Light-emitting diode or lamp

What are the opportunities for LED UV technology in the graphic arts industry?

The basics of LED UV technology (Part 1)

LED UV is a buzzword that attracted a great deal of attention among the graphic arts sector in the run-up to and during the Drupa trade fair in Düsseldorf. The presentation of an LED UV dryer for sheet-fed offset printing announced by Japanese machine manufacturer Ryobi met with particular acclaim. How can we judge the success of the UV curing of inks and varnishes using light-emitting diodes? The enthusiastic advertising claims of various suppliers sound extremely promising. Besides low energy consumption, minimal wear, a compact design and no warming-up time, the lack of infrared radiation also reduces the time and money spent on cooling. However, running counter to all these benefits is the fact that, as yet, not enough suitable printing inks and varnishes are available on the market. The aim of this series of articles – the first of which deals with the basics of the technology – is to take an in-depth look at this new technology which, in lighting applications, in particular, is part and parcel of everyday life.

The fact that solid objects can generate light when electrically excited was observed by Henry Joseph Round in a silicon carbide crystal (SiC) as far back as 1907. This phenomenon is also exploited by LED technology. LED stands for 'Light-Emitting Diode’. A diode denotes an electrical component that only lets current flow in a single direction (forward biased condition), and blocks the current in the other direction (reverse biased condition). When current flows through a light-emitting diode in the forward biased condition, the result is energy in the form of visible, infrared or even ultraviolet light.

Converting electrical current into light

Light-emitting diodes are based on semiconductor connections, which convert current directly into light. The wavelength of the light emitted by a light-emitting diode depends upon the material, and its energy gap, that is used for the electronic semiconductor. In order to cover the spectral range from infrareds to UVA, materials such as aluminium gallium arsenide (AlGaAs), gallium arsenide phosphide (GaAsP), aluminium indium gallium phosphide (AlInGaP), gallium phosphide (GaP), indium gallium nitride (InGaN) and gallium nitride (GaN) are employed for semiconductors.

As each LED is only capable of emitting light in a narrow spectral range, it cannot be used to directly generate white light. In practice, two methods are used to obtain white light from an LED. On the one hand, LEDs in the colours red, green and blue can be put together to produce white light. On the other hand, a blue or UV LED is embedded in an illuminant layer (the neon tube principle), which converts the short-wave light into white light.

Wandering electrons

There now follows a brief description of how to achieve the UV light required for curing inks or varnishes using LED technology. The semiconductor materials described above have a crystalline structure. A semiconductor crystal contains the so-called valence band, which represents the energy of the atoms’ bound electrons, and a higher energy band, called the conduction band, consisting of the energy of the electrons moving freely in the crystal. At room temperature, the thermal energy is sufficiently high to ionise some atoms in the semiconductor material. The result is a minimal but decisive amount of conductivity, which also explains the name “semiconductor”.

If sufficient energy is conveyed to an electron to allow it to escape from its parent atom, it moves randomly through the material until it encounters an ionised atom - a so-called hole - with which it recombines. This recombination process causes it to lose energy, which corresponds to the energy gap between the valence band and the conduction band. This energy can be emitted in the form of a photon, and determines the wavelength of the emitted light. Light-emitting diodes are therefore categorised as electroluminescent light sources. The same applies to traditional UV lamps.

The discharge lamps used in the graphic arts industry are primarily mercury discharge lamps. They generally consist of a glass tube filled with gas or metal vapour, with electrodes affixed to the ends of the tubes. If sufficiently high voltage is applied to these electrodes, free electrons move to the anode, where they excite or ionise further atoms. During this process, energy is stored in the atom and is released in the form of UV light when the atoms return from their excited condition to a lower energy state. The spectrum emitted by discharge lamps can be broadened to an emission continuum by increasing the pressure.

Light-emitting diodes generate monochrome light

Contrary to the process described above, LEDs have only a very narrow wavelength window (see figure 2). Inks or other coating materials must be precisely matched to this spectrum, if they are to be cured using LED UV lamps. This is the principal reason why, so far, there is still no commercially available range of suitable printing inks or varnishes on the market.

In industry, common LED systems for UV curing currently focus on the 395 nm and 365 nm wavelengths. Other possible spectral ranges are 350 nm, 385 nm and 405 nm. Scientific publications also mention the wavelengths 210, 250 nm, 275 nm and 290 nm.

UV light-emitting diodes can achieve power values of a few watts per square centimetre, and an efficiency rating of between 1 and 20 %, depending on the LED UV system used. Here, the basic rule of thumb is: the shorter the wavelength, the lower the efficiency. The wavelength also has an effect on the price of LED products. The shorter the wavelength of the emitted light, the higher the production costs.

LED UV is largely unexplored territory for the printing industry

Most practical applications are concerned with the visible light spectrum, e.g. lights in the home, indoors and outdoors and in the automotive industry. One example of the commercial use of LEDs is the polymerisation of plastics in dentistry. In contrast, most UV applications in the printing industry require considerably higher power and a larger area of illumination with LED UV. This can be achieved by packing a larger quantity of LEDs together at high density in so-called clusters. The next article will discuss how the future development of LED UV technology could manifest itself specifically in the graphic arts field and what challenges would need to be overcome in order for this technology to assert itself in a wide range of applications in the printing industry.

The basics of LED UV technology (Part 1) as pdf-file