Incoherent Light Source Physics

Incoherent Light Source Physics

Incoherent Light Source Physics

The incoherent light sources discussed here share several characteristics that distinguish them from lasers. Incoherent light gives rise to radiation, which is emitted from the source in all directions. Furthermore, unlike laser gain media where the radiating species are generated through optical or electrical excitation, the most common excitation mechanism for these sources is thermal excitation. This gives rise to spectrally broadband emission, which depends on the temperature of the source medium as described below. The broadband nature of these sources, coupled with its omnidirectional emission, makes them ideal for lighting in homes, workplaces, and vehicles. In research applications, the broadband output can be exploited for simulating solar radiation or can be spectrally filtered for applications such as spectroscopy or microscopy. These incoherent light sources are mainly categorized by the wavelength range and spectral shape of their output. These sources, which are detailed below, consist of deuterium light sources, arc lamp sources, quartz-tungsten halogen (QTH) sources, and IR emitters. LEDs are also incoherent light sources but have been described in detail earlier and will not be covered here. One key point is that LEDs have a narrower emission spectrum (see Semiconductor Light Characteristics) than the incoherent light sources discussed here and so broadband emission is typically achieved by using multiple LEDs with different center wavelengths.

Black Body Radiation

When charged particles in matter are heated, they gain kinetic energy and the resulting movement of these charged particles gives rise to electromagnetic radiation in the form of thermal energy. Thus, any material with a temperature above absolute zero emits thermal radiation. If the material system is in thermal equilibrium with its surroundings and is a perfect emitter, it is called a black body radiator. While most material systems are not true black bodies, they are often approximated as such since the laws governing the emission from a black body are simple and quantitative. Planck’s Law describes the spectral distribution of radiant energy inside a black body. Spectra generated according to this law are typically given in units of spectral radiant exitance or spectral irradiance. These spectra are smoothly varying curves with their distribution and output directly related to the temperature of the black body (see Figure 1). The inverse relationship between peak wavelength and temperature, known as Wien’s Law, is also shown in Figure 1. Sources like the sun and the material systems making up the incoherent sources described below all have black body-like emission spectra. The temperature of the sun’s surface is close to 6000 K and, as shown in Figure 1, this gives rise to a peak solar emission around 0.5 µm, which corresponds to green light. Even objects at room temperature emit thermal radiation but their peak emission wavelength is around 10 µm. Since this gives rise to no VIS radiation, this became the genesis of the term “black body”.
Spectral irradiance for various black bodies. Wien’s Law is shown, which relates the peak wavelength to the black body temperature
Figure 1. Spectral irradiance for various black bodies. Wien’s Law is shown, which relates the peak wavelength (λm) to the black body temperature (T).

Types of Sources

Arc lamps operate by passing electricity through a discharge tube containing a high pressure gas. The electricity ionizes the gas and creates an arc that emits high-intensity light. These gases typically consist of either xenon or mercury-xenon mixtures (see Figure 2). A xenon arc lamp produces a black body-like emission spectrum corresponding to 6200 K which is a bright white light. The general characteristics of arc lamps are high irradiance output with a small source arc, intense UV output, and a spectrum that closely mimics natural sunlight. These sources are therefore used as solar simulators and for lamps in movie projectors or searchlights. Arc lamps can also be made to emit extremely intense pulses of light instead of the typical CW emission for incoherent sources. These pulsed sources are often referred to as flashlamps and can be used for optically-pumping solid-state laser media. Finally, arc lamps generate strong, sharp emission peaks in addition to their black body-like emission (see Figure 2). These peaks are the result of spontaneous emission from atomic level transitions in the gases. The resulting narrow and well-defined emission lines are therefore ideal for use as spectral calibration sources.
Spectral irradiance for different lamp types and for typical infrared (IR) emitters.
Figure 2. Spectral irradiance for different lamp types (top) and for typical infrared (IR) emitters (bottom).

A deuterium lamp is a type of arc lamp in which molecular deuterium is excited to a higher energy state prior to radiatively decaying to the ground state. Consequently, the deuterium lamp is one of the few incoherent sources whose radiation is due to spontaneous emission as opposed to thermal radiation. Its emission spectrum is not black body-like but rather continuous and centered in the UV. Deuterium lamps possess the shortest output wavelength of all lamps, generate negligible output in the VIS and NIR spectral regions, and are the preferred sources for UV spectroscopy since they are both stable and have long lifetimes.

QTH lamps are a variation of the traditional incandescent lamps in which a filament of tungsten is heated to produce thermal radiation. The presence of halogen creates a regenerative cycle with the tungsten to enhance the overall lifetime and prevent the process of blackening. Since this process operates at a higher temperature than a conventional incandescent lamp, it must be housed in a bulb made of quartz since it has a high melting point. A QTH light source produces a smooth and continuous spectrum from the near UV to well into the NIR (see Figure 2). These sources are extremely stable, possess high total visible output, and are easy and inexpensive to operate. For these reasons, they are ideal as calibration sources when a known spectral irradiance is needed. Furthermore, if a QTH source is coupled to a monochromator, the system can also be used as a spectral calibration source.

Infrared (IR) emitters are useful light sources for IR spectroscopy. IR emitters provide the necessary intensity at IR wavelengths that are not emitted by arc and QTH lamps, and IR emitters are more economical and have longer lifetimes. These sources function as almost perfect black bodies and generate broadband IR light from 1 µm – 25 µm (see Figure 2) with very efficient emission.

Lambert’s cosine law indicating how intensity depends on angle of observation from the normal.
Figure 3. Lambert’s cosine law indicating how intensity (I) depends on angle of observation (θ) from the normal (left). Orientation of a QTH lamp for maximum intensity, which is along the axis normal to the filament plane (right).

The spatial emission properties of an incoherent source depend on the shape of its lamp. In a QTH lamp, the filament is effectively a planar surface, so its emission resembles that of an LED as described in Laser Diode and LED Light Characteristics. Its emission is Lambertian, that is, its intensity decreases with the cosine of the angle from the normal leading to a reduction as one goes off axis (see Figure 3). This must be considered when orienting a lamp with respect to its target since one typically wants to maximize the irradiance (see Figure 3). Conversely, the arc in an arc lamp is typically small enough that it resembles a point source. Consequently, its emission is isotropic, i.e., no dependence on the observation angle. This also has significant advantages in terms of collecting and collimating the light when using a lens system.