Abstract

The measurement of thermal radiation from ambient-temperature objects using short-wave infrared detectors and regular glass optics is described. The detectors are chosen to operate in the 2.0 µm to 2.5 µm atmospheric window. Selection of detectors with high shunt resistance along with the 4-stage thermo-electric cooling of the detectors to -85 °C results in detectivity, D*, of 4×1013 cm Hz1/2/W which is near the background limited performance at 295 K. Furthermore, the use of regular-glass commercial optics to collect the thermal radiation results in diffraction-limited imaging. The use of a radiation thermometer constructed with these elements for the measurement of a blackbody from 20 °C to 50 °C results in noise-equivalent temperature difference (NETD) of <3 mK at 50 °C. The operation at shorter wavelengths than traditional thermal sensors also leads to lower sensitivity to the emissivity of the object in determining the temperature of the object. These elements are used to construct a calibrator for an infrared collimator, and such a system demonstrates noise-equivalent irradiances of <5 fW/cm2. These results indicate that radiometers using short-wave infrared sensors could be constructed utilizing commercial glass optics with possible better performance and lower NETD than existing radiometers using cryogenically-cooled mid-infrared or thermal infrared detectors.

© 2008 Optical Society of America

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References

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  1. A. Rogalski and K. Chrzanowski, "Infrared devices and technique," Opto-Electron.Rev. 10, 111(2002).
  2. D. G. Crowe, P. R. Norton, T. Limperis and J. Mudar, "Detectors," in Electro-Optical Components, W. D. Rogatto, ed., Infrared Information Analysis Center, Michigan, 1993.
  3. L. S. Rothman,  et al., "The HITRAN 2004 molecular spectroscopic database," J. Quantitative Spectroscopy and Radiative Transfer 96,139-204 (2005).
    [CrossRef]
  4. Schott glass designations.
  5. H. W. Yoon, D. W. Allen, and R. D. Saunders, "Methods to reduce the size-of-source effect in radiometers," Metrologia 42, 89-96 (2005).
    [CrossRef]

2005 (2)

L. S. Rothman,  et al., "The HITRAN 2004 molecular spectroscopic database," J. Quantitative Spectroscopy and Radiative Transfer 96,139-204 (2005).
[CrossRef]

H. W. Yoon, D. W. Allen, and R. D. Saunders, "Methods to reduce the size-of-source effect in radiometers," Metrologia 42, 89-96 (2005).
[CrossRef]

2002 (1)

A. Rogalski and K. Chrzanowski, "Infrared devices and technique," Opto-Electron.Rev. 10, 111(2002).

Allen, D. W.

H. W. Yoon, D. W. Allen, and R. D. Saunders, "Methods to reduce the size-of-source effect in radiometers," Metrologia 42, 89-96 (2005).
[CrossRef]

Chrzanowski, K.

A. Rogalski and K. Chrzanowski, "Infrared devices and technique," Opto-Electron.Rev. 10, 111(2002).

Rogalski, A.

A. Rogalski and K. Chrzanowski, "Infrared devices and technique," Opto-Electron.Rev. 10, 111(2002).

Rothman, L. S.

L. S. Rothman,  et al., "The HITRAN 2004 molecular spectroscopic database," J. Quantitative Spectroscopy and Radiative Transfer 96,139-204 (2005).
[CrossRef]

Saunders, R. D.

H. W. Yoon, D. W. Allen, and R. D. Saunders, "Methods to reduce the size-of-source effect in radiometers," Metrologia 42, 89-96 (2005).
[CrossRef]

Yoon, H .W.

H. W. Yoon, D. W. Allen, and R. D. Saunders, "Methods to reduce the size-of-source effect in radiometers," Metrologia 42, 89-96 (2005).
[CrossRef]

J. Quantitative Spectroscopy and Radiative Transfer (1)

L. S. Rothman,  et al., "The HITRAN 2004 molecular spectroscopic database," J. Quantitative Spectroscopy and Radiative Transfer 96,139-204 (2005).
[CrossRef]

Metrologia (1)

H. W. Yoon, D. W. Allen, and R. D. Saunders, "Methods to reduce the size-of-source effect in radiometers," Metrologia 42, 89-96 (2005).
[CrossRef]

Rev. (1)

A. Rogalski and K. Chrzanowski, "Infrared devices and technique," Opto-Electron.Rev. 10, 111(2002).

Other (2)

D. G. Crowe, P. R. Norton, T. Limperis and J. Mudar, "Detectors," in Electro-Optical Components, W. D. Rogatto, ed., Infrared Information Analysis Center, Michigan, 1993.

Schott glass designations.

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Figures (10)

Fig. 1.
Fig. 1.

The spectral radiances from the use of the Planck radiance law for blackbodies at the respective temperatures. In the spectral region from 2.0 µm to 2.5 µm, a blackbody at 22 °C and a blackbody at 300 °C will differ by a factor >19,000 while at a center wavelength of 4.0 µm, such a ratio is only ~370.

Fig. 2.
Fig. 2.

The temperature dependence of shunt resistance for 1 mm and 3 mm diameter InGaAs, extended InGaAs and 1 mm diameter swMCT detectors. A shunt resistance increase of 1000 over the room temperature value is easily achieved using TE cooling. The slope of the temperature dependence is constant for both types of detectors.

Fig. 3.
Fig. 3.

The background-limited power (BLIP) restricted to f/2 field-of-view for 295 K background with the detectivity of near-infrared, short-wave infrared and mid-wave infrared sensors plotted versus their bandgap wavelengths. The detectivities plotted are for electrical bandwidth of 0.16 Hz. The detectivity of the uncooled ex-InGaAs at room temperature (RT) does not meet the BLIP curve while the detectivities of the 4-staged TE cooled detectors lie near the BLIP curve.

Fig. 4.
Fig. 4.

The NIST-measured spectral irradiance responsivity of an ex-InGaAs detector demonstrating the peak of the responsivity lies in the 2.0 µm to 2.5 µm region.

Fig. 5.
Fig. 5.

The transmittance at sea level for a 1 m path length of atmosphere showing the window between 2.0 µm to 2.5 µm calculated using HITRAN.

Fig. 6.
Fig. 6.

The transmittances of 10 mm thick BK7 (crown) glass and SF5 (flint) glass elements which are commonly used to form achromat lenses. Other optical glasses have generally similar transmittances.

Fig. 7.
Fig. 7.

The SWIR detector was constructed into a radiation thermometer configuration to determine the noise-equivalent temperature difference as a function of the blackbody temperature. The distance between the objective lens and the blackbody aperture was 500 mm with a 50 mm diameter objective resulting in roughly a f/10 collection geometry. A 6 mm diameter target spot was focused at the blackbody opening. All three lenses used in this setup were commercial, visible-wavelength-optimized achromats.

Fig. 8.
Fig. 8.

The stability of the chopped radiation thermometer measuring the radiance temperature of the 50 °C blackbody. The blackbody was constructed with a spherical geometry and a control loop was used to stabilize the blackbody temperature.

Fig. 9.
Fig. 9.

The noise-equivalent temperature difference (NETD), at an electrical bandwidth of 0.16 Hz, obtained from the standard deviation of the radiation thermometer measurements at the respective blackbody temperatures. The increase in the NETD from the low signals as the blackbody temperature approaches the laboratory ambient temperatures is observed. The NETD at human body temperature of 36 °C is below 10 mK.

Fig. 10.
Fig. 10.

The schematic of the SWIR radiometer used to measure the collimated output from an infrared collimator. The use of the two chopper positions allows comparison of “upstream” chopping at the source to “down-stream” chopping inside the radiometer.

Tables (4)

Tables Icon

Table 1. The direct-current (DC) signals from various objects at the estimated temperatures filling the field-of-view (placed directly over the top of the detector) of the 3 mm diameter, 4-stage TE cooled (-85 °C) ex-InGaAs photodiode with a preamplifier gain of 107 V/A. These signals clearly demonstrate that there is sufficient signal for thermal measurements. The differences in the spectral emissivity of the room temperature objects account for the different signals.

Tables Icon

Table 2. The specifications of the 200 mm focal length achromatic lens, optimized for infinite-conjugate imaging at 550 nm, used for the optical modeling of the SWIR performance. The last surface denotes the air-glass interface, and thus the thickness and material are intentionally left blank.

Tables Icon

Table 3. The optical performance of the achromatic lens at infinite conjugate at a wavelength of 2.45 µm. The performance was determined using lens parameters from Table 2 for a lens optimized at 550 nm. The performance is diffraction-limited as indicated by a smaller geometric, root-mean-squared (RMS) radius than the diffraction limit radius.

Tables Icon

Table 4. The noise-equivalent irradiance measurements using the SWIR radiometers with the collimator shown in Fig. 10. The blackbody source was at 300 °C, and the measurements were performed at the source aperture diameters shown below.

Equations (3)

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L 4.0 μ m ( 300 ° C ) L 4.0 μ m ( 22 ° C ) 370 .
L 2.4 μ m ( 300 ° C ) L 2.4 μ m ( 22 ° C ) 1.9 × 10 4 .
dL L = c 2 λ dT T 2 ,

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