TECHNOLOGY FOCUS - TEXTILES AND PAPER - Preferred Material: The transparent shell contains filter substances and/or transparent coloring material(s). The fibers may contain two or more cores consisting essentially of material with temperature-controlled radiation transmission. An additional core of polymer material with a high tensile strength may also be present. The fibers are obtained by melt-spinning, using an extruder die suitable for the production of multi-component fibers.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: Suitable core materials comprise known polymer mixtures, as described e.g. in EP 0181485, EP 0559113 or DE 4408156 (i.e. mixtures of polymers which become reversibly opaque at a required temperature). Suitable shell polymers comprise polyacrylonitrile, modacryl, polyesters, polyolefins, ethylene vinyl acetate, polyamides, polyurethanes, polyvinyl chloride etc. EXAMPLE - Core-shell fibers were obtained by 2-component melt spinning with a twin screw extruder, using Escorene Ultra UL 00018 (RTM: EVA copolymer) as the shell polymer, with a core mixture comprising 53.5 parts by weight (pts. wt.) styrene/hydroxyethyl methacrylate (94/4 %) copolymer, 43.2 pts. wt. polypropylene oxide (mol. wt. 4000), 2.5 pts. wt. trimethylolpropane triacrylate and 0.75 pt. wt. 2,4,6,-trimethylbenzoylphenylphosphine oxide. The shell polymer was extruded at 180 degreesC into a recess (8) in the die plate (1) at one end of a multi-component spinneret, from which it flowed through 12 holes (9, 10) in die plates (1) and (2). The core mixture was melted at 130 degreesC and injected through a threaded connector (7) into a channel (16) in a die plate (4) at the other end of the spinneret and thence (via holes 12 and 13 in plates 4 and 3) into 3 channels (14) between the 3 groups of 4 holes in die plate (2). The core and shell polymers then flowed into die plate (3) where they were combined in 3 x-shaped cavities (15) to form the core-shell structure. The combined melts were then extruded through holes (6) in die plate (4) to give 3 core-shell fibers with an outer diameter of ca. 4 mm. These fibers showed appreciable opacification on heating (opacification point = ca. 50 degreesC), due to enhanced light-scattering in the thermotropic core.