Magazine June 17, 1999  

Bimetals boost display brilliance

The trend toward larger computer monitors and HDTV brought a need for ever bigger cathode-ray tubes. But though these larger CRTs produce impressive images their size requires improved compensation to allow for greater differences in the thermal expansion of their imaging components. Otherwise, the electron beam can drift and degrade color quality. Thermal mismatches can particularly degrade image color quality or color purity. Color purity suffers when the electron beam is misaligned with the fluorescent coated glass of the monitor or TV. The aperture openings of the metal mask position the electron beam onto the screen surface, exciting the phosphor to form images. During service, however, the TV or monitor heats up. The increased internal temperatures cause the mask and metal frame holding it to expand faster than the glass screen. This thermal mismatch between adjacent components adversely affects geometric tolerances necessary for good color purity. Thermal-compensation devices help maintain alignment of the mask to the glass screen allowing the electron beam to hit the right locations over the entire pixel-array. Various material design approaches have been developed to solve the panel/mask distortion problem in CRTs.
Thermal-compensation devices based on advanced bimetal thermostat technologies respond to temperature variations within the CRT and act both as a thermal sensor and actuator in one economical package.