelength.
6. An optical sensor for determining a chemical composition of a sample, the optical sensor comprising:
a light source comprising a resistively heated element;
a power circuit configured to provide a pulse width modulated voltage signal to the resistively heated element to heat the resistively heated element to a temperature selected to emit black body radiation in a continuum spectral range;
an optical delivery system to direct an illumination light to a sample;
a sample cavity containing the sample and configured to receive the illumination light;
an optical collection system to collect a sample light;
a detector synchronized to the pulse width modulated voltage signal, the detector configured to measure the sample light from the optical collection system; and
an Integrated Computational Element (ICE) to separate the sample light into a first light portion and a second light portion.
7. The optical sensor of claim 6 , wherein the first light portion comprises a sample light reflected from the ICE; and the second light portion comprises a sample light transmitted from the ICE.
8. A method of determining a chemical composition of a sample using an optical sensor, the method comprising:
generating an illumination light from a light source using a pulse width modulated voltage, wherein generating the illumination light comprises heating a resistively heated element of the light source to a temperature selected to emit black body radiation in a continuum spectral range;
collecting a sample light;
measuring the sample light with a detector, wherein a first portion of the sample light is measured with a first detector and a second portion of the sample light is measured with a second detector;
the first detector producing a first signal that is representative of the sample light measured by the first detector;
the second detector producing a second signal that is representative of the sample light measured by the second detector;
processing circuitry determining, based on the first and second signals, a chemical component in the sample, wherein the pulse width modulated voltage comprises a plurality of pulses having a first frequency and a duty cycle;
passing the illumination light through an optical modulator operating at a second frequency lower than the first frequency; and
synchronizing the two detectors to a combination frequency including a sum or a difference of the first frequency and the second frequency.