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A diving mask (also half mask, dive mask or scuba mask) is an item of diving equipment that allows underwater divers, including, scuba divers, free-divers, and snorkelers to see clearly underwater.〔''NOAA Diving Manual, 4th Edition'' CD-ROM prepared and distributed by the National Technical Information Service (NTIS)in partnership with NOAA and Best Publishing Company〕 Surface supplied divers usually use a full face mask or diving helmet, but in some systems the half mask may be used.〔 When the human eye is in direct contact with water as opposed to air, its normal environment, light entering the eye is refracted by a different angle and the eye is unable to focus the light. By providing an air space in front of the eyes, light enters normally and the eye is able to focus correctly. ==Construction== There are two basic categories of diving mask: The half mask covers the eyes and nose, and the full face mask covers eyes, nose and mouth, and therefore includes a part of the breathing apparatus. The half mask is described here. Diving masks may have a single, durable, tempered glass plate, or two lenses in front of the eyes and a "skirt" of synthetic rubber or silicone elastomer to create a watertight seal with the diver's face.〔 Some masks with two lenses allow the user to fit prescription lenses directly into the frames, otherwise it is possible to have prescription lenses glued to the inside of the original flat glass, either full frame, or partial, which function as bifocals. In the case of freediving masks, which need to be low volume to minimize the amount of breath needed to equalize the change of pressure that occurs with depth, the lenses may be made of polycarbonate plastic. All masks have means to keep them in position, such as an elastic rubber strap〔 or a velcro fabric strap. The elastomer skirt provides a seal between the lenses and the diver's face. The material may be almost transparent, translucent or opaque. A nearly transparent skirt provides a greater peripheral vision, though somewhat distorted, and may reduce the feeling of claustrophobia in some divers, but in some cases the light entering through the sides may cause distracting internal reflections. The skirt also encloses the nose, usually by means of a nose pocket, so that air can be exhaled into the mask to equalise the pressure during descent. A rigid frame may be used to connect the lens(es) to the skirt, or they may be permanently bonded together without using a frame. Masks that are used at depth must be constructed so that the diver can exhale through the nose into the mask to prevent the "squeeze" caused by increasing pressure during descent in water. The nose section of the mask also needs to be flexible to allow the diver to block the nostrils to perform a maneuver such as the valsalva maneuver or the frenzel maneuver when equalising pressure in the middle ear. Some masks have a one-way purge valve under the nose to let water out. While common in snorkeling, this feature is less favored by scuba divers because of the possibility of the valve failing at depth and leaving the user with a leaking mask. Divers may test whether a mask is a good fit by placing it on their face, without using the straps, and gently inhaling through their nose. If the mask stays on without any help this indicates that no air is being drawn in and that the fit is good. The straps are usually made of an elastomer that can sometimes cause hair tangling. This can be prevented by replacing the rubber strap with a customized neoprene strap or by covering the existing strap with a neoprene cover. Sometimes masks are sold in conjunction with snorkels and/or swimfins. Low quality snorkelling masks may have a low-quality plastic or glass faceplate, and are not recommended for anything but rare, casual use. Masks that have the snorkel built in are considered hazardous as well. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Diving mask」の詳細全文を読む スポンサード リンク
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