UV and visible (Vis) light sources for spectrophotometry come in a range of different varieties that each have their own advantages and disadvantages. Important aspects to consider when choosing a light source for your spectrophotometer are wavelength range, emission intensity, stability, service life and cost. You may need more than one light source to cover the full range of spectra required for your experiments. Read the following guide to gain a better understanding of the different common light sources, their properties and their applications.
Halogen Lamps
Halogen lamps, also called tungsten-halogen lamps, contain a tungsten filament and a halogen gas along with an inert gas. The lamp works as an incandescent lamp, with the filament producing light as it is heated up, but the tungsten and halogen react to produce a halogen cycle that causes the tungsten to return to the filament as it evaporates. This gives the lamp a longer lifetime than typical incandescent bulbs and allows it to maintain its brightness over time.1
Halogen lamps can produce light within a wavelength range between 350 nm to 3500 nm, but are most effective within the visible and near IR ranges. Thus, halogen lamps are frequently paired with UV emitting sources such as deuterium lamps to cover a wider wavelength range. Halogen lamps are commonly used due to their relatively low cost, high stability over time and long service life, which is typically around 2,000 hours.
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Deuterium Lamps
Deuterium lamps, or deuterium arc lamps, produce light through the excitation of deuterium within the bulb. The deuterium is excited by an arc that forms between a tungsten filament and anode, and the isotope emits radiation as it transitions back to its original state. The filament will require about 10-20 seconds of preheating before starting the arc discharge.
Deuterium lamps can continuously and stably emit short wavelengths between about 190 and 370 nm, making them a good partner to halogen lamps for covering the UV-visible spectrum.2 These lamps will require a large power supply and are typically more expensive than halogen lamps. A typical lifespan for this type of lamp is about 1,000 hours, although some designs offer an extended lifetime.
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Xenon Lamps
Xenon arc lamps produce a bright light that is similar to natural sunlight, through ionization of xenon gas atoms under high pressure. Ionization is triggered by an electrical charge between thoriated tungsten electrodes. These lamps can produce a continuous spectrum of light between 190 and 1100 nm at a much higher intensity than the halogen lamp. However, they are also more expensive and exhibit more fluctuations in output than their halogen and deuterium counterparts. These lamps are typically used in applications where a high light intensity is needed, and can vary in lifespan all the way from 200-400 hours to 2000-3000 hours depending on factors such as temperature management and cathode design.3,4
Xenon flash lamps are a more compact version of the xenon lamp that generate less heat by emitting light in short pulses rather than continuously. These lamps do not require preheating and have increased efficiency and an extended lifetime. However, xenon flash lamps typically have greater fluctuations and less stability than xenon arc lamps, and may require more downstream data processing.
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Mercury Lamps
Mercury lamps produce light through the ionization of vaporized mercury between electrodes. Mercury lamps provide high intensity in the far UV and visible ranges at distinct emission lines in the UV, blue, green and yellow spectral regions that can be up to 100 times brighter than the rest of the lamp’s continuous emissions.3 Therefore, these lamps are most useful for calibration purposes. Emission lines at 254 nm, 365 nm, 436 nm and 546 nm can be used to calibrate displayed wavelength values.1
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LEDs
Light-emitting diodes (LEDs)consist of a semiconductor that releases light when an electric current passes through it. This is due to electrons releasing energy in the form of photons as they recombine with electron holes inside the semiconductor. Each LED can emit only a narrow band of wavelengths within the spectral region between about 375 and 1000 nm.5 Therefore, they can be used in certain applications where only a specific wavelength is needed, and usually don’t require a monochromator. They are incredibly inexpensive and have a very long lifetime.
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Knowing the types of light sources available for your UV-Vis spectrophotometer will give you an idea of the wavelength range and intensity you can expect, but exact specifications may vary from product to product, so it’s a good idea to compare numbers before making your final selection. Other specifications to look for are guaranteed and maximum lifetime for your light source, power supply requirements and preheating requirements. Remember that you may need to switch light sources to achieve the full wavelength range needed for your experiments, such as when using a deuterium lamp in conjunction with a halogen lamp.
References
1. "Light Sources for Spectrophotometers," Shimadzu. https://www.shimadzu.com/an/service-support/technical-support/analysis-basics/fundamentals-uv/lightsources.html
2. "Glossary of UV Vis Spectrophotometry," biochrom. http://www.biochrom.co.uk/content/1/91/glossary-of-uv-vis-spectrophotometry.html
3. "Fundamentals of Illumination Sources for Optical Microscopy," Carl Zeiss Microscopy Online Campus. http://zeiss-campus.magnet.fsu.edu/articles/lightsources/lightsourcefundamentals.html
4. "A Guide to Selecting Lamps," Photonics Media. https://www.photonics.com/Articles/A_Guide_to_Selecting_Lamps/a44487
5. "Inside the UV/Vis Spectrophotometer," Chromedia Analytical Sciences. https://www.chromedia.org/chromedia?waxtrapp=fotjtbEsHiemBpdmBlIEcCDrB&subNav=aqljabEsHiemBpdmBlIEcCDrBJ