Abstract: A method for reading of information, which comprises printing information on a metal substrate or a substrate capable of transmitting infrared rays of 2-10 .mu.m wavelength, with a marking ink containing tin oxide, indium-tin mixed oxide or a mixture of tin oxide or indium-tin mixed oxide and at least one compound selected from the group consisting of alumina, barium sulfate, silicon dioxide and calcium carbonate, as necessary coating a clear coating or a coloring ink on at least the information-printed area of the substrate, and identifying the information by the use of a CCD camera sensitive to infrared rays of 2-10 .mu.m wavelength or an infrared detector. No noise is generated even when the reflectance of a light from the substrate surface is large, and the information can be identified (read) easily.

Abstract: Information is printed on the surface of a substrate with a marking ink containing a mixture of an indium-tin mixed oxide and at least one compound selected from the group consisting of alumina, barium sulfate, silicon dioxide and calcium carbonate, to prepare an invisible marking-printed material; and infrared rays of 2-10 .mu.m wavelength are applied onto the invisible marking-printed material to measure the infrared absorptivities of the invisible marking area and other area of said material and identify the information signified by the invisible marking.

Abstract: An image-transmitting fiber includes at least one first fiber element for transmitting light power therethrough, and a plurality of second fiber elements disposed around and extending along the first fiber element. The second fiber elements cooperates with one another for transmitting an image of an object therethrough. The first fiber element is greater in diameter than each of the second fiber elements.

Abstract: An optical fiber includes light-transmitting means comprising at least one optical fiber element, and a heat-resistant and radiation-resistant coating formed around the light-transmitting means. The coating comprises an inner buffer layer formed around the light-transmitting means and an outer layer of polyether ether ketone formed around the inner layer.

Abstract: A fiberscope has a light source and a light guide for leading light emitted from the source onto an object disposed in liquid helium. A transmission system for transmitting the image of the object has an image guide. A body tube is fixed to one end of the image guide and faces the object. Objective lenses are disposed in the body tube to project the image of the object onto the one end face of the image guide. An eyepiece is fixed to the other end of the image guide, through which the image of the object is viewed. The interior of the body tube is kept under vacuum.