Optics: measuring and testing – By dispersed light spectroscopy – With background radiation comparison
Patent
1998-05-15
1999-12-14
Evans, F. L.
Optics: measuring and testing
By dispersed light spectroscopy
With background radiation comparison
356319, G01J 342
Patent
active
060024771
DESCRIPTION:
BRIEF SUMMARY
This invention relates to spectroscopy and is particularly concerned with ultraviolet/visible/infrared spectrophotometers.
Conventional ultraviolet/visible/infrared spectrophotometers use one or more light sources that continuously emit light. For example, a popular combination is a deuterium arc lamp and a quartz halogen filament lamp to cover the ultraviolet and visible/infrared portions of the spectrum respectively. In order to obtain high quality readings from an instrument, it is necessary to obtain three separate measurements. A measurement of the intensity of the light source without any sample present (called the reference measurement), a measurement of the intensity when the sample is present (called the sample measurement) and a measurement of the signal when no light from the source reaches the detector (called the dark signal). A measurement of the fraction of light not absorbed by the sample can then be computed as:
In order to obtain such measurements it is common practice to use a mechanical chopping mechanism which, depending on it's position, directs the beam from the source along either of two paths. One path bypasses the sample and goes to a detector for reference measurement whereas the other path passes to the sample and then goes to a detector for sample measurement. The chopping mechanism also functions to block the light beam from the source to the detector for dark measurement.
Such an arrangement has some significant disadvantages. Firstly, since the chopper is a mechanical device there is a practical limit at which the system can switch from one measurement to the next measurement (typically more than 1 millisecond and frequently about 10 milliseconds). Any changes in the system conditions during this switching time will not be correctly eliminated. For example, if the light source fluctuates in output with time, changes in effective intensity between sample and reference measurements will not be eliminated. Furthermore, a major component of the dark measurement is room light entering the optical path between light and detector, and the intensity of room light can fluctuate significantly and rapidly with time especially if fluorescent lights are used in the room. Changes in this intensity between dark and sample or dark and reference measurements will not be cancelled correctly. The problem is particularly evident in spectrophotometers in which the spectral resolution subsystem is placed before the sample. In such arrangements only a very small fraction of the source light passes through the spectral resolution subsystem and consequently the source light intensity on the sample is relatively low. Room light ingress is therefore a significant factor and in a conventional instrument it is necessary to take steps to rigidly exclude all room light.
The aforementioned problem is not so evident in spectrophotometers in which the spectral resolution subsystem is placed after the sample and thus illuminate the sample with white light since the total light intensity from the source is usually much larger than the room light accepted into the optical path. On the other hand, such instruments have very significant limitations when measuring some types of samples. Light at wavelengths other than the wavelength of interest can excite fluorescence which causes the sample to emit additional light at the wavelength of interest. A second problem is the total amount of light incident on the sample which can be sufficient to affect the sample and thereby cause errors in the measurement process.
Another disadvantage of the conventional arrangement is that the light source usually needs some time to stabilise (for the operating temperature to equilibrate) and is therefore left on while the instrument is switched on, regardless of whether or not measurements are being made. Sample measurement occurs over a small portion of the time the instrument is switched on and as a result continuous energisation of the light source wastes power and shortens the useful life of the light source. In addition the sam
REFERENCES:
patent: 4076421 (1978-02-01), Kishner
patent: 4857735 (1989-08-01), Noller
patent: 5175697 (1992-12-01), Kawagoe et al.
Berkowitz Edward H.
Evans F. L.
Varian Inc.
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