handbook of modern sensors

3.13 Light 111
[38] whose emissivity is over 0.999. A cavity body is fabricated of solid copper
with a cavity of any shape; an inversed cone is preferable. An imbedded temperature
sensor and a thermoelectric heater/cooler with a control circuit (not shown) form a
thermostat that maintains the temperature of the cavity on a preset level. That may be
above or below the ambient temperature. The inner portion of the cavity should be
painted with organic paint. The visible color of the paint is not important, because in
the infrared spectral range, there is no correlation with reflectivity in the visible range
(which determines visible color). The most troublesome portion of a cavity is located
near the aperture, because it is very difficult to ensure that the temperature of the left
side of the blackbody (as in Fig. 3.46B) is independent of ambient and equal to the
rest of the cavity walls. To minimize the effects of ambient temperature and increase
the virtual cavity size, the inner surface of the front wall around the cavity is highly
polished and gold plated. Thus, the front side of the cavity has very low emissivity
and, thus, its temperature is not that critical. In addition, the gold surface reflects
rays emitted by the right-side parts of the cavity walls that have high emissivity and
thus enhances the cavity effect. The entire copper body is covered with a thermally
insulating layer. It should be noted that the blackbody surface is the virtual surface
of the aperture, which, in reality, is a void.
3.13 Light
Light is a very efficient form of energy for sensing a great variety of stimuli. Among
many others, these include distance, motion, temperature, and chemical composition.
Light has an electromagnetic nature. It may be considered a propagation of either
quanta of energy or electromagnetic waves. Different portions of the wave-frequency
spectrum are given special names: ultraviolet (UV), visible, near-, mid-, and farinfrared
(IR), microwaves, radiowaves, and so forth. The name “light” was arbitrarily
given to electromagnetic radiation which occupies wavelengths from approximately
0.1 to 100 µm. Light below the shortest wavelength that we can see (violet) is called
ultraviolet, and higher than the longest that we can see (red) is called infrared. The
infrared range is arbitrarily subdivided into three regions: near-infrared (from about
0.9 to 1.5 mµ), mid-infrared (1.5 to 4 µm), and far-infrared (4 to 100 µm).
Different portions of the radiation spectrum are studied by separate branches of
physics. An entire electromagnetic spectrum is represented in Fig. 3.41. It spreads
from γ -rays (the shortest) to radiowaves (the longest). In this section, we will briefly
review those properties of light which are mostly concerned with the visible and
near-infrared portions of the electromagnetic spectrum. Thermal radiation (mid- and
far-infrared regions) are covered in Section 3.12.
The velocity of light c 0 in vacuum is independent of wavelengths and can be
expressed as µ 0 = 4π × 10 −7 henrys/m and ε 0 = 8.854 × 10 −12 farads/m , which are
the magnetic and electric permitivities of free space:
c 0 = 1 √
µ0 ε 0
= 299,792,458.7 ± 1.1 m s . (3.145)