page_title: Diving mask
section_title: Construction
hierarchical_section_title: Diving mask / Construction
caption_attribution_description: English: Scuba diver with bifocal lenses in mask
context_page_description: A diving mask is an item of diving equipment that allows underwater divers, including scuba divers, free-divers, and snorkelers, to see clearly underwater. 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 on the retina. By providing an air space in front of the eyes, the eye is able to focus nearly normally. The shape of the air space in the mask slightly affects the ability to focus. Corrective lenses can be fitted to the inside surface of the viewport or contact lenses may be worn inside the mask to allow normal vision for people with focusing defects.
  When the diver descends, the ambient pressure rises, and it becomes necessary to equalise the pressure inside the mask with the external ambient pressure to avoid the barotrauma known as mask squeeze, This is done by allowing sufficient air to flow out through the nose into the mask to relieve the pressure difference.
context_section_description: 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 faceplate, or two lenses in front of the eyes. These may be supported by a relatively rigid plastic or metal frame, or they may be permanently bonded to the rim of the skirt, in a construction known as "frameless". In the case of freediving masks, which need to have a low internal 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. There is usually a "skirt" of synthetic rubber or silicone elastomer to support the frame and lenses and create a watertight seal with the diver's face.
  The skirt 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 through the nose into the mask to equalise the internal pressure during descent and thereby avoid possible barotrauma of the enclosed area of the face.  The section of the mask covering the nose must allow the wearer to block the nostrils while equalising pressure in the middle ear. All diving masks have means to keep them in position, usually an elastomer strap of similar material to the skirt, but occasionally an expanded neoprene pad with velcro straps is used. Mask straps are usually wider at the back or split into an upper and lower strap at the back of the head for stability and comfort.
  Some masks had a one-way purge valve under the nose to let water out, but this is no longer common as they were neither necessary nor reliable, and often leaked.
caption: Scuba diver with bifocal lenses in half mask
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