A comparison of thermal imaging at microwave and infrared wavelengths

Infrared Physics 1975. Vol. IS. pp. 12S131. Pergamon Press. Printed in Grcal Enlain

A COMPARISON OF THERMAL IMAGING AT MICROWAVE AND INFRARED WAVELENGTHS G.!~IAERER Institute of Applied Physics, University of Berne, Berne. Switzerland (Receiwd 19 August 1974) Abstract-A comparison of thermal imaging at mm-wave and i.r. wavelengths shows that a passive microwave system may be very useful for applications concerning the mapping of moist soils, water surfaces and metals. A typical mm-wave thermal image is compared with an i.r. image of the same scenery.

Studies on passive mm-wave sensing have been made at the University of Berne for several years. The increasing interest in passive microwave sensing is due to the better atmospheric transparency of microwaves under bad weather conditions (fog, rain), compared to the infrared. For wavelengths longer than 15 cm the atmospheric influences are very small. The principal disadvantage in this spectra1 range is however the bad angular resolution, when a small antenna is used: the resolution 0 of a microwave antenna is diffraction limited and is given for an aperture diameter D and an operating wavelength 1 approximately by e = A/D.

At the shorter limit of the mm-wave range (1 mm < 1 < I.5 cm) a much better angular resolution may be achieved. The atmospheric attenuation may however not be neglected any more. Only the transmission windows at i. = 1.2, I = 3 and I. = 8 mm allow theqnal sensing over large distances. Figure 1 shows the atmospheric absorption at mm-wave and at i.r. wave1engths.“-3’The strong influence of rainfall is shown in Fig. 2.‘4*5’The principal Clear

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A comparison of thermal imaging

127

6-

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Fig. 3. ‘Temperature variation of the radiance ratio r at the infrared (8-13 pm) and at mm-waves (3 mm wavelength, I GHz bandwidth).

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The system noise temperature T, of the receiver varies from about 10°K (antenna pointing to the cold sky) to thousands “K, depending on the frequency and type of antenna and receiver. The ideal background limited radiometer (T, = TA) has a maximum sensitivity of O.Ol”K for a 500°K blackbody calibration source and a post detection bandwidth of 1 Hz (radio frequency bandwidth 4 GHz). The noise equivalent power NEP at microwaves is then given by NEP = kTS.J(2.Av,,.AvpD). Thus for the above mentioned situation the noise equivalent power is between lo- l6 and lo- l9 W, depending on the radiofrequency bandwidth. The chopping frequency has no influence on the noise equivalent power within the range 30Hz-1OkHz. A comparison of the ultimate sensitivity of a background limited ideal thermal infrared detector gives (‘) NEP = 4&k. A. a). T”’ J(Avpr,) where A = area of the detector CJ= 5.67 lo- l2 W cmm2 Ke4 (Stefan-Boltzmann

constant).

For A = 1 cm2, Av,, = 1 Hz and T = 500”K, NEP = 5 x lo- lo W. For an ideal photo detector, the noise equivalent power is still one order of magnitude below this value. A microwave receiver is similar to an i.r. receiver in many respects. Figure 4 shows a block diagram of a typical scanning mm-wave radiometer with heterodyne detection. The received noise level is compared with the constant reference noise temperature of a ferrite switch (chopper) at a rate of 10kHz. This allows integration times in the order of msec. The mechanical switch enables the calibration of the instrument by a hot load. The center frequency of 92 GHz with a bandwidth of 1 GHz is transposed in a Schottky barrier mixer stage to an intermediate frequency of 10.5 GHz. The amplification is done by three tunnel diode amplifiers. The tunnel diode detector delivers the video signal, which is amplified and detected phase coherently. The data handling system corresponds entirely to an i.r. system.

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128

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An important difference between mm-waves and the i.r. concerns the emissivity. The radiation temperature of a target is composed of two different contributions TR=eT,+(l

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where e and (1 - e) are emission coefficient and reflection coefficient, respectively of the object (for a given polarization). Tk is its real or kinetic temperature and T, is the radiation temperature of the surroundings, which is the average over a solid angle determined by diffraction, depending on the surface of the object. The mm-wave emissivities show characteristic differences to the i.r. emissivities in many respects. The frequency dependence for many materials is more pronounced at mm-waves than at the ix. Some typical values are given in Table 2.@-l”) It may sometimes be difficult at the i.r. to distinguish between small differences in temperature and between small differences in emissivity, because the changes are in the same order of magnitude. The relatively large emissivities (e > 04) and roughnesses of many materials give a very low polarization dependence of the emissivity. At mmwaves however, the large polarization dependence of the emissivity may be used for an identification of objects. This would be especially suited for objects on a water surface (ice, pollutants). F igure 5 shows the reflection coefficient of water at a wavelength of 3.2 mm. Table 2. A comparison of emissivities at different spectral regions 3cm

3mm

Steel

0-00

0.00

Water Sand (dry) Concrete Asphalt

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4pm 06-09 0.96 0.83 0.9 1 0.71

A comparison

of thermal

129

imaging

0.6

Fig. 5. Reflection coefficients of water at 33°C and 9°C as a function of the incident angle (0 = vertical) for horizontal and vertical polarization. Measuring points and best-fit Fresnel curves.

At mm-waves, changes in emissivity are much better recognized than changes in temperature. A mm-wave thermogram gives therefore the material properties (and the reflections from the surroundings) rather than the temperature distribution. The best materials to detect at mm-waves are moist soils at different moisture contents, water surfaces, ice and metals. Metals are nearly perfect reflectors within the whole mm-wave spectrum, irrespective of the oxide layer or camouflage paintings. When a metal reflects the cold sky, the contrast between the metal and the terrain is maximum. The effective radiation temperature of the metal is then just the sky noise temperature, which is about 3°K at wavelengths longer than 1.5 cm. For wavelengths shorter than 15cm the sky noise is however increased, due to the absorbing atmosphere. The state of the cloud cover and the humidity must then be considered. Table 3 gives some values of the sky noise temperature at 3 mm wavelength. Figure 6 shows the comparison of a thermal image of a partly snow-covered mountain (distance 75 km) at mm-waves (3 mm) and at the i.r. (2-5.4 pm). Figure 6(a) shows the mmwave thermogram. Black points represent the highest temperatures of the image (about 300°K) and white points represent the lowest temperatures (40-240°K). The extremely clear recognizability of the snow fields is due to the high reflectivity of snow and the fact that the mountain slopes preferably reflect the sky noise that emerges under small zenith angles (low sky noise temperatures). The temperature resolution is 3°C the post detection bandwidth 0.5 Hz and the angular resolution is 3 mrad. Figure 6(b) shows the infrared thermal image, taken with the Dynarad Thermo-Imager. The temperature resolution is 0*3”C (at 30”(Z), the post detection bandwidth is 300 kHz and the angular resolution is 1.7 mrad.

Table 3. Measured

zenith sky noise temperature for various cloud covers at 3.26 mm wavelength Thun (altitude 565 m above sea level)

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temp. Weather

condition

measured

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Humidity (XJ

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25-30

5&60

8&120

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20-26

5&60

I2&200

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16-20

5&60

200-240

Dark thunderstorm clouds

l&15

60-70

from

G. SCHAERER

Fig. 6. Comparison of a mm-wave with an i.r. image (a) mm-wove image (3 mm wavelength) (b) ix. image (wavelength 2-54 ,um) (c) photographjc identifi~tion of (a) and (b).

A comparison of thermal imaging

131

REFERENCES

1. KNEUB~~HL, F. private communication, Swiss Institute of Technology, Ztirich (1974). 2. FURASHOV,N. I. Opt. Spectroscopy 20,234 (1966). 3. MEYER,J. W. Proc. IEEE 44,484 (1966). 4. Mm, G. Ann. Physik 25, 377 (1908). 5. RYDE,J. W. and RYDE,D. Gen. Elect. Res. Lab. Rev. Wembley (1945). 6. KRAUS,J. D. Radio Astronomy. McGraw-Hill, New York (1966). 7. SMITH,R. A. The Detection and Measurement of Infrared Radiation. Oxford University Press, London (1968). 8. Handbook ofMilitary lnj?ared Technology. Office of Naval Research Department of the Navy, Washington, D.C. (1965). 9. SCHANDA,E. and HOFER,R. Proc. Ninth Int. Symp. on Remote Sensing of environment. Ann Arbor, Michigan (1974). 10. MKTZLER,C. Diploma-Thesis, University of Berne (1970).