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E15. Describe the proper technique for handling the light source of a light-cured system when placing a Class II restoration, and explain the consequences of improper utilization of the light.

The light intensity of different light sources differs considerably. Modern light-sources have a light intensity often exceeding 600 mW/cm², while older light-sources often had light intensities below 200 mW/cm². Even more important is to realize that the light intensity of a lamp is not stable. It changes as the bulb ages and the reflector oxidizes. It is therefore important to check the quality of the light source regularly by use of a light meter. We used to say that a 2 mm thick composite layer requires a cure time of 40 s. This recommendation is based on the use of the older light sources, but still seems to be valid because of light absorption in the material. With some of the modern light sources, the cure time can be decreased. If, however, the tip cannot be placed close to the composite surface, the cure time needs to be increased. For example, if the tip is one inch away from the surface, only 7% of the light intensity exist at that distance. Also, be aware that darker colors have a lower depth of cure. Because the composite material has a tendency to shrink toward the light source, a transparent matrix band and light scattering wedges should ideally be used when a Class II restoration is placed. With such a band and wedge, it is possible to start the polymerization process in the cervical region, helping the material to shrink in the right direction. If, on the other hand a metal band is used, one increases the risk for shrinking the material away from the cervical margin. Today, there is some controversy regarding fast versus slow cure. The reason is simply that it seems as the use of high energy light sources induce more polymerization stresses than a slower and gentler cure. Some manufacturers of light sources have developed light sources that start with a low intensity and increase the intensity as the curing process proceeds.

The left column represents the distance from the light source to the composite surface. The right column represents the hardness values measured at the bottom of a 2 mm thick composite layer.