| Light-cured resin composites have been widely used in modern dentistry. Curing lights were used to activate the curing reaction and polymerize composites in common temperature. Curing-light units influence the ultimate properties of resin composite and the effectiveness of restoration. There are 4 types of curing lights available: quartz tungsten halogen (QTH), light-emitting diodes (LED), plasma arc (PAC) and Argon ion laser light units. The most commonly used dental curing lights are QTH and LED lights. Numerous factors may influence polymerization of resin composites, such as sufficient power density, adequate exposure time, correct wavelength, the size of the curing tip, distance between light guide tip and surface of the resin composite, size and situation of cavity,thickness of the increment and characteristics of resin composite. Sufficient power density, adequate exposure time, correct wavelength are critical variables for maximum polymerization of the resin composites. There are several surveys reported on the intensity of light units, but no related data of clinical curing-light units used in China. Most of these surveys were about the light intensity of QTH and few surveys were related to LED. LED units have many advantages compare with QHT and used increasingly widespread. This survey choose Chang Chun city as the representation in China and investigate the type of curing-light unit, distribution of power density and other information about the curing-light units that correlate to the power density. We analyzed the information and provided some reference recommendations to clinical dentists about the correct usage and maintaining of cuing-light units in order to achieve the best effectiveness of restoration.The investigation analyzed 270 lights from 2 stomatology hospitals, 6 department of stomatology in general hospitals, 111 private dental offices. There were 174 QTH and 96 LED units. The distribution of power density was from 0 to 1702mW/cm~2. The mean power density was 413.2mW/cm~2, the power density of QTH and LED were 298.9mW/cm~2, 620.4mW/cm~2, respectively. Using two independent-samples t test, we found that the power density of LED was significantly higher than that of QTH (P<0.001). The mean number of years usage of the light unit was 4.74 years. Bivariate correlations of power density and light unit age, revealed a significant negative correlation (r=-0.736, P<0.001). Classification analysis showed that the mean years of QTH was 5.82 years, a negative correlation was noted between power density and the increasing age of the units (r=-0.613, P<0.001). The mean years of LED was 2.79 years, a negative correlation was recorded (r=-0.782, P<0.001). The survey showed that every light-curing unit only had one light guide. The situation of light guide builded-up by composite resin or damaged was very severe and most of the light guides were builded-up or damaged. The mean number used everyday was 7.1 times. Bivariate correlation of power density and light unit frequency revealed no significant negative correlation (r=-0.087, P>0.05). Most of clinical doctors didn't know that the light-curing unit needed to be monitored and maintained regularly. A bulk of light-curing units didn't change lamps, light guides and filters.A survey of the efficiency of power density from 270 light-curing units, the following conclusions could be drawn:1. The mean power density of light-curing units used in Chang Chun city was adequate for clinical application. However, QTH units showed relatively low power density and should be increased exposure time.2. Most of the light-curing units were QTH. Some QTH units degenerate severely and need to be changed with the new ones.3. There is a general lack of awareness among dentists of the need for maintenance of these units. The situation of light guide builded-up by composite resin or damaged is very severe. Doctors should pay attention to check and replace the curing lights regularly.
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