Physics of Thermography - Infrared radiation

1. Physics of Thermography
Infrared radiation

2. The infrared ray is a kind of electromagnetic wave with a frequency higher than the radio frequencies
and lower than visible light frequencies. The Infrared region of the electromagnetic spectrum
is usually taken as 0.77 and 100 mm. For convenience, it is often split into near infrared (0.77 to 1.5
mm), middle infrared (1.5 to 6 mm) and far infrared (6 to 40 mm) and far far infrared (40 to 100 mm).
Infrared rays are radiated spontaneously by all objects having a temperature above absolute
zero. The total energy ‘W’ emitted by the object and its temperature are related by the Stefan
Boltzman formula,
W = s OET4 (i)
where W = radiant flux density and is expressed in W/cm2
OE = Emissivity factor
s = Stefan-Boltzman constant = 5.67 10–12 W/(cm2 × K4)
T = Absolute temperature

3. Equation (i) shows that the amount of infrared energy emitted varies with temperature of the
object. Figure 24.3 shows the spectral distribution of intensity of radiation from black bodies. The
wavelength of the energy peak and the absolute temperature is given by the Wien formula,
lmax =
2897( m)
(K)
m
T
The human body has a temperature of 37°C (310 K), therefore,
lmax(human body) a 10 mm
From equation (i), Fig. 22.2, the energy density of the infrared radiation of the human body = 4.6
10–2 W/cm2. Assuming the surface area of the human body to be 1.5–2.0 m2, the amount of
infrared energy radiating from the whole body is approximately 700–1000 W.