The incoherent light sources discussed in this article have some properties that distinguish them from lasers. Incoherent light produces radiation emitted from a light source in all directions. In addition, unlike the laser gain medium, which produces radiant species by photoexcitation or electrical excitation, the most commonly used excitation mechanism for these light sources is thermal excitation. This results in broadband spectral radiation, which depends on the temperature of the light source medium as described below. The broadband nature of these light sources, combined with their omnidirectional emission, makes them ideal for home, workplace, and vehicle lighting. In research applications, broadband output can be utilized to simulate solar radiation, or spectral filtering can be performed for applications such as spectroscopy or microscopy. Incoherent light sources are classified primarily by the wavelength range and spectral shape of their output. The following is a detailed description of these light sources, including xenon light sources, arc light sources, quartz tungsten halogen (QTH) light sources and infrared emitters. LEDs are also incoherent light sources, and the emission spectrum of LEDs is narrower than the incoherent light sources discussed here, so it is critical that multiple LEDs with different center wavelengths are often used to achieve broadband emission.
The charged particles in the substance gain kinetic energy when they are heated, and the resulting motion of the charged particles causes electromagnetic radiation in the form of thermal energy. As a result, any material with a temperature above absolute zero emits thermal radiation. If the material system is in thermal equilibrium with the surrounding environment and is an ideal emitter, then it is called a blackbody emitter. Although most material systems are not true blackbodies, such approximations can often be made because the laws that control blackbody emission are simple and quantitative. Planck's law describes the spectral distribution of radiant energy inside a black body. The spectra produced according to this law are usually characterized by spectral radiance or spectral irradiance. These spectra are smooth-varying curves, and their distribution and output are directly related to the temperature of the blackbody (see Figure 1). The inverse relationship between peak wavelength and temperature, known as Wien's law, is shown in Figure 1. The light source composed of the sun and the material systems that make up the incoherent light sources described below have an emission spectrum similar to that of a black body. The surface temperature of the sun is close to 6000 K, as shown in Figure 1, which produces 0Peak solar radiation of around 5 m, corresponding to green light. Even objects at room temperature emit thermal radiation, but their peak emission wavelength is around 10 m. Since no visible light radiation is produced, this became the origin of the term "black body".
Figure 1.Spectral irradiance of various blackbodies. Wien's law is shown in the diagram and correlates the peak wavelength (m) with the blackbody temperature (t).
The working mechanism of an arc lamp is to pass an electric current through a discharge tube containing a high-pressure gas. An electric current ionizes the gas and creates an electric arc that emits high-intensity light. The gas is usually xenon or a mercury-xenon mixture (see Figure 2). Xenon arc lamps produce a black-like radiation spectrum corresponding to 6200 K, which is bright white light. General characteristics of xenon arc lamps include high irradiance output, small arc of the light source, high intensity of ultraviolet output, and a spectrum that is highly similar to natural light. This type of light source is therefore used as a solar simulator and can also be used in movie projectors or searchlights. Arc lamps can also emit extremely strong pulses of light, which differ from the continuous emission typical of incoherent light sources. This type of pulsed light source is often referred to as a flash lamp and can be used to optically pump a solid-state laser medium. Finally, in addition to radiation similar to that of a blackbody, arc lamps produce intense and sharp emission peaks (see Figure 2). These emission peaks are caused by spontaneous emission from atomic level transitions in the gas (see Figure 3). The resulting specific emission lines are ideal for use as a spectral calibration source.
Figure 2.Spectral irradiance of different lamp types (top) and typical infrared (IR) emitters (bottom).
A deuterium lamp is a type of arc lamp in which the molecular deuterium is excited to a higher energy state before radiation decays to the ground state. Therefore, deuterium lamps are spontaneous emissions in a small number of incoherent light sources in which the radiation process is the opposite of thermal radiation. The deuterium lamp emission spectrum is not a black-like spectrum, but a continuum centered on ultraviolet light. Deuterium lamps have the shortest output wavelength of all lamps, and the output in the visible and near-infrared spectral regions is negligible. Deuterium lamps are stable and long-lasting, making them the preferred light source for ultraviolet spectroscopy.
QTH lamps are a variant of traditional incandescent lamps in which tungsten filaments are heated to produce thermal radiation. The presence of halogens causes the tungsten to undergo a regenerative cycle, which increases the overall life and prevents the blackening process. Since this process operates at a higher temperature than conventional incandescent lamps, it must be confined to bulbs made of quartz, which has a high melting point. The QTH light source produces a smooth continuous spectrum from the near-ultraviolet to just into the near-infrared. The light source is extremely stable, has a high total visible light output, and is easy and inexpensive to operate. For these reasons, QTH light sources are ideal for calibrated light sources when a known spectral irradiance is required. In addition, the system can also be used as a spectral calibration source if the QTH light source is coupled to a monochromator.
Infrared (IR) emitters are a useful light source for infrared spectroscopy. Infrared emitters emit infrared radiation of a certain intensity, while arc lamps and QTH lamps do not emit infrared wavelengths. Infrared emitters are more economical and have a longer lifespan. The infrared emitter has the function of almost perfect blackbody, and can produce broadband infrared light from 1 to 25 m (see Figure 2), and the emission efficiency is very high.
Figure 3.Lambert's cosine law shows how the intensity (i) varies with the observed angle ( ) of the normal (left). The direction in which the intensity of the QTH lamp is maximum, along the axis perpendicular to the filament plane (right).
The spatial radiation characteristics of an incoherent light source depend on the shape of the lamp. In a QTH lamp, the filament is actually a flat surface, so the emission is similar to that of an LED. Its emission follows Lambert's law, which means that the intensity decreases with the cosine value of the angle to the normal, and therefore decreases as the light deviates from the axis (see Figure 3). Radiation characteristics must be taken into account when determining the orientation of the lamp relative to the target, as the highest irradiance is often desired (see Figure 3). Conversely, the arc in an arc lamp is usually small enough to be considered a point light source. Its radiation is isotropic and independent of azimuth. When using a lens system, there are also significant advantages in collecting, collimating light.