Sutech Optical
Click here to download a PDF of this application.

Table of Contents:
I. Introduction
II. Reliability of Sutech Optical Choppers
III. Chopper Design Considerations
IV. Chopper Drive Circuits
V. Recommended Chopper Frequencies with Various Detectors
VI. Chopper Special Apertures
VII. Examples of Chopper Applications
1. Color Fiber Optic Spectrometer with Chopping and Synchronous Detection
2. Open Path Remote Sensing Transceiver with Integrated Reference and Chopping
3. Photoacoustic Gas Detection with Integrated Reference and Chopping
4. NDIR Gas Monitoring with Integrated Reference and Chopping
5. Material Thickness Monitoring with Synchronous Detection and Chopping
6. Multicolor Thermal Temperature Detection with Synchronous Detection and Chopping

I. Introduction:
The addition of optical chopping in many light detection instruments will significantly improve lower detection limits. This is accomplished by using light chopping in combination with synchronous detection (or lock-in amplifier). By chopping the light at a specific frequency, two benefits are realized. First, it permits the selected detector to work at its highest D* value for best performance. Second, the source or detector is constrained to respond only to light energy that is synchronous to that frequency. Light energies not synchronous are heavily rejected thus permitting increased detection sensitivity for the optical signal.

For example, a modulated light beam can propagate through free space with increased rejection to atmospheric and background aberrations. In chopped receiver applications (e.g., low temperature non-contact infrared measurements), the input signal is continually toggled between maximum signal and the “zero signal” baseline for increased performance. In both examples, light chopping improved the probability of detecting the true signal over background noise.

System detection performance with optical chopping can easily be 10-20 times higher than unchopped designs. A benchmark application is an infrared spectrometer with PbSe detector, optimized at 570Hz chopping frequency. The light source is beamed through the sample cell and detected by a PbSe sensor. The electrical signal from the PbSe detector is then bandpass filtered and sampled by a synchronous detection circuit. At the precise peak phase angle of the signal, a FET gate (microprocessor controlled) is opened to capture the detector voltage onto a small holding capacitor. This peak voltage is then A/D converted and time averaged by a microprocessor ( ~ 1.0 second). Typical improvements in S/N are shown by the following table for various asynchronous noise frequencies:

Back to top of page

II. Reliability of SUTECH OPTICAL Choppers:
Optical resonant magnetic choppers provide many unique advantages over a motorized chopper wheel or reciprocating blade. In an industrial product, the compact size, frequency stability and reliability permit many tough applications to become possible. The aperture motion or chopper window is produced by a high-Q resonating tuning fork that is highly resistant to vibration and shock. There basically are no moving parts to jam or to wear down.

Through conducted test, the nomimal service life of Sutech Optical magnetic choppers are estimated well over 20 years. The biggest issue observed in the field has been poor filtration of the air around the equipment leading to excessive dirt and oil buildup on the chopper. If the optical chopper is kept clean, it will often exceed the service life of the instrument.

At the chopping frequencies recommended for specific detectors (Si, InGaAs, PbSe, PbS), Sutech Optical choppers will quickly reach stability after “power-on” within 2.0 seconds. Power consumption is typically around 20mW. Due to the compact size and special alloys, Sutech Optical choppers can be mounted in close proximity to many hot filament sources. This enables compact and efficient designs to be possible in table-top or portable instruments.

When ordered with specific options, Sutech Optical choppers can be used in low vacuum (to 10 -10 Torr) or at extended high and low temperatures. The optical chopper blades can also be ordered with special vanes to perform reflective or multiplexing operations. Please refer to the general specifications table and examples for details.
Back to top of page

III. Chopper Design Considerations:
Chopping Frequency vs. Aperture Size:
The selection of an optimum chopping frequency is a balance between several variables. As chopping frequencies increase, the aperture size of the chopping window decrease and the passing light beam diameter must become smaller. Chopping frequencies beyond 1,000 Hz are typically not used because of this reason. At 570Hz, the minimum aperture window is 0.20” (5 mm) and at 800Hz, the window is 0.08” (2 mm).

High Q for Shock Resistance:
For high rejection against vibration and shock, a higher chopping frequency offers improved performance with Q values exceeding 600. This factor is of greater importance in portable instruments. Sutech Optical recommends 570Hz or 800Hz frequencies in both bench and portable instrument designs.

Select the Right Light Source and Detector:
Optical chopping of transmitted or received light generally falls into four wavelength bands. These applications are UV-VIS (300nm-700nm), Near IR(700nm-3µm), Med IR(3µm-6µm) and Long IR(6-15µm). For UV-VIS, light source selections are halogen, tungsten or UV bulb. Sutech Optical offers such sources fully integrated into the resonant magnetic chopper assembly. Matching detectors are silicon PiN diodes (normal or UV enhanced) or InGaAs PiN diodes.

For near infrared applications, the light source should produce strong radiation up to ~3µm wavelength. The integrated IR source from Sutech Optical is designed to operate between 800K and 1200K with a near blackbody emission profile. It is well suited for wavelength applications between 0.8µm to 4µm. Detectors of choice are Lead Sulfide or Lead Selenide. PbS and PbSe detectors are both quantum type, photoconductive detectors. When light enters the detector, the resistance across the detector will drop.

For long wavelength applications, glow bars, Nernst lamps and ceramic ignitors are effective sources. Pyroelectric detectors are often the detector of choice because of cost and sensitivity. Please contact Sutech Optical application engineering for more information.

Blackbody Emission Curves (Temp Kelvin)
PbS Detectors:
The recommended spectral range of PbS detectors is between 1 to 2.5um and at a chopping frequency of 570Hz. The expected signal to noise performance (at 25C) is ~500:1. Increased S/N performance up to 1500:1 is offered by Sutech Company with addition of thermoelectric cooling (1 to 3 stages) into the PbS detector package. Uncooled detectors are TO5 and cooled detectors are TO-8 or TO-66. The inclusion of TE elements will increase the size of the detector package and overall chopper dimensions. Please contact the factory for additional information.

PbSe Detectors:
The recommended chopping frequency for PbSe detectors is 800Hz and the expected signal to noise performance (at 25C) is ~ 500:1. Increased S/N performance up to 1500:1 is offered by Sutech Optical with addition of thermoelectric cooling (1 to 3 stages) into the PbSe detector package. Uncooled detectors are TO5 and cooled detectors are TO-8 or TO-66. The inclusion of TE elements will increase the size of the detector package and
overall chopper dimensions. Please contact the Sutech Optical for additional information.

Chopping Frequency:
Selection of an optimum chopping frequency is important to permit the best D* (D-star) performance in a detector. This term is a measure of photo sensitivity per unit area of a detector and is expressed in the format of D*(A,B,C) where A is temperature (K) or wavelength (µm) of a radiant source, B is the chopping frequency in Hz and C is the bandwidth of the detector. It is desired to operate a detector at the highest D* value that is practical. Sutech Optical recommends using 570Hz or 800Hz chopping frequency in the majority of optical designs.
Low Chopping Frequency:
Light chopping at low frequencies have special challenges as well as mechanical limitations. Low frequency magnetic choppers must be larger in size with longer tine forks that are less immune to vibration and shock. They are not recommended in portable applications when measurements must be made with the instrument in motion. Please contact Sutech Optical for more details.

Special Applications:
Sutech Optical choppers can also be ordered with special tines in multiplexed applications. For beam switching between two detectors, the chopper vanes can be fitted with special apertures or optical elements without compromise to size or reliability. Gold coated mirrors and grating can be mounted to the chopper vanes (see applications section). Please contact Sutech Optical on special applications.
Back to top of page

IV. Chopper Drive Circuits:
The drive voltage to the main chopper coil must be a 50% duty cycle square wave waveform. The amplitude will vary from +/- 4 to +/- 10VDC depending on the desired chopper drive level. Higher drive voltages will produce a larger aperture window at the given chopper frequency. In the majority of applications, aperture size should match the beam size for optimum results.

Many different techniques may be used to drive the chopper coil. If synthesized methods are used (D/A control with output drivers), the software must limit the maximum coil drive voltage to +/- 10V (20Vpp) at startup. After around 0.2 seconds, there will be sufficient feedback signal to lock onto. Once in lock, the drive voltage required for most applications will be around +/- 5VDC.

The feedback signal from the chopper can range between 1Vpp to 3Vpp. If a microcontroller is used, the input feedback A/D signal can be sampled for peak amplitude and then used to maintain feedback loop lock. A stable feedback signal will result when the chopper aperture is locked at constant size.

The Sutech Optical choppers (CF models only) are factory frequency tuned to a very tight tolerance close to perfect resonance. This precision permits the microcontroller to directly drive the chopper coil at the specified resonant frequency. For example, the 800Hz chopper can be directly driven by the microcontroller with a synthesized 800Hz (+/- 0.2 Hz) square wave. Very stable operations can be achieved by monitoring the feedback signal with a time constant of 20 milliseconds.

The recommended coil drive output circuit is shown below. The input square wave drive signal is connected to [D] and this is current amplified by a unity gain push-pull stage to the output terminals. The power supplies are at +12 and -12VDC. The two diodes preserve linearity and compensates for the Vbe drop across the PNP and NPN output transistors.

Coil Drive Output Circuit

An alternative drive option is to use an analog closed loop control circuit. This is now described below. In this system, the feedback signal is sensed and in turn governs the output drive to maintain constant aperture size. In this design, output sync signals are also provide to permit external devices to synchronize to specific phase angles of the chopper blade motion. This sync signal is often used to initiate sampling of an optical signal. It can also be used to phase synchronize several choppers to one master in parallel applications. For most applications, however, only one sync output is required.

The first step in the analog design approach is to process the feedback coil signal. The circuit below will low-pass filter the feedback signal and pre-amplify it with correct polarity for feedback loop control. This signal, later, will also drive the final output stage. Note that all circuits in this design are based on quad op amp packages such as the commonly available LM324. Supply rail voltages are +12VDC and -12VDC. Typical drive current peaks into the chopper drive coil are under 10mA.

Feedback Signal Filter and Amplification
The output of the above pre-amplification stage is further filtered to reject noise. A standard second order bandpass filter is used shown in the figure below.

Bandpass Filter of Feedback Signal
This bandpass filter is set for a center frequency of 800 Hz. Parameters are Gain = 1 and Q = 4. For different chopper frequencies, the best way to tune this filter is to change the three resistor values before considering changing the capacitor. The equations for this filter can be found in any operations amplifier handbook.

Once the feedback signal is filtered, the output is sensed using peak detection. One such approach is shown in the circuit below.

Negative Peak Detection Circuit Sensing Bandpass Output
The negative going excursion of the bandpass filter output waveform is now detected and stored on the 0.22 µF capacitor. The RC time constant of 0.02 seconds works well to track the feedback signal. This peak signal is then buffered by a voltage follower before going to the next stage.

The peak signal is now a DC level that can be compared against an internal DC reference for proper chopper drive level control. The voltage follower output is around (-4) volts for most applications. The negative peak error signal is compared with the reference circuit shown below.

The peak signal enters a summing junction where the input current is summed against the DC reference input current. The differential current is then amplified. If the bandpass filter feedback signal is small in magnitude (such as at startup), the output of the summing amplifier will go negative. If the bandpass output is too large from excessive swing of the chopper, the voltage will go positive. The damping in the system will produce stable operation usually within 2.0 seconds after power on. This circuit is the heart of this feedback control design.

Balance Bridge for Feedback/Reference and Autoclamp Circuit
A simple bipolar clamping circuit is used to regulate the output drive stage. The inverting gain stage signal (left op-amp) is used to derive two clamping voltages that will limit the output stage drive signals. This is done with two more op-amps (right side of circuit), one in a buffer configuration and the other in an inverting unity gain configuration. As the common input to these two stages goes negative, the lower op-amp buffers this value and uses it as the (V-) negative clamp rail. At the same time, the upper inverting buffer stage inverts this same voltage to drive the (V+) clamp rail. A signal line connected to this junction between the two diodes will therefore be clamp to active values of (V+) and (V-).

As the input level to these two stages approach zero, the (V+) and (V-) rails will track and collapse proportionally. The feedback signal therefore regulates this autoclamp window to increase or decrease drive levels to the chopper.

An example of stable operation is a 2Vpp signal at the feedback input that produces a -4
V value at the peak hold output. When this is summed against a 3VDC reference, the autoclamp rails would be around +/-5V thus producing a 10Vpp output drive to the chopper coil. If higher chopper drive is needed, the 3VDC reference voltage is simply increased. A potentiometer circuit can be used instead of a fixed voltage to permit setting of the chopper aperture size to match an application. The potentiometer range recommended is 3VDC to 5VDC.

In the above circuit, the feedback diode across the left op-amp permits faster collapse of the clamping window compared to the speed of expansion. For a brief time after power turned on, the autoclamp circuit will permit maximum chopper drive for faster starts. The capacitor across the feedback controls this time delay.

To finish this feedback control circuit and tie it back to the output stage, another amplification stage is added between the bandpass output and output driver as shown in circuit below. This extra amplification ensures that basic drive levels are always high enough for the final output stage. This drive signal is then regulated by the autoclamp circuit to appropriate levels. With this circuit, the feedback control loop is now complete.

Signal Amplifier Boost to Output Driver Stage

In summary after power on, maximum drive signal is presented to the chopper coil. As the feedback signal increases, the output of the bandpass filter also increases. The negative peak signal from this bandpass output is then compared to the reference voltage or setpoint. As the chopper swing increases, the drive voltage quickly reduces as the autoclamp circuit comes into action. The loop response is set to allow the system to find equilibrium without oscillation. Final aperture size is set by trimming the setpoint or reference voltage.

This final addition to this circuit is a sync generator for chopper blade phase position. This trigger will signal an external microcontroller to read the optical signal. Since the feedback signal is locked to the mechanical phase of the chopper blades, this signal is used to derive the sync trigger signal.

The circuit below shows two identical sync output circuits. The bandpass filter output is phase delayed by an RC circuit and enters the positive input of the op-amp stage. The negative input, however, sees the same signal but without a delay. This circuit will produce an output trigger waveform phased to a specific chopper blade angle. By increasing or decreasing the RC delay, this trigger phase angle can be adjusted.

Figure 8
The phase shifted signal is squared by a single transistor output stage. This output level can be CMOS, TTL or open collector depending on design.

This phase trigger is set normally at 180 degrees (full open) for many applications. If two phase sync signals are required, a second channel is added and set accordingly.

Sutech Optical offers complete stand alone controllers or open-frame driver boards for embedded applications. Please
Contact Us for additional information.
Back to top of page

V. Recommended Chopper Frequency with Specific Detectors:
Type 1: 800 Hz Chopper and PiN Detectors:
For applications using a silicon PiN or InGaAs PiN detector, the recommended chopping frequency is 800Hz. This frequency offers a large 0.080" aperture size for easy coupling into free space optics or fiber optic cables. The range of wavelengths for this application category will span between 350nm to 900nm. Many applications in color spectrometers or liquid cell readers operating from UV to near-IR will use this combination of chopping frequency and detectors.

The purpose of chopping light in a photovoltaic detector system is the rejection of ambient light and/or the need to lower interference from background fluctuations. For maximum InGaAs performance, the detector should be cooled and combined with synchronous detection to achieve the best signal to noise performance. All Sutech Optical choppers provide an integrated feedback signal to drive synchronous detection circuits. The 800 Hz Sutech Optical chopper is recommended in applications with PiN and InGaAs detectors.

Type 2: 800 Hz Chopper and PbSe Detectors:
For PbSe detectors, the recommended chopping frequency for best 1/f noise rejection and good aperture size is 800Hz. This chopping frequency (at 25C) will yield a signal to noise performance of ~500:1. With optional thermoelectric cooling (1 to 3 stages), the S/N can be increased to ~1500:1. These special detectors are in TO-8 or TO-66 packages. The basic PbSe uncooled detector is a TO-5 package. Sutech Optical offers standard and custom machined mounts that will integrate chopper and detector into one compact and efficient package. These housings can interface easily to optical fiber or free space optics.

Type 3: 570 Hz Chopper and PbS Detectors:
For applications using lead sulfide (PbS) photoconductive detector, the recommended chopping frequency is 570Hz. With this detector type, noise goes down as chopping frequency increases but signal intensity also begins to drop after 300 Hz. With this response characteristic, the maximum S/N is achieved at 570 Hz. The Sutech Optical 570 Hz chopper offers a large 0.100" aperture and permit easy coupling to fiber or free space optics.
Back to top of page

VI. Special Chopper Aperture Designs:
Sutech Optical resonant magnetic choppers have the unique advantage that the vibrating tines can be integrated with reflective mirrors or gratings without compromise to performance or reliability. This feature enables unique ratiometric spectrometer designs to be possible.

A diffraction grating element can be substituted as one vane of the resonant magnetic chopper. In this configuration, a very accurate multicolor infrared thermometer is produced that can be coupled into free space or fiber optics. This design has the unique ability of measuring the total emissive energy from an unknown target while also sensing two side bands of color (see applications section for details).

Sutech Optical offers custom apertures mounted to chopper tines. This will produce special light profiles useful as reference or scanning signals in special applications. One example is the matching slit window where each tine contains a narrow slit. Light can pass only when the slits are aligned. This shutter configuration will produce very narrow light pulses at twice the chopper rate. Please contact Sutech Optical for additional information.
Back to top of page

VII. General Examples of Chopper Applications:
1) Fiber Spectrometer for Liquid or Gas Cell Analysis:
This application uses chopped light and a tunable monochromator to test samples inside a liquid sample cell. The spectral scan produced is high resolution and has strong rejection to ambient light. This application can also be used for gas cells with minor modifications.
A chopper modulated halogen bulb (or appropriate IR source) serves as the light source. Halogen sources are available with typical color temperatures from 3000K to 7000K, depending on application. As color temperature is increased, more source intensity is produced but at a cost of lower filament life.

The Sutech Optical SC100-KIT can be ordered for general product development. The kit allows easy design development on an optical bench. The Sutech Optical chopper includes an integrated Type I infrared light source with a built-in parabolic reflector (ideal for applications in the 0.8µm to 4µm wavelength range). Normal source temperature is ~1150K with expected life of over 30,000 hours. A matching sensor is also offered in the Sutech Optical CR100-KIT. A choice of a PbS or PbSe detector is available. Halogen sources and custom mounts are also available. Please contact Sutech Optical for more details and application assistance.

The light is collected and focused by a sapphire ball lens designed to couple into an optical fiber with NA of 0.5. The chopper aperture is positioned between the ball lens and the optical fiber.

A multimode fiber is connected to the Sutech Optical integrated source through an integrated FC connector. This compact source is then coupled to a tunable monochromator controlled by a microprocessor. A silicon PiN, InGaAs or IR detector (depending on application) will monitor the light passing through the sample cell.

Sutech Optical also offers a patent pending custom designed smart pixel sensor array that has the ability for gas signature recognition. Lower detection limits and immunity to false alarms are significantly improved. Please contact Sutech Optical for additional details and specifications.

As the monochromator is scanned, the output spectra from the detector is digitized and analyzed by the microprocessor system. Spectral information is compared to stored library data for qualitative and quantitative measures. All wavelengths from the monochromator are chopped at the same frequency. This permits the same synchronous detection parameters to be used on full or partial scans. The chopper drive coil is synchronously driven by the microprocessor at 570Hz (or 800 Hz) to match the detector response. A feedback coil signal from the Sutech Optical chopper provides timing input to the synchronous detection circuit.

Calibration of the system is accomplished by using standard white cells and calibration gasses.
Back to top of page

2) Open Path Remote Gas Sensing:
This application projects a beam of chopped light through the air for remote open path gas sensing. A reflecting telescope design is used to provide transmission and receiver functions within one instrument. For a field measurement, a retroreflector array is place distal to the transceiver telescope at distances from 100 meters to 300 meters depending on the size of the mirror system used. Typically, a 6" diameter mirror can reach 100 meters while an 8"system will work to 300 meters.

The principle of open path detection is based on absorption of light energies by gas molecules (from vibration and rotation excitation of bonds) in the path of the monitoring light beam. The degree of absorption at specific wavelengths is directly proportional (Beer-Lambert law) to the equivalent path length that the gas is occupying.

Light Absorption Beers Lambert Law:
I = intensty at pathlength; Io = Source intensity; n = concentration; l = pathlength; s = absorption crossection

-dI = I*n*s*?dl
I = Io*exp(-n s l)

A gas plume at 1000ppm over 10 meters has the same degree of light absorption as a 10ppm plume covering 1000 meters. The advantage of a telescope system and a retroreflector is the doubling of path length for increased sensitivity. For example, if a plume of target gas is 20 meters in length, the optical beam of the telescope will travel through this plume, reflect back from the remote retroreflector panel and traverse through this gas plume a second time for a total path length of 40 meters. The resulting absorption (at specific wavelengths) is used to derive gas concentration in ppm-meter units.
Open path monitoring is cost effective and has high sensitivity to many target gasses in applications, such as remediation of land fill sites or monitoring of outgassing rates from chemical facilities. A dedicated system to monitor sulfur oxides and nitrous oxides (SOx and NOx) emissions is described below. This compact solution offers very high reliability and is more cost effective than FTIR systems.

The source of the reflecting telescope is composed of a chopper modulated IR source (in this case, a Nerst heater or glow bar). The IR source is selected to produce effective light output in the 1 to 3µm range where SOx and NOx gasses have strong absorption. The output beam is focused by a sapphire IR ball lens to produce an f-matched point source located at the focal point in the telescope (the selection of the ball lens must match the f-number of the mirror used). In many designs, an f-number of 1.5 will produce a very compact telescope design for 6" or 8" systems.

The source light beam enters a 50/50 splitter where 50% of the light is sent to the primary mirror. This light is projected into free space and then returned to the primary mirror by the external retroreflector. The returning beam is also split again by the 50/50 splitter. The resultant 25% energy beam is further split into two equal signals and sensed by detectors A and B. Each detector will sense 1/8 of the original light energy produced by the main light source. Even with beam splitting, the advantage gained by a one piece ratiometric transceiver with no moving parts is still dominant. It is possible to eliminate the beam splitter in the final detector by using a stacked detector with dual wavelength selectivity or improve performance with use of wavelength selective beam splitters, but these advanced techniques are beyond the scope of this introduction.

Detector-A is designed with a bandpass type-A filter where the target gasses will have strong absorption. Detector-B is fitted with a bandpass type-B filter sensitive to a wavelength window outside of the absorption range. This detector-B signal will serve as the reference baseline for the system. Outputs from detectors A and B are then processed as a ratiometric signal. This technique produces high detection sensitivity with very strong rejection of atmospheric effects such as from smoke, rain and fog.

A standard white cell can be used to simulate gas concentrations and produce a look-up table of gas levels and detector ratios. This data set is then stored in the instrument to permit very fast gas alarms. For toxic gas monitoring, this ratiometric speed advantage is very important.

A microprocessor system controls the Sutech Optical chopper frequency and performs synchronous detection. For PbSe detectors, the recommended chopping frequency is 800Hz. The Sutech Optical SC100-KIT optics bench chopper can be ordered for general product development. Please contact Sutech Optical for additional information.

Sutech Optical also offers a patent pending custom designed smart pixel sensor array that has the ability for gas signature recognition. Lower detection limits and immunity to false alarms are significantly improved. Please contact Sutech Optical for additional details and specifications.
Back to top of page

3) Photoacoustic Gas Detection:
Photoacoustic (PA) gas detection is a simple, yet powerful, technique for detecting low concentrations of a target gas with high immunity to cross interferences. The basic concept is to use the absorption properties of a target gas to detect its own presence.

If carbon dioxide, for example, is the target gas of interest, a sealed cell “A” serves as the reference cell and is filled with a high concentration of CO2. A second cell “B” or sample cell holds the unknown sample. This sample gas can diffuse into the chamber through a gas membrane or for remote locations, is pumped in. A sensitive differential pressure gauge connects between the two cells. For illumination, a chopped source is
split into two beams where one beam is projected into chamber A and the second beam into chamber B. Wide bandpass filters matched to the application prefilters the input light. The CO2 gas in the reference cell will absorb some light energy per its characteristic spectra at 4.3µm. This energy converts to heat and causes a pressure rise inside the reference cell. Similarly, if CO2 is present inside the sample cell, a pressure rise will also occur. The differential pressure between the two cells is sensed by the pressure gauge or a microphone. A signal maximum is reached when no CO2 is inside the sample cell. As CO2 concentration grows in the sample cell, the differential pressure signal will decrease following a linear relationship. When this concentration equals the reference cell, the differential pressure signal will be zero. A calibration curve is developed that translates this signal into PPM concentration of CO2. The sensitivity of photoacoustic detection can be 10 times higher than NDIR techniques.
Another photoacoutic design with very high immunity to interference gasses is shown above. In this arrangement, the chopped light is prefiltered only with a broadband filter and enters directly into the sampling chamber before entering the reference chamber. In this example, the reference chamber is filled with CO2. If no CO2 is present in the sampling chamber, the pressure developed in the reference chamber will be at a maximum. If CO2 enters the sampling chamber, it will absorb the same spectral energy and reduce this energy entering the reference cell. The signal from the differential pressure sensor only responds to variations in CO2 concentration with very high rejection of all other gasses.

The pressure sensors can be two simple microphones, one monitoring each cell. The signals from these microphones are processed individually and differentially to gain different information.

This high sensitivity of detection is possible because the target gas absorption spectrum is used to detect itself. The amount of heat produced in the photoacoustic reference cell is predominantly caused by the absorption characteristics of the CO2 gas inside. This specificity of aborption and conversion to heat is the core reason for photoacoustic sensitivity. The total energy in the input light beam for CO2 absorption must always be shared between the sample and reference chambers. Any robbing of this spectral energy in the sampling chamber decreases the heat generation in the reference chamber. This is true regardless of the complexity of spectral lines within the spectrum.

When no CO2 gas is present in the sample cell, the differential signal is peaked with maximum pressure inside the reference cell. When CO2 concentrations increase in the sample cell, the differential pressure will drop. It will decrease to zero when the CO2 absorption in the sample cell equals the absorption in the reference cell. Generally, this concentration level is the maximum detectable level of the instrument. Due to the specificity of absorption in the photoacoustic design, it is highly immune to interference gasses even when such absorption lines falls very close to that of the target species.

When detecting gas types with many closely spaced absorption bands, photoacoustic detection will extract much more energies from a sensing beam. With nondispersive infrared (NDIR), bandpass filters have limitations that makes them impractical or impossible to pass enough target spectra energy without also passing a high level of background spectra or noise. The more complex is the target spectra, the greater is the photoacoustic advantage over NDIR.

The pressure waves developed inside the reference and sample cells are low frequency sound waves, thus the name, photoacoustic detection. In designing a chamber, detection limits can be further increased by using reflective resonance chamber designs (at the chopping frequency) to increase the amplitude of the target gas signal.

The Sutech Optical light source will use a coated tungsten bulb or a glow bar light source, depending on application. For longer IR wavelengths, the glow bar choice is superior. The light beam is focused by a sapphire ball lens and directed evenly into the two gas cells through appropriate windows. Zinc selenide is often the window material of choice. Pre-filters can be used to improve system performance. The recommended chopping frequency is 800Hz. This frequency offers fast response with synchronous detection and strong immunity to vibration.

The Sutech Optical SC100-KIT can be ordered for general product development. It is designed for mounting onto an optical bench or used as a stand alone component. Please contact Sutech Optical for additional details and specifications.
Back to top of page

4) NDIR Gas Monitoring:
This application use chopped light and bandpass filtered detectors to sense concentrations of one or more target gasses. A sample tube is coated on the inside with a reflective material such as gold for high infrared reflectivity. The target gas is monitored by an approximate sensor (e.g., lead selenide, lead sulfide) and bandpass filter. The center wavelength of the bandpass filter is matched to the strongest absorption peak in the target gas spectra. For many target gasses, a wide bandpass filter will yield more light energy for detection but at a cost of less wavelength discrimination. Proper balance in center wavelength selection and filter passband window is critical for optimum performance and sensitivity.

A separate wavelength channel is also monitored outside the target gas absorption band to serve as a reference. The ratio between this reference channel and the target gas channel is used to calculate the PPM concentration of the target gas.
The advantage of NDIR analysis is the relative design simplicity and the ability to detect several gasses with multiple detectors. In some applications where sensitivity and interferences are non issues, NDIR can be an ideal choice. For highest sensitivity, the NDIR light source must be chopped at the optimum detector frequency for the best performance. The recommended chopping frequency is 570 Hz for lead sulfide detectors and 800 Hz for lead selenide detectors.

Sutech Optical also offers a patent pending custom designed smart pixel sensor array that has the ability for gas signature recognition. Lower detection limits and immunity to false alarms are significantly improved. Please contact Sutech Optical for additional details and specifications.

The Sutech Optical SC100-KIT can be ordered for general product development. The choice of light source will depend on the target gas. This technique is extremely effective for gasses not affected by water or CO2 interference. Path length can also be easily optimized for an application. Please contact Sutech Optical for additional details and specifications.
Back to top of page

5) Material Thickness Monitoring:
This application shows the use of focused chopped light to determine the thickness and uniformity of translucent materials. Many materials will exhibit light absorption properties that can be used to assess its thickness and general properties. Such applications may be the thinning of silicon wafers for semiconductor devices or in quality control of products, such as paint, fabric, plastics and paper
The Sutech Optical chopper, integrated light source and probe head is positioned on one side of the scanned material. The collimated light from an IR source is coupled into a fiber by a sapphire ball lens matched to a NA of 0.5. The aperture of the scanning fiber is large enough to average the surface texture of the material being scanned. Too small of a diameter may result in unwanted signal variations. Once optimized, materials can be monitored in real time at high speeds for process feedback and/or quality control. The recommended chopping frequency is 800 Hz.

The Sutech Optical SC100-KIT can be ordered for general product development. Please contact Sutech Optical for additional specifications.
Back to top of page

6) Multicolor Non-Contact Temperature Detection:
This application uses non-contact infrared light measurements to determine the temperature of a target surface. Many infrared temperature systems are susceptible to errors due to emissive and absorption variability as a function of wavelength. The multicolor system described here can provide higher accuracies for difficult materials in process control systems

Infrared Radiation and Blackbody Primer:
All materials above zero degrees Kelvin will emit infrared radiation. A perfect blackbody surface by definition is a surface that will absorb all wavelengths and will appear completely black. Radiation emission from a material can never exceed that from a blackbody. At any point of the wavelength spectrum, the ratio of the emission from a surface to that from a blackbody at the same temperature is called emissivity and has a value range from 0 to 1.

Three properties of a material determine the emissivity fraction. They are reflectance, absorbance and transmittance. Any level of reflectance or transmittance will lower the emissivity number. A perfect blackbody has emissivity of 1.0 because it has zero reflectance and zero transmittance. Therefore, the equation for emissivity is:

Emissivity = 1 – (Reflectance + Transmittance)

A bright metal surface will have high reflectance, zero transmittance, and therefore, very low emissivity. On the other hand, glass (at lower wavelengths) has low reflectance but high transmittance but also very low emissivity.

Emissivity can also be stated as the absorbance property of the material. If all absorbed energy is turned into thermal radiation, it is essentially a blackbody. Any reflectance or transmission factors will therefore lessen this intensity. These factors can also be wavelength dependent resulting in vastly different emissivities at different bands of the spectrum.

Glass is a good example. At wavelengths below ~2.5µm, glass has excellent transmission properties and emissivity is very low at 0.03 to 0.04. Very little infrared signal is sensed by a probe in this band. However, at longer wavelengths beyond 4.5µm, glass transitions into a strong absorber and the emissivity value jumps to over 0.95 with strong infrared emissions. By exploiting this property in glass, temperatures can be measured at the glass surface or deeper into the bulk just be changing the wavelength region sensed.

In applications where emissivity can vary substantially as a function of wavelength, techniques utilizing multi-zone monitoring and ratiometric processing will produce significantly more accuracy in temperature readings. For applications where the target material can be simplified as a “grey body,” a single infrared reading may yield sufficient temperature accuracy. This is possible because blackbody curves are monotonic functions where a single point can define its shape or temperature. As temperature increases, the area of the blackbody curve grows while the peak shifts to a shorter wavelength.

Several laws govern the characteristics of black body emissions but will not be discussed in detail. Wien’s law governs the light wavelength where the strongest emission is observed at a given temperature. Planck’s law describes the shape of the blackbody emission spectra and Stefan-Boltzmann’s law governs the total power emitted per unit area from that surface.
This application uses the aperture motion of the Sutech Optical chopper to duty cycle (multiplex) the input light from the observing telescope between a direct detector A and a pair of secondary spectrum detectors (B and C). When the chopper aperture is open, the incident light from the observing telescope (through a slit aperture) is directed onto detector A to measure the total energy of the input spectrum. When the chopper apertures are closed, a diffraction grating on the outer chopper vane produces a spectrum that is sensed by detectors at locations B and C. This technique permits three color channels of information to be used in calculating the surface temperature of the object in view.

The use of 3 bands of spectra information can generate multiple ratios to identify material types or sense phase properties in certain materials as it gets hotter. During first time use, calibration is performed on the materials to be sensed. Color ratios are improved further by an “E-slope” correction factor for materials that exhibits changing emissivity as a function of wavelength. First time calibration will compare thermocouple readings against the IR spectrometer values to optimize E-slope parameters.

The microcontroller controls the recommended 800Hz operating frequency of this lead selenide detector system. The high chopping frequency maintains good correlation between sequential reads from detectors A versus B and C.

Proper selection of wavelength bands for detectors B and C is important for optimum performance. The field of view is set by the telescope to be as full as possible. This multicolor technique can sense accurate temperatures even when the FOV is not quite fully filled. The use of multiple ratios instead of absolute amplitude also preserves temperature reading accuracy as signal levels drop compared to single color systems.

In difficult applications such as hot steel processing where the emissivity factors in the FOV can be changing as a function of temperature, the presence of slag can be differentiated from the molten metal by monitoring the ratios of the three color signals from the three detectors. Excessive amounts of slag will decrease steel quality.
The higher emissivity of the slag can be sensed and a calibration table developed for process monitoring.

A table of emissivity for a range of common materials:
The Sutech Optical SC100-KIT can be ordered for general product development. Please Contact Us for additional specifications.

Sutech Optical also offers a patent pending custom designed smart pixel sensor array that has the ability for gas signature recognition. Lower detection limits and immunity to false alarms are significantly improved. Please contact Sutech Optical for additional details and specifications.

Back to top of page

Sutech Optical
7980 Kingsbury Drive
Hanover Park, Illinois 60133
P: (630) 855-5068 F: (630) 855-5068