appliance DESIGN IAM : Page 8Reality Traditional mercury arc lamps indeed emit a high level of energy at the wavelength peak of 365 NM (OWEVER THISDOESNOTMEAN that an LED light source with a 365 nm peak will provide the best performance. The reality is that many light-curable chemistries, including acrylated adhesives, use photo initiators that respond best to the narrow, bell-shaped spectral curve (Figure 3). Myth No. 5— Manufacturers’ LED power ratings are comparable and correspond to performance. Reality A thorough review is required to properly interpret the claimed power ratings and associated advantages of LED curing systems. LED curing-equipment manufacturers may use LED power ratings as a sales tool, claiming that a higher power rating equates to better performance. Qualifying direct comparisons of power can be achieved by focusing on the following areas: Power Expressed as Intensity Intensity is the measure of electromagnetic radiation of all frequencies at the surface of an object. In reference to light-curable chemistry, the electromagnetic radiation is that in the visible or UV range of frequencies or wavelengths. Standard units of measure of intensity are milliwatts per square centimeter (mW/cm2 ). Compare Intensity at the Surface of the Substrate This measurement represents delivered power at the bond line. Determining the intensity at the substrate surface removes process variables and misinformation such as claimed power, transmission losses, and lens variables such as divergence angle. Intensity is measured with a radiometer. When measuring intensity with a radiometer, keep in mind that considerable variation can exist in the measurements depending upon the brand of radiometer. When comparing irradiance among prospective light-curing systems, use a radiometer that is specifically designed to measure intensity levels at the frequency(ies) emitted from the curing-light source, or, at the very least, use the same radiometer for all comparative measurements. Incorporation of a radiometer that is specifically designed and optimized to address the LED source frequencies will ensure accurate readings. Claimed Power Versus Useable Intensity The claimed power of LED curing lights is almost always quoted as power emitted from THELIGHTGUIDE (OWEVER TWO factors influence how much radiation or light actually reaches the substrate surface: (1) the divergence angle of the energy emitted from the light source and (2) the inverse-square law in radiography. Divergence Angle Decreases or Increases Power The emitting end of the lightguide is the final optical component in the light-delivery path. The divergence angle determines the degree to which the energy spreads out after it leaves the light source. As the light diverges, the power per unit area (expressed as mW/cm2 ) on the substrate surface is diminished. Inverse-Square Law The inverse-square law describes the transmission losses that occur due to the distance between the emitting end of the lightguide and the substrate. Less power is transmitted to the substrate surface as the emitting end moves further from the substrate. As you double the distance between the emitting end of the lightguide and your substrate, the light intensity or energy impinging on the substrate decreases by a factor of one quarter. This principle illustrates why the most valuable information is determining the intensity level at the bond-line point of cure. Comparing Manufacturers’ Power Ratings Requires Comparing Spectral Distribution Delivered power or intensity provides only a partial comparison of LED light-curing systems. Spectral distribution of the prospective light sources must also be evaluated. Furthermore, the spectral 8 International Appliance Manufacturing 2010 Publication List |
