Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1992-01-29
1993-08-24
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250205, 25022723, G01J 350
Patent
active
052391752
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to apparatus and methods for monitoring colour changes in what will be referred to herein as a target, i.e. anything in or on which a change of colour may take place. The term "colour" is to be understood to mean the wavelength of light, emitted or reflected according to the context, whether or not it happens to be in the visible part of the spectrum.
2. Discussion of Prior Art
The problem of detecting over a substantial period of time the changes in chemical composition occurring in a gas or liquid flow, or indeed the detection of the change in any physical quantity which can be made to manifest itself as a colour change, is advantageously addressed by apparatus for monitoring colour changes. Such apparatus requires a high level of stability and fault tolerance if it is to be in operation over a long period of time.
One known system that can be applied to the measurement of absolute colour intensities over a period of time is an optical device for measuring physical quantities and is disclosed in UK Patent Application No. 2,025,608 (corresponding to U.S. Pat. No. 4,281,245). This device comprises generally two sources of different wavelengths of light which are optically connected to an optical transducer. The transducer comprises an optical filter having spectrally varying absorbing, transmitting or reflecting properties which are influenced by the physical property in question. The transducer is assembled in such a way as to ensure that one of the wavelengths of light (the reference signal) is substantially less dependant on the measured quantity than the other (the measuring signal). The corresponding two output signals of the transducer are then divided and in this way a stablised measurement is obtained.
This known device is capable of measuring a sequential series of absolute values of the physical quantity over a period of time. Errors common to the paths of both wavelengths of light are removed by the divider, however other errors which vary with time will be incorporated into the measurements obtained obscuring genuine changes in the value of the physical quantity. Such errors can be caused by drifts in the sensitivity of the sensors and in the intensity of the light source due to ageing and other inconsistencies which are likely to occur over a substantial period of time. One way of overcoming this is by repeated calibration procedures which would necessarily halt the continuous monitoring of the target and is therefore inconvenient.
SUMMARY OF THE INVENTION
The present invention overcomes this problem by the provision of signal processing means for comparing successive output signals from the divider means to give a resulting signal representing the degree of change or rate of change of successive measurements. This eliminates errors which change with time but whose rate of change with time is relatively constant, as is the nature of the errors that are likely to occur over a substantial period of time, as mentioned above. Furthermore this apparatus provides a versatile system for continuously monitoring colour changes in a target, reliably, accurately and with simple signal processing.
According to the invention in a first aspect, such an apparatus comprises: each having effectively a single emission wavelength different from that of the other source or sources,
timing means for causing each source to operate in turn, and but reflected from the target,
characterised in that the apparatus additionally comprises:
a direct light sensor substantially identical to the reflected light sensor, for receiving light directly from the sources, the direct light sensor being connected in a closed energy loop with the light sources and with reference means for stabilising the source intensity of the emission from each source at a predetermined level,
a plurality of data stores, each arranged to receive from the reflected light sensor a signal representing the intensity of the reflected light at a respective one of the emission
REFERENCES:
patent: 3910701 (1975-10-01), Henderson et al.
patent: 4281245 (1981-07-01), Brogardh et al.
patent: 4529875 (1985-07-01), Brogardh et al.
patent: 4681454 (1987-07-01), Breemer
patent: 4760250 (1988-07-01), Loeppert
patent: 4845647 (1989-07-01), Dils et al.
patent: 5021647 (1991-06-01), Tatsuno et al.
Alder John F.
Jawad Sajad M.
Nelms David C.
The Secretary of State for Defence in Her Britannic Majesty's Go
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