Light Curing Units

A diversity of dental light curing units (LCU) are available in the market. These include quartz-tungsten-halogen light (QTH), plasma-arc light (РАС), argon-ion laser (AL), and light-emitting diode (LED).

Quartz-Tungsten-Halogen Light (QTH)

Quartz-Tungsten-Halogen Light (QTH) was the first visible light used to cure RBCs. Its bulb consists of a tungsten filament enclosed in a clear, crystalline quartz casing, filled with a halogen-based gas. As electricity flows through the filament, heat develops because of the wire resistance. The heat developed is sufficient to cause tungsten atoms to vaporize from the wire surface. When this happens, tremendous amounts of electromagnetic energy are released, mostly occurring in the infrared (IR) spectral region. Thus, these types of light units typically require filtering to remove heat, as well as excess visible light not required for photopolymerization.

Plasma-Arc Light (РАС)

Plasma-Arc Light (РАС) was introduced to photopolymerize RBCs. These units utilize two tungsten rods, held at a specified distance, encased in a high- pressure envelope of xenon gas, with a sapphire window through which emitted radiation escapes. When high voltage is applied across the electrodes, a spark forms, which produces a tremendous amount of electromagnetic radiation over a wide spectral range: from IR to short wavelength UV. Because of the massive amount of radiation emitted falling outside of the narrow limits needed for dental photopolymerization, a substantial amount of filtering is required in this light. Additionally a special liquid helps to further reduce unwanted IR, UV, and visible light (Rueggeberg, 2011).

Argon-Ion Laser (AL)

Argon-Ion Laser (AL) was used to enhance vital tooth bleaching before being used for intraoral photopolymerization of dental RBCs. The initial delivery of power from the laser to the tooth was directly through the end of a fiber optic cable. The radiation coming out of the fiber optic cable had a divergent nature, and therefore other methods were developed in an attempt to make a collimated beam of coherent energy, whose target power was not related to the curing light guide-to-tooth distance. However, these units were large, heavy, and expensive. Over time, the unit became much smaller, and could easily fit into a clinic, yet, it needed to be on a cart when moved from room-to-room due to its weight. Additionally, after the use of the device, room temperature would be elevated. Due to the above reasons, this curing system became outdated in a short time (Rueggeberg, 2011).

Light-Emitting Diode (LED)

The Light-Emitting Diode (LED) technology has been proven to be an efficient, cost- effective lighting source. It has been widely used shortly after the blue LEDs became available using indium-gallium-nitride (InGaN) substrates (Rueggeberg, 1999). These devices rely on the forward-biased energy difference (band gap) between two dissimilar semiconductor substrates (n-type conduction band and p-type valence band), to determine the wavelength of emitted light (Rueggeberg, 1999). Electrons are forced to traverse from one side of a semiconductor material (the “N” material, having an excess of electrons) to a substrate having an electron deficiency (the “P” material). When electrons travel through this potential energy “gap”, they emit light with wavelengths depending on the composition of each semiconductor substrate (Nakamura, 2015). The spectral emission from such units could successfully photo- activate CQ-based products.

Development in Light-Emitting Diode

LED technology was borrowed from other industries and was incorporated in the dental field in the 1990s. The concept that the spectral emission from such units could successfully photoactivate CQ-based products has been proven. LED technology requires low power, no filament, no optical filter, and therefore provides much greater photon-generating efficiency than any competitive light source. In addition, these units can be powered by a battery. LED sources are claimed to last for thousands of hours without needing replacement (Rueggeberg, 1999).

First-Generation LED

The first introduced blue LED curing lights were mainly experimental. They were built to test whether they could generate light at the correct wavelength and deliver a sufficient number of photons needed to successfully photopolymerize dental RBCs. The individual LED available at that time had a very low output power, which necessitated the different arrangement of multiple LED elements into a physical array, and a turbo-tip was used, resulting in sufficient output, which was enough for photopolymerization of CQ-based RBCs.

These closely packed arrays generated a significant amount of thermal energy, and therefore heat dissipation was needed. Heat sinking technology was incorporated in some units to draw heat away from the LED chip. Other units have used metal body castings that provided a large area for the thermal dissipation as well as structural durability.

The first-generation LED did not produce adequate output for curing RBCs resulting in insufficient depths of cure of dental RBCs (Ernst et ah, 2004). However, if the curing unit was used for extended exposures, the output was found to be comparable with QTH source (Ernst et ah, 2004). The irradiance value of this LED generation varied greatly between different units. Nickel-cadmium (NiCAD) battery was used, but careful recharging routines had to be followed or the useful lifetime of these power sources was significantly reduced. The spectral emission of this light was effective for activation of CQ and PPD, but it was not suitable with TPO (Rueggeberg, 1999).

Second-Generation LED

In the illumination industry, high emission area LEDs became available and were adapted in dentistry. One-Watt chips were available, all on one body, consisting of four main areas of illumination, each consisting of four, bar-shaped emitting surfaces: a total of sixteen emission areas. Incorporation of these chip types greatly boosted irradiant output and allowed blue LEDs to be able to accomplish effective photopolymerization in a much shorter time. A higher-power LED became available shortly thereafter containing 5-Watt devices and rapid advances have led to chips reaching 10 and 15 Watts, which were incorporated in dental LEDs. These resulted in a greater photon density increasing irradiance values and allowed lower exposure time to achieve optimal photopolymerization of CQ-containing restorative materials (Uhl, Sigusch. & Jandt, 2004).

Concurrently, battery technology also advanced, allowing incorporation of the longer-lasting nickel metal hydride (NiMH) units. Higher-rated LED chips produced a lot of heat, and therefore advanced methods such as internal fans and large metal heat sinks were used to dissipate thermal power from the LED arrays (Rueggeberg, 1999).

As for the emission spectra of this generation, it is much greater than the first one. However, the peak emission is located at a shorter wavelength than previously seen leading to an increased overlap w'ith PPD while still providing activation of CQ. No interaction w'ith APO was possible.

Third-Generation LED

The need to provide radiant energy to activate TPO as well as Ivocerin® drove manufacturers to incorporate more than one color into the LED chipset. These LCUs are known as polywave as they emit irradiation with different peaks of wavelengths. Different arrays were used to provide a simultaneous combination of violet and blue wavelengths. Among the chip arrangements was an array with centrally positioned, high-wattage blue LED, surrounded by four lower-powered, converging violet LEDs. Another array incorporated two blue LEDs and one violet LED. This array arrangement is seen in Bluephase Style light. A third array was the arrangement of three different color chips into the single array set: two blue (emitting near to 460 nm), a shorter wavelength blue (emitting near to 445 nm), and one violet (emitting close to 400 nm).

With the inclusion of the violet emission near to 407 nm, photons are delivered to a wide bandwidth for all photoinitiators used in dental restorative materials, particularly for TPO. PPD, and CQ. However, in the third generation, the blue emission is reduced compared to that of an all-blue emitting light, which means less potential for CQ activation at RBC depths. In materials that contain Lucirin® TPO or Ivocerin® in addition to CQ, improved curing is possible, even with the lower blue light present. Despite the differences in the amount of violet and blue light emitted by the poly wave LED. previous evidence supports the idea that the spectral output of this polywave LED used was enough to efficiently cure bulk-fill RBCs containing either CQ or CQ associated with alternative photoinitiators (Schneider, Pfeifer, Consani, Prahl, & Ferracane, 2008). However, the light beam profile received by the specimen should not be ignored.

The third-generation curing lights are present in two different designs. The first is the traditional gun-style light, which has the chipset inside the gun body and uses fiber optic light guides to transmit emitter photons onto the target area. Another concept is the use of a pencil-style body, which can still use removable fiber optic guides or can have the emitting chipset placed directly at the distal guide end of the unit. This allows the light to directly shine onto the target, without the use of fiber optic light guides. It also has the advantage of greater ease of placement intraorally facilitating light guide position and allowing more direct illumination and maximum transfer of light to the restoration. Battery technology has been advancing, and the use of lithium-ion batteries is common in most LED units of this generation. They are stable and durable allowing long-usage energy storage sources providing a reliable output over extended clinical operation time.

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