Sources of Light and
3.1. Thermal Radiation Sources for Visible and IR
Thermal radiation sources, like ﬁlament lamps or Nernst rods, have been exploited
for many years and are still in use in many optical systems, primarily in those
intended for imaging. The modern quartz tungsten halogen (QTH) lamps and IR
emitters are just technologically improved versions of the older sources.
The operation of these sources is based on thermal radiation laws described in
detail in Chapter 6. We will address here some speciﬁc features of thermal sources
that affect their use in practice.
A QTH lamp has an electrically heated tungsten ﬁlament positioned inside a
transparent bubble made of fused silica and ﬁlled with halogen gas. This gas causes
a chemical reaction between the tungsten atoms evaporated from the ﬁlament and
deposited on the bubble wall and the halogen molecules improving in such a
manner both the lifetime of the lamp and the transparency of the QTH envelope.
A QTH lamp is a source of broadband radiation: actually the tungsten itself
emits at all wavelengths, but the transparency of the envelope limits the useful
emission to visible and near-IR wavelengths (up to about 2.5 μm). Actually some
UV radiation is also available, in the wavelength interval from 240 to 400 nm,
although this is of low intensity.
Usually the “optical strength” of the light source is characterized by its irradi-
, measured as the radiation ﬂux, per 1 nm wavelength band, incident on an
area of 1 m
at a distance of 0.5 m from the source. The second important feature of
96 3 ♦ Sources of Light and Illumination Systems
Figure 3.1 Spectral irradiance of QTH lamps (The Book of Photon Tools, Oriel
Instruments (2003), with permission of Spectra Physics Ltd).
the lamp is its color temperature, T
(see deﬁnition in Section 6.2). There exist QTH
lamps with color temperatures of 2,850 K and of 3,200 K (even up to 3,400 K).
Although the emissivity of tungsten is strongly selective, it increases rapidly in
the visible where it achieves a value of 0.8, providing continuous radiation which
is close to that of a black body. A typical graph of irradiance of QTH lamps of 100
and 250 W is shown in Fig. 3.1. The radiated spot in a QTH lamp usually has the
shape of a rectangular (lower power) or a coiled ﬁlament (larger lamps) of several
millimeters in size.
The lifetime of the lamps varies from 50 to 1,000 hours and it is evidently a
critical parameter. One can improve it signiﬁcantly by reducing the voltage (which
is accompanied by decrease of the temperature). A voltage reduction of 6% might
result in a doubling of the lifetime. However, a reduction of more than 10%
becomes problematic as it could spoil the halogen cycle inside the lamp bubble.
Much more intense radiation than that of QTH lamps is produced by arc lamps
where an electrical discharge arc is created in surrounding xenon, mercury, deu-
terium, or other inert gas. The color temperature of such lamps can be 4,000 K
and even as much as 6,000 K (xenon lamp). Another important feature is a great
number of spectral lines in the UV. With a deuterium arc lamp wavelengths as
short as 160 nm can be obtained. The brightest portion of the arc is usually of
several millimeters in size, but its location might be unstable.
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