Practical Optics

Chapter 3

Sources of Light and

Illumination Systems

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-

ance, E

λ

, measured as the radiation flux, per 1 nm wavelength band, incident on an

area of 1 m

2

at a distance of 0.5 m from the source. The second important feature of

95

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

c

(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.