Infrared (IR) absorption is one of the most reliable and accurate measurement techniques for gas sensing. Traditional IR absorption sensors for gas monitoring contain a small cabinet full of discrete components to produce IR illumination and measure the amount of IR light that the gas under study absorbs at specific wavelengths. Although accurate, such IR instruments have historically been too large, too expensive, and sometimes too fragile to provide attractive solutions for very large mass market applications, such as automotive and appliance monitoring. For example, carbon monoxide monitoring for automotive emissions checks has been a standard IR application for a decade, with accuracies down to the level of several hundred parts per million. The instruments used, however, are typically cart-mounted and cost end users several thousand dollars.

Thanks to recent innovations in their components, instruments are now becoming smaller, cheaper, more sensitive, more accurate, and less power consumptive. This paradigm shift illustrates the larger trend: A suitcase-sized unit with a $2000 end-user price achieves a place in every new car dealership, but a new, more accurate, $500 handheld unit might find a place in every toolbox. The same shift is occurring in the process-control, environmental, HVAC, and medical instrumentation arenas. To understand how this revolution in size and price is being made possible, you must take a look inside both the traditional and new-style instruments.

Many units built in the past several years have replaced the glowbar and chopper with incandescent specialty lamps or even ordinary panel lamps. Although commonly used, they are not a good solution because they emit too wide a spectrum of light. Besides being a waste, light that is outside the desired measurement band produces "parasitic" heat, which warms detectors, preamplifiers, and other components. This, in turn, interferes with measuring the targeted gas spectrum and degrades the sensitivity and accuracy of the reading. The incandescent bulb's inefficiency also dissipates excess power and heat and requires a higher drive power for the unit. An alternate approach was needed.

To simplify design and improve overall economics, Ion Optics developed the pulsIR, an IR emitter that incorporates source and chopper functions in a single element. Based on surface-modification and spectral-control technologies originally developed for the space program, the pulsIR line of efficient, pulsable IR sources is presently used in commercial instruments for gas detection, industrial process control, indoor air quality monitoring, automotive exhaust testing, and anesthesia gas monitoring. The secret of the pulsIR's success is an ion-milled microfoil radiator with spectrally tuned emission whose negligible mass (a few milligrams) permits it to heat and cool in milliseconds (see Figure 1).

Figure 1. The view from a cross-section scanning electron microscope (SEM) of an Ion Optics IR radiator (left) illustrates the component's fundamental operating principle. The entire radiator is only a few micrometers thick, so that it heats and cools rapidly in response to an electronic drive signal. The structure observed on the top and bottom surface of the cross section is the ion-beam texturing shown in the plain-view SEM (right).

Conventional glowbars and blackbody sources require long lead times for warmup and stabilization, and, during these times, they place a parasitic heat load on components in the optical train. The source-induced heat load is a problem because it is continuous and provides a steady-state (DC) noise source within the optical train, even when the detector is shuttered. Ion Optics' miniature sources, on the other hand, rely on electrically heated filaments of such low thermal mass that their temperature at all times correlates precisely with the current flowing through them. The filaments can be heated and cooled in milliseconds, delivering a current-following temperature profile exactly matching the drive pulse. Because they rely on electrical pulse shaping and current control rather than thermal mass for stability, these miniature sources have two important advantages:

• They need be powered only for a short time, thus minimizing parasitic heating.
• Their low thermal mass and high radiant output allow them to achieve a very high temperature-slew rate with virtually no thermal hysteresis.

This means that at any instant, filament output is directly related to electrical drive power. Effectively, this converts the difficult thermal stability problem into a straightforward matter of assuring electrical stability.

The critical feature of the pulsIR radiator is its specially constructed filament surface that is designed to improve conversion of thermal energy to in-band radiation. Figure 1 shows the characteristic features that result from ion-beam milling; their size and spacing can be adjusted to control both total radiated power and its spectral composition. Micron-sized features convert flat surfaces to selective emitters with high emissivity in the desired measurement band and low emissivity at wavelengths outside this measurement band (see Figure 2).

Figure 2. Ion Optics makes IR emitters for gas detection by ion beam texturing the radiator surfaces. This wavelength scan of the emission from an Ion Optics pulsIR shows the spectral narrowing achieved. This device is producing a spectrum useful for monitoring hydrocarbon compounds, by concentrating illumination on the carbon-hydrogen absorption band at 3.4 micron wavelength.

Wavelength Specialization
By concentrating electrical drive power into the wavelength range needed for the measurement, the pulsIR radiator assumes a portion of the task ordinarily performed by an interference filter. An interference filter window is typically used to filter radiation outside the required wavelength. Ion Optics' future development plans include radiators with a sufficiently narrow illumination band to replace the filter entirely, in effect combining the functions of a blackbody and a thin film interference filter in a single element. Such a tuned-band radiator, by offering a surface dark (high emissivity) in the appropriate absorption band and shiny (low emissivity) outside that band, would maximize the ratio of usable bandpass flux to total flux and therefore maximize instrument sensitivity. This maximization is particularly important for gases, such as carbon monoxide, that have only a weak IR absorption signal but must be measured at very low concentration levels.

Many chemicals important for industrial process and indoor air quality monitoring, such as chlorofluorocarbon refrigerant compounds, have strong absorption bands in the long wavelength (LWIR: 8- to 12-micron wavelength) IR. These compounds have historically been difficult to measure with IR panel lamps because of the low signal levels available in this waveband.

Figure 3. The thermal images of a tungsten filament bulb were recorded when the bulb was in two different states. The image on the left is of the bulb in the fully ON state; the one on the right was taken 300 ms after it was shut off.

Figure 3 shows ON and OFF 8- to 12-micron thermal images of an ordinary tungsten-filament panel-lamp pulsed at 1Hz. The left image was recorded with the lamp fully ON, the right after the lamp had cooled for 300 ms. The bulb was driven over its rated power and is brilliant white in the visible range; temperature modulation was a few degrees Celsius (indicated by the color shift from a white central zone in the ON image to a bright yellow one in the OFF condition). Most of the energy from the filament is used to heat the glass bulb envelope, which is opaque to radiation in the 8- to 12-micron waveband. The glass bulb reaches a steady-state temperature of ~100F (34C) and changes its temperature only a few degrees over the course of a single drive pulse.

Figure 4. The thermal images of a pulseIR source were recorded when the source was in two different states. The image on the left is of the source in the fully ON state; the one on the right was taken 300 ms after it was shut off.

By comparison, an identically driven pulsIR filament (Figure 4) cools from >300C to <100C in the same time, yielding a substantially better signal for an LWIR measurement. Because the surface has been specially treated to emit efficiently in the long wavelength band, this LWIR pulsIR filament is ideal for measurements of refrigerant gases. Notice also that the filament precisely follows its drive current and provides sharp temperature contrast between ON and OFF.

At the same time, the pulsIR reduces out-of-band radiation (especially short wavelength visible light) so that during this test sequence, the filament stayed black and produced no visible light at all. In a gas and chemical sensor system, this reduces parasitic heating of detectors, amplifiers, and other components to greatly improve system SNR and stability.

Already the Ion Optics pulsIR IR source allows sensor OEMs to reduce their parts count, power consumption, and integration complexity while dramatically improving sensor performance. In addition to providing a clear price and performance edge over current-generation IR sensors, this device is also an important steppingstone for development of next-generation gas and chemical sensors.

By integrating traditional functions of discrete components, such as IR source emission, chopper modulation, and spectral filtering, while minimizing size, power consumption, and heat dissipation, the Ion Optics pulsIR IR source is the first of a new generation of highly capable integrated sensor components that allow IR instruments to penetrate new mass market gas and chemical sensor applications.

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