Multiple applications for UV LEDs
UV electromagnetic radiation is used in a variety of applications, such as: germicidal air and water purification, surface disinfection, currency validation, medical, military, industrial (photo-chemical) curing, printing, instrumentation, effect lighting and forensic analysis. The market for UV equipment of all types is conservatively estimated at over $5 billion. The predominate method used to produce UV electromagnetic radiation today is based on tube technology developed nearly 100 years ago. Though UV lamps are able to generate considerably higher power output levels than today’s existing UV LEDs there are several drawbacks of UV lamps such as:
The emerging UV LED technology has an opportunity in the coming years to provide a competitive technology in a manner similar to ongoing events in solid-state lighting using visible LEDs. UV LEDs will be an enabling technology in the future to drive new innovative applications.
Spanning the UV
The ultraviolet spectrum lies between the visible light range the human eye is able to detect and x-rays as shown in Table 1. The term Ultraviolet refers to all electromagnetic radiation with wavelengths in the range of 10 to 400 nanometers. In addition, there are several classifications inside of the UV range: UV-A (315-400 nm), UV-B (280-315 nm), UV-C (200-280 nm) and Vacuum UV (10-200) nm.
Wavelengths in the UV-A range are used for currency validation, industrial curing, phototherapy, and for forensic / analytical instruments. UV-A wavelengths from 315 to 345 nm are used for sun tanning and are a suspected cause for premature aging of human skin. UV wavelengths below 385-390nm can not be detected by the human eye; therefore it is essential to take precaution to protect your eyes and skin when working with UV light sources.
The UV-B range is more hazardous than UV-A, and it is largely responsible for sunburn. It is used in forensic and analytical instruments and for the more recent narrow band UV-B phototherapy skin treatments for Psoriasis (308-311nm). UV-B does not penetrate as deeply in the skin as UV-A, however, the deadliest types of skin cancer (malignant melanomas) start in the epidermis, an upper layer of the skin. UV-B is largely blamed for these cancers although shorter UV-A wavelengths are considered possibly cancer-causing as well.
The UV-C range refers to shorter UV wavelengths, which is sometimes referred to as the Deep UV Range. Wavelengths in this range, especially from the low 200’s to about 275 nm, are especially damaging to microorganism’s DNA. UV-C is often used for germicidal applications for water, air and surface decontaminations. The earth’s atmosphere absorbs most of the UV-C radiated by the sun.
Vacuum UV has the shortest wavelengths and highest energy level and is absorbed by the atmosphere. Strong absorption of vacuum UV in the Earth’s atmosphere is due to the presence of oxygen. Semiconductor photolithography processes seek to use shorter UV wavelengths for the next generation of smaller IC chips.
Killing germs
UV germicidal technology has been established in Europe for nearly 100 years, and the first use of UV light to disinfect drinking water occurred in 1910 in France using mercury based lamps as the UV-C light source. Around the same time, UV-C light from mercury based lamps was being used to disinfect the air of pathogens such as tuberculosis. These applications were based upon the key discovery in 1877 by Dr. Arthur Downes and Thomas P. Blunt of the germicidal properties of direct sunlight. They correctly identified the increasing germicidal effectiveness (ability to inactivate pathogens) with shorter electromagnetic wavelengths (from visible blue, to violet and then to ultraviolet electromagnetic wavelengths).
More recently, the U.S. Environmental Protection Agency (EPA) has recognized the use of ultraviolet electromagnetic radiation as a proven technology to inactivate pathogenic microorganisms without forming regulated chlorinated disinfection byproducts in public water supplies. UV can also be used to disinfect surfaces and is used in the food, beverage, medical and semiconductor industries to deliver a sterile environment. The next section will review how UV electromagnetic radiation accomplishes these germicidal effects starting with a very brief review of biology.
All living organisms contain nucleic acids, the two most commonly known are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The former provides the genetic code information for all living organisms to develop and function, and the latter facilitates translating the genetic information of DNA into proteins. Generally, DNA is a double stranded helix structure as shown in the “before section” of Figure 1. The individual rungs of the DNA ladder shown in Figure 1 are made up of Nucleotides. The Nucleotides for DNA have nitrogenous bases of adenine, cytosine, guanine and thymine and the Nucleotides for RNA have bases that consist of adenine, cytosine, guanine and uracil.
Nucleic acids (DNA and RNA) readily absorb UV electromagnetic radiation, especially in the range of 240nm to 290nm. The UV absorption in DNA peaks around 260nm which is very close to the primary emission line of a low pressure mercury lamp at 253.7nm. Further examination showed that nucleic acids in DNA absorb 10 to 20 times the amount of UV electromagnetic radiations as equal weights of the protein component of DNA; whereas the sugar and phosphate components of DNA do not absorb UV above 210nm. It should also be noted that both the rate and peak absorption occurs at different levels for each of the Nucleotides of DNA (adenine, cytosine, guanine and thymine) and the pyrimidines portion (thymine and cystosine) have been shown to be much more sensitive to UV electromagnetic radiation. Three possible pyrimidine dimers that can be formed in DNA are (thymine-thymine, cystosine-cystosine, and thymine-cystosine). The absorption of UV light by nucleic acid (3 types of pyrimidine dimers) is what leads to alterations in the genetic material; the smallest of which can ultimately lead to the death of a living organism. A microorganism that can not replicate is not capable of infecting a host.
The earth has been exposed to UV-B for millions of years; in some cases UV has performed a helpful role in forming the essential Vitamin D and likewise a harmful role in causing sunburn, skin cancer and cataracts. Ultraviolet electromagnetic radiation harms DNA in different ways.
An illustration describing one method how UV can alter DNA is shown in the “after UV-B” exposure portion of Figure 1. In this common damage event, adjacent bases bond with each other instead of across the nucleotide ladder. This creates a bulge and the distorted DNA molecule does not function properly. If the distorted DNA molecule can not produce the correct proteins the cell can die. Over millions of years, living cells have adapted to an environment exposed to UV-B electromagnetic radiation and have evolved by sending an enzyme in an attempt to repair the damaged DNA. These enzyme driven microbial repairs can be derived from light energy (photorepair) or chemical energy (dark repair). However, as the time for UV exposure increases for the cell; the risk for an incorrect DNA repair increases as well.
Exposure of DNA to a higher energy level UV-C light source coupled with the fact that this is where the DNA peaks in absorbing UV energy (240nm to 290nm) will result in even greater levels of molecular damage. DNA with increased levels of disruption to cellular processes due to incorrect repairs is more likely to be inactivated and possibly die. High energy UV-C radiation from a typical low pressure mercury lamp emitting at 253.7 nm is very effective at inactivating viruses, bacteria, mold and protozoa that can be harmful to humans. Some extremely lethal pathogens like anthrax, typhoid fever, diphtheria, cholera, dysentery, salmonella and tuberculosis can be inactivated at energy levels measured in millijoules per square centimeter.
Many health officials worldwide are concerned with the potentially pandemic situations posed by the avian influenza virus (H5N1), more commonly known as Bird Flu. Health officials take steps to develop a vaccine before any major outbreak occurs, though there was difficulty with production for the Swine flu vaccine this year. The effects of the SARS virus from a few years ago on the worldwide economy and resulting loss of life are only part of the reason for these preemptive actions.
The worldwide Spanish Flu influenza (H1N1 virus) pandemic that occurred between1918-1920 is shown in Figure 2. The Spanish Flu mortality estimates ranged upwards of 5% of the human population (50-100 million) people being killed and infecting up to 400 million people world-wide at the time. A greater portion of the Spanish Flu deaths occurred in healthy young adults than normally is associated with influenza, in as little as one to two days. The Avian “Bird Flu” is a more virulent influenza strain with high fatality rates. If one considers the greater travel speeds and higher amount of international travel of today when compared to 1918; the pandemic concerns appear to be warranted. UV radiation can inactivate and kill the Avian Flu virus and measures can be taken to install UV systems in hospitals, office buildings, planes and homes to minimize the spread of a pandemic influenza. As the relative size of the target organism increases, generally so will the amount of UV electromagnetic radiation required to cause disruption to cellular processes.
The amount of UV required to inactivate a specific target organism involves many different factors in addition to the relative size of the target. The specific DNA chemical composition and accordingly the amount of UV absorption will vary between the DNA of a virus, bacteria, mold or protozoan. The different rate of UV absorption in DNA is based on the Nucleotides of DNA (adenine, cytosine, guanine and thymine) and the pyrimidines portion (thymineand cystosine) have different rates of UV absorption. The particular shape of the microorganism will help determine the specific amount of UV required to damage the cell. Possible shapes include but are not limited to being spherical, spiral, rod-like or filamentous and should also include other construction factors (cyst).
Ultraviolet radiation must be able to strike the microorganism in order to inactivate the target which is challenging in a very large UV air or water treatment system. Scattering can also be a factor, when the size of the target microorganism is much less than that of the UV wavelength then Rayleigh scattering is present. When the target microorganism is larger than the wavelength then empirical adjustments are generally made to account for this including the shape of the target microorganism (rodlike versus spherical). Harmful microorganisms can withstand considerably more UV radiation in water than in dry air. Consequently, higher dosage levels are required to kill the exact same type of pathogen in water than in air.
The largest UV disinfecting water treatment facility in the world is being implemented for the city of New York. The New York City UV water treatment facility is designed to process up to 2.2 billion gallons per day and serves over 9 million consumers daily. The UV disinfection treatment facility will cost on-quarter of what a comparable filtration plant would cost and it will require approximately onetenth of the space.
The UV treatment facility will be comprised of 56 separate processing units capable of disinfecting 50 mgd (million gallons per day) under worst-case conditions. The city adopted a very conservative (higher) UV dose of 40mJ/cm3 that will insure a 99.9999% UV kill rate for the deadly Cryptosporidium protozoa. The contact time to inactivate microorganisms and disinfect the water is approximately 20-30 seconds in a single pass.
UVC water treatment can be used in a variety of applications to disinfect water for drinking, processing wastewater, in pools and spas, beverages and industrial processing. Industrial processing would include ultra pure water for pharmaceutical, cosmetic & semiconductor industries and for obscure applications like maritime ballast water and eliminating sulfate-reducing bacteria in offshore oil drilling. According to 2007 statistics from the American Water Works Association, there are more than 2,000 UV drinking water treatment systems operating in Europe and over 1,000 UV systems in the United States.
All of the UV water treatment facilities that have been discussed are based on UV lamps. UV-C LED power output levels are at present several orders of magnitude lower than needed to inactivate microorganisms. However, in the coming years, improved LED chip design coupled with higher density packaging and improved thermal management will make inroads. Water treatment applications for UV LEDs is unique since the high volume of flowing water in the systems could utilize the water to remove a significant portion of the heat generated in the LEDs. As mentioned earlier, low pressure UVC lamps operate best at a wall temperature of approximately 40°C and begin to lose efficiency at a temperature below –orabove 40°C. HVAC systems also could utilize the high velocity cool air to enhance the LED performance.
Treating psoriasis
Another major use for UV Technology is Phototherapy to treat Psoriasis and other skin conditions. Phototherapy describes a broad range for medical treatment using light. Psoriasis is a persistent and chronic skin disease which has a tendency to be genetically inherited. Psoriasis can range from a small localized area to covering the entire body and can be treated with UV-A or UV-B wavelengths. UV-A is done in conjunction with a photosensitizing agent which allows for a lower UV dose to be used. After several treatments, improvement can be seen in as little as 3 weeks with maintenance therapy thereafter. UV dental applications include curing (UV-A to Blue Visible LED) for cavity fillings, brightening and UV-C for toothbrush and medical instrument sterilization.
The medical analytical instrument market also utilizes UV light sources in fluorescence spectroscopy and ultravioletvisible spectroscopy. Fluorescence spectroscopy is a type of electromagnetic spectroscopy which analyzes the fluorescence emitted from a sample being irradiated and evaluated. The light source is generally UV to excite the electrons in the specimen to emit light of a lower energy level usually in the visible spectrum. In fluorescence spectroscopy, the sample is excited, by absorbing the higher energy UV light, causing the sample to move from its ground electronic state to one of the various vibrational states in the excited electronic state. Analysis of the emission spectrum will permit the identification of the substance (chemical compound, tumor, food processing).
Fluorescence spectroscopy is also used in forensics and chemical research fields. Ultraviolet-visible spectroscopy (UV/ VIS) uses multiple wavelengths of light in the visible, ultraviolet and near infrared ranges. The absorbance of light in a solution is directly proportional to the solution’s concentration (Beer-Lambert Law).
UV light sources are fundamental tools for forensic investigative work. The US Department of Justice in the Revised Processing Guide for Developing Latent Fingerprints which includes UV light sources for all types of surfaces (porous & non-porous) issued the FBI Laboratory Division in 2000. UV light sources have vastly improved collecting human DNA evidence (oils, amino acids, blood) at a crime scene by making the evidence highly visible to investigators. UV light can also be used by police to discover former wounds, bite marks and bruises not revealed by the visible spectrum for up to 6 to 9 months after the injury was inflicted that would not otherwise be visible. Exposing counterfeits Protecting the integrity of paper currency and other important financial documents such as stock and bond certificates against counterfeiting is fundamental to a sound monetary system. The United States Treasury Department and specifically the Secret Service Bureau was established in 1865 by Congress for the purpose of controlling counterfeiting. The mission was to prevent and prosecute counterfeiting activity and thus maintain the public’s confidence in the nation’s currency. Over the years many different features were used to deter counterfeiting US currency. In 1861, the first circulation of paper money issued by the federal government occurred to finance the Civil War. These non-interest bearing demand bills were green in color and the popular nickname “greenbacks” has been in use since that time. Many additional anti-counterfeiting measures have been taken since the first currency bills were issued such as the paper texture, paper weight, imbedded fibers, intricate images and serial numbers. Stock and bond certificates also adopted these same features.
The US Treasury Department has recently completed the security upgrade of US currency that was initiated with the twenty dollar bill in 2003 and completed with the release of the five dollar bill in 2008. The new anticounterfeiting measures implemented include watermarks, new colors, micro printing and a security thread that emit a different color under ultraviolet radiation based on the specific denomination. The color coded stripe can be seen by holding the bill in front of a strong source of white light. However, when illuminated with UV-A light, the security thread glows a bright: Blue-$5, Orange-$10, Green-$20, Yellow-$50 and Red-$100 bill. Figure 3 shows US and British currency illuminated with fluorescent lighting and also with 365nm UV-A light emitted from UV LEDs in a dark room.
UV-A LEDs are now being investigated as replacements for mercury based UV tubes. US passports and many credit cards have implemented UV threads and materials in their anti-counterfeiting efforts. A very practical application is to include a UV-A LED emitter into a cell phone allowing consumers to conveniently validate the integrity of their currency. These measures will greatly increase both the technical challenge and financial costs to forge currency and financial instruments; thus maintaining the integrity and validity of the world-wide monetary system.
The material presented in this feature is based on one of the chapters from the recent book: Optoelectronics: Infrared-Visible- Ultraviolet Devices and Applications. This publication that was launched late last year expands on the groundbreaking work of its 1987 predecessor.
The second edition is fully revised to reflect current developments and practical considerations for those working in the field. Claimed to be a comprehensive mini-encyclopedia, this treatise reviews essential semiconductor fundamentals, including device physics, from an optoelectronic industry perspective.
The co-editor of this book, Dave Birtalan, began his career at General Electric’s Semiconductor Division and held various engineering, product marketing and sales management positions involving Optoelectronics, MOSFETs, and Laser Diodes including working on the Strategic Defense Initiative Program. He received his bachelor of science in electrical engineering from Penn State University and conducted his graduate studies at Syracuse University. In addition, he has held leadership positions with the Mitsubishi-General Electric power semiconductor joint venture, Vishay Telefunken and TT electronics involving RF, LEDs, ICs, IrDC and Sensor products. He can be reached at: dbirtalan@aol.com
