Light Sources for Spectroscopy

Most optical spectroscopy techniques will require a calibrated and reliable light source in order to conduct trustworthy experiments.  Most spectroscopy equipment can use a range of different light sources.

Spectroscopy light sources produce monochromatic or broadband emission. Generally, monochromatic sources are used to excite materials (for example, in fluorescence measurements) whereas broadband sources can be used for simple absorption, transmission and reflection measurements. 

Browse Light Sources

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Monochromatic Light Sources

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Tunable Light Sources

There are two ways of creating monochromatic light:

• You can use a light source which emits over a narrow wavelength range such as lasers or specific LED light sources.
• Alternatively, you can combine a broadband light source with a monochromator to select individual wavelengths.

Using a tuneable light source gives you flexibility with your measurements, which is especially useful in a lab where many researchers work on different materials requiring varied experimental conditions. Tunable light sources also allow you to sequentially scan through a spectrum, nanometer by nanometer. By building an absorbance or fluorescence spectrum this way, you can get more accurate spectrum with higher resolution.

However, one issue with using a tunable light source is that the power is distributed over the entire wavelength range.  As a result, the intensity at any given wavelength is lower than from a monochromatic light source (such as a laser or single-wavelength LED) operating at the same total power. This reduced intensity at specific wavelengths can become the limiting factor in the sensitivity of your measurements, particularly when detecting weak signals or small changes in absorption or emission.

Light Emitting Diodes (LEDs)

LED light sources are a popular monochromatic light source due to their narrow emission, low power consumption, high stability, long lifetime and fast switching. Mostly, these are made from inorganic semiconductors, such as gallium arsenide (GaAs) and gallium arsenide phosphide (GaAsP). However, organic LEDs (OLEDs) have now become popular due to the broad range of colors which they are able to produce; by adding different functional groups to organic molecules, it is possible to alter their emission wavelength, making it relatively easy to fabricate LEDs of any color.

Lasers

Lasers produce monochromatic, coherent, collimated light through the process of stimulated emission (hence LASER, or “Light Amplification by Stimulated Emission of Radiation”).

There are two types of lasers, continuous wave lasers and pulsed lasers. Continuous wave (CW) lasers produce a constant beam of photons with no fluctuation in power over time. Diode CW lasers are similar in design to LEDs, and are often used for measurements where very high powers are not necessary, such as fluorescence measurements. Compared to continuous wave lasers, diode lasers are much more affordable.

Pulsed lasers are very powerful as they are able to deposit very high amounts of energy in a short space of time. The pulse length used for fast spectroscopy is usually on the order of picoseconds (10^-12 s) or femtoseconds (10^-15 s), though attosecond (10^-18 s) pulses are also possible. Pulsed lasers are often used in time-resolved measurements, such as transient absorption (pump-probe), or measurements that require very high energies - for example, as an excitation source for other lasers.

Pulsed lasers can also be used in non-linear optics to produce pulses of different wavelengths, such as in second harmonic generation (frequency doubling) or optical parametric amplification.

Broadband Light Sources

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Broadband LEDs

Although individual LEDs tend to produce light with a very narrow spectrum, LEDs can be used to create a broadband emission. Some white light LEDs are simply single LEDs with a phosphor layer. Coating LEDs with phosphors, a material that absorbs UV and blue light and re-emits in the visible, can expand emission across more of EM spectrum. Other broadband LEDs combine multiple LEDs to create a broader, more continuous spectrum. 

The power output, spectral distribution and smoothness of a broadband LED's emission varies significantly from device to device. Small LED light sources, like the Ossila USB Broadband White Light Source can produce light covering a spectral range of 360 - 900 nm, but is also USB-powered, small and can be easily connected to an optical fiber. These light sources are perfect for use with USB Spectrometers. 

 

Alternatively, the Ossila LED Light Source has a more continuous spectrum and delivers a higher overall power output. This light source is designed to be compatible with rail-based optical systems and can be used in combination with a monochromator to make a tunable light source.

Broadband LED light sources are typically more expensive than incandescent and gas discharge lamps, but their extended lifetimes mean they need to be replaced much less often, making them cheaper in the long run. They are also significantly more efficient, no energy is lost through heat and their “warm up” and “cool down” times are instantaneous. Broadband LEDs are also less fragile than other lamps and do not contain hazardous gases.

Tungsten Halogen

Tungsten halogen lamps (also referred to simply as halogen lamps or as quartz iodine lamps) are a type of incandescent lamp that emit from the UV-visible light boundary to the infra-red region. The exact spectral range depends on the temperature of the filament, but they are generally not suitable for measurements in the UV.

Tungsten halogen lamps consist of a tungsten filament inside a glass bulb. For this, quartz glass is used as it has a high melting point and is capable of withstanding high pressures without breaking. The capsule is filled with a mix of an inert gas, such as krypton or xenon, and a halogen, such as iodine or bromine.

The tungsten filament is heated by passing an electric current through it so that the filament becomes incandescent (it emits light). Most of the energy is emitted in the infrared, making tungsten halogen lamps very inefficient for day-to-day lighting but suitable for spectroscopy measurements in the IR region.

The inert gas in the glass bulb reduces the evaporation and oxidation of the tungsten filament, while the halogen helps to redeposit the tungsten particles back onto the filament through the “halogen cycle”. This increases the lifetime of the filament compared to incandescent lamps that do not contain any halogen, and reduces blackening caused by the deposition of tungsten particles on the inside of the glass.

Gas Discharge/Arc Lamps

Arc lamps are a type of gas discharge lamp which produce light by sending an electric discharge current through a plasma (an ionized gas). Generally, an electric field is applied between two electrodes inside a heat-resistant glass tube filled with the gas. The atoms become excited through ionization or through collisions with electrons or other gas atoms or ions. When the atoms or ions relax back to the ground state, a photon is emitted. The wavelength of this photon is characterized by the gas used.

Deuterium arc lamps are commonly used in UV spectroscopy as they produce a continuous spectrum from around 180 - 370 nm (though there is non-continuous emission up to 900 nm). They are almost always combined with a tungsten halogen lamp to allow measurements in the UV, visible, and NIR.

Xenon arc lamps typically produce a continuous spectrum over a wavelength range of 190 - 1100 nm. This makes them more efficient than deuterium/tungsten halogen lamps as they can cover the same spectral range with only one lamp. However, they are both more expensive and less stable.

Light from discharge gas lamps is unpolarized and incoherent. Often they take a while to reach full light output power, but despite this, they are still more efficient than incandescent lamps.

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