Types of UV and Visible Light

8/25/2017

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Types of UV and Visible Light Visible light and UV light are specified by their wavelength in “nanometers” which is 10-9 or one billionth of a meter. The discovery of UV radiation in 1801 by German physicist Johan Wilhelm Ritter was associated with the observation that silver salts darken when exposed to sunlight and that invisible rays just beyond the violet end of the visible spectrum gave a chemical reaction which was effective at darkening silver chloride soaked paper. The term “Chemical Rays” was adopted shortly thereafter, then eventually dropped in favour of “ultraviolet radiation”. The name means “beyond violet” (Latin ultra, = “beyond”), and violet being the color of the shortest wavelengths of the Visible Light. UV light has a shorter wavelength than that of violet light. Physicist and chemist Richard Küch joined Heraeus, a precious metals and technology company in Germany in 1890 and reached the fundamental discovery in 1904 determining that mercury vapor emits an intensive greenish light when it is induced to make electrical discharges in a quartz glass tube and developing the first UV quartz lamp which laid the foundation for lamp technology that is still used today. Progress in LED performance has been so rapid that it has been described by a logarithmic law, akin to Moore’s law for microelectronics. Although everyone with a passing interest in science seems to have heard of Moore’s law (which states that the number of transistors in an integrated circuit doubles every two years), far fewer seem to be aware of ‘Haitz’s law’ named after Roland Haitz, a now-retired scientist from Agilent Technologies. The law forecasts that every 10 years the amount of light generated by an LED increases by a factor of 20, while the cost per lumen (unit of useful light emitted) falls by a factor of 10. What is astonishing is that not only have these forecasts been fulfilled but over the past five years there are signs that they have actually been over achieved.

Types of UV Light This table follows the adhesive/coating UV curing definition by RadTech. It also states using UVV to represent the very long wavelength band (400-450nm). One argument of extending the long wavelength upper limit to 450nm is that there is no precise boundary between UV and visible light. Visible light and UV light are specified by their wavelength in “nanometers” which is 10-9 or one billionth of a meter. The UV region is divided into four parts; Ultraviolet A (UVA 315-400nm) is used for low energy UV polymerization reactions in the bonding and curing of various materials, and is also used in non-destructive fluorescent inspection methods. Photons in the UVA (315-400 nm) promote through cure, especially with thicker film layers. Ultraviolet B (UVB 280-315nm) is used along with UVA for polymerization and since it is the most energetic region of natural sunlight, for accelerated light aging of materials Photons in the UVB (280 – 315 nm) contribute to bulk cure. Ultraviolet C (UVC 100-280nm) is used for rapid surface cure of UV inks and lacquers, and is also used in the sterilization and germicidal processes and applications, the most energetic of the wavelengths used in UV curing. Photons in the UVC are important for surface cure and promote surface properties such as hardness, stain resistance, and abrasion resistance. Vacuum Ultraviolet (VUV 10-200nm) Vacuum UV can only be used in a vacuum, however, they have very significant commercial importance. As is well know from Moore’s law, microelectronics can address smaller and smaller dimensions. Currently 193nm and 157nm are most important for making 65-30nm nodes. [Nitrogen does not strongly absorb VUV. Actually nitrogen is the most popular "purging gas" to prevent VUV being absorbed by oxygen.] Although UV is classified as invisible light it is commonly referred to as “light” because it conforms to the optical rules of visible light. In the electromagnetic spectrum, UV is located in the higher frequency range than the visible band:

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8/25/2017

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Light is composed of its basic unit known as “photons”. Planck’s law describes the energy of a photon of a particular frequency as: E = hv; where E = energy of the photon, h = Planck’s constant, and v = frequency As frequency increases, a point is reached in the electromagnetic spectrum where photons have enough energy to power photochemical reactions. This is what gives UV light its commercial usefulness. There are a number of organic compounds which absorb in the UV region and which have the chemical capabilities to use this energy to promote a photochemical reaction. The most common photochemical reaction is photosynthesis – where green plants absorb visible light photons from the sun and convert carbon dioxide and water to carbohydrates. For convenience sake, the upper end of the electromagnetic spectrum is specified by wavelength. Name Ultraviolet A (long wave, or black wave) Ultraviolet B (medium wave) Ultraviolet C (short wave, or germicidal) Vacuum Ultraviolet

Abbreviation

Wavelength (nm)

Energy/Photon (eV)

UVA

315mn – 400nm

3.10 – 3.94eV

UVB

280nm – 315nm

3.94 – 4.43eV

UVC

200nm – 280nm

4.43 – 12.4eV

VUV

100nm – 200nm

6.20 – 124eV

Electromagnetic Spectrum

Visible Light (400-700nm) is also subdivided with different colors being associated with different wavelengths, as shown in the diagram of the electromagnetic spectrum.Visible light has also found commercial use in curing. Light in the 400-420 nm region (violet) has been used to cure white pigmented coatings. Light in the 440-460 nm region (blue) has been used in dental offices for tooth repair. The choice of the right output spectrum is very important for successful curing applications. Not only must the lamp output match the absorption spectrum of the photoinitiator, but the effects of pigments and other additives must be taken into consideration. In general terms, the thicker or more heavily pigmented the UV curable layer is, the longer the wavelength should be. This is because longer wavelengths tend to penetrate deeper.There are three commonly used lamp spectra for UV curing: The most common is the Mercury spectrum, also known as the “H” spectrum. This is produced by using only Mercury as the fill material of the lamp. The Mercury spectrum output has a series of peaks distributed throughout the UV spectrum and is used as a general purpose lamp. Most printing applications use the Mercury spectrum. Strong output in the UVC region makes the Mercury spectrum the lamp of choice where surface cure properties are very important. An example of this is UV curing on vinyl flooring. When other additives are mixed with the Mercury fill, the output spectrum of the lamp changes dramatically. The “D” spectrum is formed when Iron halides are mixed with the Mercury. This spectrum has most of its output in the UVA region. The Iron lamp is used in applications which require curing of a thick layer of UV material. When Gallium halides are mixed with the Mercury, the Gallium ( “V” ) spectrum is produced. This lamp has a characteristic purple hue, due to the location of most of its output in the violet region (400 – 425 nm) of the visible spectrum. It is used to cure thick white pigmented coatings. This is because the lamp output very closely matches a cure window in the coating formed by the absorption of the photoinitiator and the transmission curve of the white Titanium Dioxide pigment. Other metal halides or combinations of metal halides are used for special applications. An example is Indium halide which has a strong output peak in the 440 – 460 nm (blue) region. This spectrum is used in dental offices to cure coatings on teeth.

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