Two-dimensional thermography image retrieval from ...

Two-dimensional thermography image retrieval from zig-zag scanned data with TZ-SCAN Hiroshi Okumura*a , Ryohei Yamasakib and Kohei Araia a Department

b Graduate

of Information Science, Saga University, Honjou 1, Saga, Japan School of Information Science, Saga University, Honjou 1, Saga, Japan ABSTRACT

TZ-SCAN is a simple and low cost thermal imaging device which consists of a single point radiation thermometer on a tripod with a pan-tilt rotator, a DC motor controller board with a USB interface, and a laptop computer for rotator control, data acquisition, and data processing. TZ-SCAN acquires a series of zig-zag scanned data and stores the data as CSV file. A 2-D thermal distribution image can be retrieved by using the second quefrency peak calculated from TZ-SCAN data. An experiment is conducted to confirm the validity of the thermal retrieval algorithm. The experimental result shows efficient accuracy for 2-D thermal distribution image retrieval. Keywords: TZ-SCAN, thermography, cepstrum, second quefrency peak, simple thermal imager

1. INTRODUCTION Thermal imaging (thermography) has been widely used in environmental measurement, administration, construction, military and medical fields.1 Even in a movement body and the targets that are hard to approach, temperature can be easily measured because of remote and non-contact measurement.2 In addition, observation results obtained by thermography cameras are easy to interpret for any person because these are displayed as 2-D color / grayscale image. Most of these thermography cameras, however, are too expensive to purchase them in small facilities. In this study, we develop a simple and low cost thermal imaging device, TZ-SCAN (Thermal imager by Zigzag SCANning). The system consists of a single point radiation thermometer on a tripod with a pan-tilt rotator, a DC motor controller board with a USB interface, and a laptop computer for rotator control, data acquisition, and data processing. TZ-SCAN acquires a series of zig-zag scanned data and stores the data as CSV file. Characteristics of TZ-SCAN in comparison with a commercial thermography camera is shown in Table1. A 2-D thermal image retrieval algorithm from a series of zig-zag scanned data acquired with TZ-SCAN is also developed. The algorithm is based on the second quefrency peak calculated from TZ-SCAN data. In this paper, The details of structure of TZ-SCAN system and the image retrieval algorithm are described. An experiment result which is conducted to confirm the validity of the thermal retrieval algorithm in comparison with a commercial thermography camera is also shown.

Table 1 Characteristics of TZ-SCAN in comparison with a commercial thermography camera Commercial thermography camera TZ-SCAN

Spatial Resolution High Relatively high

Real-time Observation Available Not available

Cost Too expensive Not so expensive

2. STRUCTURE OF TZ-SCAN SYSTEM TZ-SCAN is a simple and low cost thermal imaging device which consists of three subsystems for temperature measurement, scanning and data acquisition / processing / displaying. The details of each part are described in the following sections. * [email protected]; phone 81 952 28 8568; fax 81 952 28 8650 Image and Signal Processing for Remote Sensing XIV, edited by Lorenzo Bruzzone, Claudia Notarnicola, Francesco Posa, Proc. of SPIE Vol. 7109 710914 · © 2008 SPIE · CCC code: 0277-786X/08/$18 · doi: 10.1117/12.799928 Proc. of SPIE Vol. 7109 710914-1 2008 SPIE Digital Library -- Subscriber Archive Copy

2.1. Temperature measurement subsystem In this subsystem, a single point measurement radiation thermometer PT-U80 of OPTEX Corp. is employed for temperature measurement sensor. The advantages of PT-U80 are as follows: 1) Small and lightweight 2) Measurement range: −30.0 ∼ +600.0[˚C] 3) Measurement field of view: 30 × 30 mm from a distance of 1 m 4) Coaxial laser marker 3) Connection to a PC via USB for control and data acquisition. The appearance and the measurement field of view of PT-U80 are shown in Fig.1 and Fig.2.

—— —— Fig.1 Appearance of radiation thermometer PT-U80

Iota Eftil

Q!,!!

Fig.2 Measurement field of view of PT-U80

Proc. of SPIE Vol. 7109 710914-2

2.2. Scanning subsystem This subsystem consists of a tripod with remote-controllable pan-tilt rotator and a DC motor controller board with a USB interface. 504QF-II of SLIK Corp. is employed as the tripod of TZ-SCAN because of its light weight and toughness. A motor-drive alt-azimuth mount of Mizar Corp. is employed as the rotator of TZ-SCAN. It has two rotating speed mode. The rotating speeds in low mode and high mode are 0.5[deg/sec] and 1,0[deg/sec], respectively. The rotating angles in pan and tilt are 360[deg] and −40 ∼ +90 [deg], respectively. In addition, YNDC and YN-USB of Sun-Mitec Corp. are employed as the DC motor controller board and the USB interface board, respectively. The appearance of these boards are shown in Fig.3 and Fig.4.

I

ur4!J

;1

Fig.3 Appearance of DC motor controller board YNDC

Fig.4 Appearance of USB interface board YN-USB

2.3. Data acquisition / processing / displaying subsystem This subsystem consists of a laptop PC with one or more USB ports of version 1.1 or later for the connection to the scanning subsystem. An original control software for zig-zag scanning and an original 2-D thermal image retrieval software are written in C and C++ language. The schematic diagram and the appearance of the total system of TZ-SCAN are shown in Fig.5 and Fig.6, respectively.

Thermometer Tilt

PC

USB Interface USB Board

DC motor Controller Board

Pan

Alt-Azimuth

Rotator Tripod

Fig.5 Schematic diagram the total system of TZ-SCAN


Fig.6 Appearance of the total system of TZ-SCAN

3. 2-D THERMAL IMAGE RETRIEVAL ALGORITHM TZ-SCAN acquires a series of zig-zag scanned data and stores the data as CSV file shown in Fig.7. The first line (header) of this CSV file shows the measurement day and the measurement start time. After the second line, each line shows temperature data record. Each data record has three fields. The first field shows the progress time from the measurement start. The second one shows obtained temperature (˚C). The third one shows the outside temperature around PT-U80 at the time of the measurement.

!Y*t

a

— an

nc1 e

O•SLO• •

•U•1

L

si

o U.

Fig.7 An example of CSV file acquired with TZ-SCAN


A 2-D thermal image can be retrieved from a series of zig-zag scanned data with the following algorithm. 1) Calculation of cepstrum from zig-zag scanned data (CSV file) with TZ-SCAN 2) Detection of the second quefrency peak 3) Estimation of the data fold position (= number of samples per line) 4) Conversion to raster scanning data by reversing the homeward line data 5) Matching between adjacent lines based on cross correlation coefficient 6) Generation of 2-D thermal image (Raw, BMP and GIF format available) by using appropriate look-up table The Fourier transform of a time-domain function f (t) and the inverse Fourier transform of a frequency-domain function F (f ) denote F [f (t)] and F −1 [F (f )], respectively. A cepstrum3 is the result of taking the Fourier transform of the decibel spectrum as if it were a signal. Its name was derived by reversing the first four letters of ”spectrum”. There is a complex cepstrum and a real cepstrum. In this method, a real cepstrum is employed because it is not necessary to apply phase unwrapping. The real cepstrum is calculated as follows. C(q) = F −1 [ln |F [f (t)] |] , where C(q) and q denote cepstrum and quefrency, respectively. The word “quefrency” is the anagram of “frequency”. The quefrency is a measure of time. The quefrency position of the second peak of cepstrum (called the second quefrency peak) shows high autocorrelation position in time domain. Fig.9 shows the cepstrum distribution calculated from zig-zag scanned data obtained from an image shown in Fig.8.

Fig.8 Sample image (64×64[pixel])


3.6

3.4

3.2

3

c.

,_ 64

2

3.2

Fig.9 Cepstrum distribution calculated from zig-zag scanned data

4. NUMERICAL SIMULATION To evaluate the performance of TZ-SCAN, An experiments using actual target is examined. Fig.10 shows the observation target which consists of a coffee pot heater as a high temperature target and a metal stand as a relative low temperature target.

Fig.10 Sample Target (Coffee pot heater with metal stand)

Fig.11 shows an example of commercial thermography camera which is used for confirmation of the performance


of TZ-SCAN. This thermography camera is named Mob IR M3 of Wuhan Guide Infrared Technology. This camera is not so expensive and has relatively high performance in comparison with other cameras. Observation result with Mob IR M3 is shown in Fig.12.

Fig.11 An example of commercial thermography camera Mob IR M3 of Wuhan Guide Infrared Technology.

Fig.12 Observation Result with Mob IR M3.

Observation result with TZ-SCAN is shown in Fig.13. As shown in this result, generated 2-D thermal image is almost same as the one shown in Fig.12. The result is reasonable because a price of TZ-SCAN is about one-tenth of Mob IR M3.

Fig.13 Observation Result with TZ-SCAN


5. CONCLUSIONS A simple and low cost thermal imaging device, TZ-SCAN is developed. The system consists of a single point radiation thermometer on a tripod with a pan-tilt rotator, a DC motor controller board with a USB interface, and a laptop computer for rotator control, data acquisition, and data processing. TZ-SCAN can acquire a series of zig-zag scanned data and can store the data as CSV file. A 2-D thermal image retrieval algorithm from a series of zig-zag scanned data acquired with TZ-SCAN is also developed. The algorithm is based on the second quefrency peak calculated from TZ-SCAN data. To evaluate the performance of TZ-SCAN, an experiment is examined. The experimental result shows that the generated 2-D thermal image using TZ-SCAN data is almost same as the one using much expensive commercial thermography camera. Future work is improvement of the system and 2-D thermal image retrieval algorithm.

REFERENCES 1. G.C. Holst, “Common Sense Approach to Thermal Imaging”, SPIE Press Book, 2000. 2. P.A. Jacobs, “Thermal Infrared Characterization of Ground Targets and Backgrounds, Second Edition”, SPIE Press Book, 2006. 3. D.G. Childers et al., “The Cepstrum: A Guide to Processing”, Proceedings of the IEEE, Vol. 65, No. 10, pp. 1428- 1443, 1977.
