298
Weathering Degree Survey of MAGAI-BITSU
by Infra-red Thermograph
Toshihiko SAKAYAMA (OYO Corporation)*
Masaaki SAWADA (Nara National Cultural Properties Institute)**
Yoshito MIYATSUKA (Shin Air Survey Co.,Ltd.)***
*** 988, Bessho, Hachioji City, Tokyo, 192-03, JAPAN
phone; +81-426-77-5480
fax; +81-426-77-5030
Key Word; Archaeology, Infra-red, Thermograpy
1,Introduction
The stone Buddhist image of ONO DERA temple is a great MAGAI-BUTSU (cliff image) engraved on the stonewall,
measuring 14m in height 4m in width, and is said to have been made A.D.1207 at early KAMAKURA Era.
For nearly 800 years, weathering and deterioration have progressed over the rock surface, while physical and chemical
factors are interrelated in a complicated fashion. The surface of the rock has shown phenomena of becoming like an
eardrum, floating up and peeling-off from the surface. In order to consider the countermeasures of preserving and
repairing of the stone image, it is necessary to grasp scientifically the current weathered condition and to clarify
progressing mechanisms of weathering and deterioration. At this time, a survey of weathering degree by remote-sensing
using an Infra-red thermograph has been carried out in order to find out the expansion of the portion with deterioration
such as the rock surface becoming like an eardrum and floating up.
2,Measuring Principles of Infra-red Thermograph
Infra-red thermograph measures the temperature of the object by measuring heat radiation, which is constantly limited
from all objects. Thermal radiation is considered to have been generated by rotation or vibration of molecules or by
vibration of the solid body lattice. The vibration spectrum appears in the extreme-infra-red ray area. These infra-red rays
are electromagnetic waves longer that visible rays and shorter than the microwaves, and their wavelenght ranges from
0.75mm to 1mm.
Theoretical research on the relation between temperature and thermal radiation was completed by Planck as the theory
of blackbody radiation in the early years of the 20th century, and the theoretical foundation of measuring thermal
radiation was established. When the object is a blackbody, the relation between its radiation strength and the temperture
is given by Planck's equation of radiation. The infra-red thermograph detected rays irradiated from the object body, using
at detector, which has sensitivity in the limited wavelegth area, and directly indicatest the detected temperature
obtained by nonlinear conversion on the basis of Plank's equation of radiation. The infra-red thermograph used for this
measurement is the TH1101 (Thermo-tracer) made by Nippon Electric SAN-EI. The infra-red thermograph carries out
optically, in the horizontal and vertical directions, the infra-red radiation energy, which is naturally irradiated from the
surface of an object, and indicates the temperature distribution of the measured object as a color static picture. The
infra-red detector, which converts incomming infra-red rays into electric signals, is HgCdTe (Mercury Cadmium
Tellurium), which has a wavelength of 8 to 13um.
3,Measured Results by the Infra-red Thermograph
Measurements by the infra-red thermograph were carried out in winter, during mid-December. The detecting part of the
infra-red thermograph was installed in the precincts of ONO DERA temple, which commanded the entire view of the stone
Buddha statue, and the temperature distribution of the stone Buddha statue was continuously measured at 30-minute
intervals from 9:30 to 18:00. At this time, the distance between the stone Buddha statue and the detector part of the
infra-red thermograph was about 50m. Fig. 1 shows the temperature distribution on the surface of the Buddha statue at
14:00, and Fig.2 shows the distribution of the temperature change during the period from 14:00 to 17:59.
The external air temperature measuring day were 2.5'C at 9:30, but in the daytime, the temperature rose until it reached
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995
