Chapter 25, Problem 110PP

Visible-Light Curing in Dentistry

An essential part of modern dentistry is visible-light curing (VLC), a procedure that hardens the restorative materials used in fillings, veneers, and other applications. These “curing lights" work by activating molecules known as photoinitiators within the restorative materials The photoinitiators, in turn, start a process of polymerization that causes monomers to link together to form a tough, solid polymer network. Thus, with VLC a dentist can apply and shape soft restorative materials as desired, shine a bright light on the result as shown in Figure 25-50 (a) and in 20 seconds have a completely hardened—or cured—final product.

Figure 25-50

An intense beam of light cures or hardens, the restorative material used to fill a cavity. (Problems 109, 110,111, and 112)

The most common photo initiator is camphoroquinone (CPQ). To cure CPQ in the least time, one should illuminate it with light having a wavelength that matches the wavelength at which CPQ absorbs the most light A rough plot of the relative absorption of CPQ for light of different wavelengths is shown in Figure 25-50 (b) Many VLC units use a halogen light, but there are some drawbacks to this approach. First, the filament in a halogen light is heated to a temperature of about 3000 K, which can cause heat degradation of components in the curing unit itself Second, less than 1% of the energy given off by a halogen bulb is visible light, so a halogen bulb must have a high power rating to produce the desired light intensity at the wavelengths that CPQ will actually absorb.

More recently, VLC units have begun to use LEDs as their light source These lights stay cool, emit nearly all of their energy output as visible light at the desired wavelength, and provide light with an intensity as high as 1000 m W/cm², which is about 10 times the intensity of sunlight on the surface of the Earth.

110. • Suppose a VLC unit uses an LED that produces light with an average intensity of 400 m W/cm^? What is the rms value of the electric field in this beam of light?

1. A.    1230 N/C
2. B.     390 N/C
3. C.     1700 N/C
4. D.     2.1 x 10⁵ N/C