NON-DISPERSIVE INFRARED GAS DETECTOR, AND METHOD OF STABILIZING INFRARED EMISSION OF AN INCANDESCENT LAMP

An NDIR gas detector includes a photodetector for detecting a portion of stray visible light emitted from an incandescent lamp so as to generate an induced electrical signal, which is compared with a preset reference signal associated with a predetermined constant level of the stray visible light corresponding to a constant temperature of the lamp so as to obtain a level difference between the induced electrical signal and the reference signal. Electrical power supplied to the lamp is repeatedly regulated based on the level difference until the induced electrical signal and the reference signal have the same level, thereby stabilizing IR emission of the lamp in response to the lamp being kept at the constant temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 1, the preferred embodiment of an NDIR gas detector according to the present invention is shown to include an incandescent lamp1, an IR detector2, a gas chamber3, a photodetector4, an electronic control unit5, and a power unit6.

The incandescent lamp1serves as an IR emitter for NDIR gas detection. The incandescent lamp1has a resistive incandescent filament capable of being heated electrically to emit IR radiation with a continuous spectrum that is a wide-range radiation spectrum covering various IR signatures of gases of interest. The continuous radiation spectrum is a blackbody-like spectrum (seeFIG. 3a) based on Planck's radiation formula expressed as follows.

Here, c1is a first radiation constant equal to 37415 W/cm2/μm4, and c2is a second radiation constant equal to 14388 μm/K. According to Wien's displacement law, the continuous radiation spectrum has a peak wavelength (λm) at a temperature (T), wherein the relationship between the peak wavelength (λm) and the temperature (T) is expressed as follows.

Namely, the product of the absolute temperature of the incandescent filament of the incandescent lamp1and the peak wavelength of the IR radiation is a constant. For example, to produce a peak wavelength at 4 μm for detection of CO2gas, the incandescent filament should be heated up to about 700° K (i.e., 427° C.) for maximum emission efficiency based on Equation (5). As shown inFIG. 3a, at such temperature, extremely weak light is generated near a visible range in comparison to that generated at 4 μm-wavelength, according to Planck's radiation law. The weak radiation in visible wavelength is amplified in log scale, as shown inFIG. 3b. Nevertheless, the shape of the radiation spectrum is fixed if the temperature of the incandescent filament is precisely kept constant. In other words, if light intensity generated in a visible wavelength is kept constant precisely, the temperature of the incandescent filament is also constant such that the intensities of all other wavelengths including the infrared to be used are kept constant.

The IR detector2selectively senses the characteristic wavelength of a wave radiated from a specific gas to be detected, e.g., 4 μm-wavelength of the wave radiated from CO2gas. In this embodiment, the characteristic wavelength of the wave sensed by the IR detector2is within a wavelength range from 1.2 μm up to 50 μm to which a silicon photodiode is virtually insensitive.

The gas chamber3houses the incandescent lamp1and the IR detector2for allowing the specific gas to flow between the incandescent lamp1and the IR detector2.

The photodetector4is received in the gas chamber3, and is located in proximity of the incandescent lamp1to detect a minute fraction of stray visible light emitted from the incandescent lamp1so as to generate an induced electrical signal corresponding to the stray visible light detected by the photodetector4. The photodetector4can be located at a position (P1) close to the incandescent lamp1or another position (P2) close to the IR detector2, as shown inFIG. 2. However, the photodetector4is preferably disposed as close as possible to the IR detector2such that it reveals closely the exact IR radiation quantity entering the IR detector2. In this embodiment, the photodetector4is a silicon-based photodetector, such as a silicon photodiode. The visible light emitted from the incandescent lamp1and detected by the photodetector4has a cutoff wavelength of up to 1.1 μm. As commonly known, a silicon photodiode is inexpensive, and is an ultra-sensitive and fast-response device over its governing spectrum compared to the dummy sensor of the prior art, and thus, is able to detect extremely lower levels of light signal. Furthermore, a silicon photodiode of tenths of square-millimeter areas is able to induce sufficient photocurrent of micro-amp magnitude, even when the silicon photodiode receives only 1 millionth of the visible light emitted from a miniaturized milliwatt IR lamp. This means, if the photodetector4is placed close to the incandescent lamp1, the photodetector4still can provide a photo-induced signal as a feedback signal to precisely control the incandescent lamp1at constant filament temperature and constant IR emission intensity, which is useful for NDIR gas detection described in this invention. In other embodiments, the photodetector4can be one of a photodiode, a photo-transistor, a photoconductor and a Schottky photodiode that are of GaAs, InGaAs, Ge or Si—Ge type. The Schottky photodiode is fabricated on a silicon wafer.

The electronic control unit5is connected electrically to the photodetector4for receiving the induced electrical signal therefrom. The electronic control unit5compares the induced electrical signal with a preset reference signal so as to generate a feedback control signal indicative of a level difference between the induced electrical signal and the preset reference signal. The preset reference signal is associated with a predetermined constant level of the stray visible light corresponding to a constant temperature of the incandescent filament.

The power unit6is connected electrically to the electronic control unit5and the incandescent lamp1. The power unit6is operative to repeatedly regulate electrical power supplied to the incandescent lamp1based on the feedback control signal from the electronic control unit5until the induced electrical signal and the preset reference signal have the same level. When the induced electrical signal and the preset reference signal have the same level, the incandescent filament is kept at the constant temperature such that stable IR emission (i.e., constant IR emission intensity) of the incandescent lamp1is obtained, regardless of incandescent filament aging and ambient temperature variation.

To sum up, because of the ultra high sensitivity and fast response speed of the silicon photodetector4, feedback control of constant-filament-temperature can be conducted with extremely high precision. In addition, due to this high sensitivity, the location of the silicon photodetector4is not critical. It is noted that the silicon photodetector4has to be mounted outside the IR detector2because a narrowband IR window on a package (not shown) of the IR detector2screens away all incident light except the signature IR wavelength to be detected. Therefore, due to the presence of the silicon photodetector4, which can be regarded as a substitute for the dummy sensor of the aforesaid dual-element NDIR detector, the NDIR gas detector of the present invention can be fabricated with less complexity at relatively lower costs compared to the prior art with the dual-element NDIR detector.