| tapetal light reflex | The glow from the eyes of some animals in the dark when a light illuminates the retina; due to the reflection of the light from the tapetum, an iridescent layer (containing guanidine crystals) in the choroid. (05 Mar 2000) |
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| EDTA light chain | <protein> Myosin light chains (18 kD) from scallop muscle (two per pair of heavy chains), easily extracted by calcium chelation. Although the EDTA light chains do not bind calcium they confer calcium sensitivity on the myosin heavy chains. (18 Nov 1997) |
| transmitted light | <microscopy> The usual method for illuminating transparent microscopic specimens. The light is concentrated on the specimen by the substage condenser. Objects appear in outline (refraction images) or coloured on a bright field (colour images). (05 Aug 1998) |
| unit of light | See: candela, lux. (05 Mar 2000) |
| unpolarized light | <microscopy> A bundle of light rays having a common propagation direction but different vibration directions. (05 Aug 1998) |
| usable light range | <microscopy> The ratio of the maximum to the minimum levels of illuminance over which a video camera or camera tube can provide a usable signal. Being aided by automatic irises, gray-wedge wheels, etc., in addition to varying electrode voltages where permissible, the usable light range can be several orders of magnitude greater than the intrascene dynamic range. See: intrascene dynamic range, condenser, variable-focus. (05 Aug 1998) |
| flux, light | <microscopy> Sometimes called luminous flux, the visible portion of the radiant energy emitted by a light source. It is measured in lumens per solid angle. In electrical engineering, it is analogous to the lines of force in a magnetic field, spoken of as magnetic flux. (05 Aug 1998) |
| light | <microscopy> Light is radiant energy of such wavelength that, falling on the retina, it stimulates the rods and cones of the eye and produces the sensation of vision. The foregoing is a physiological (subjective) definition which tells what light does, under certain conditions, rather than what it is. According to the (objective) Maxwell theory, all radiant energy is electromagnetic in character, the generation of the radiation depending upon the portion of the spectrum under consideration. For that part of the spectrum lying within the visual range, approximately 400 to 700 nanometres (nm) the release of light energy from externally stimulated sources may be thought of as due to atomic or molecular vibration or to the passing of electrons from high to lower energy levels accompanied by the spasmodic release of energy as the electronic orbits decrease in diameter. See: quantum theory. (05 Aug 1998) |
| light-activated resin | A resin which uses visible or ultraviolet light to excite a photoinitiator which interacts with an amine to form free radicals and initiate polymerization. Used mainly in restorative dentistry. Synonym: light-activated resin. (05 Mar 2000) |
| light adaptation | The visual adjustment occurring under increased illumination in which the retinal sensitivity to light is reduced. See: light-adapted eye. Synonym: photopic adaptation. (05 Mar 2000) |
| light-adapted eye | An eye that has been exposed to light, with bleaching of rhodopsin (visual purple) and insensitivity to low illumination. Synonym: photopic eye. (05 Mar 2000) |
| light bath | Therapeutic exposure of the skin to radiant light. (05 Mar 2000) |
| light cells of thyroid | Cell's present between follicles or interspersed among follicular cell's; they are rich in mitochondria and are believed to be the source of thyrocalcitonin. Synonym: C cell, light cells of thyroid. (05 Mar 2000) |
| light chain | <immunology, protein> The lighter of the two types of polypeptide chains that are found in immunoglobulin and antibody molecules. Also used as a non-specific term for the smaller subunits of several multimeric proteins such as immunoglobulin, myosin, dynein, clathrin. (14 Oct 1997) |
| light chain-related amyloidosis | A form of primary amyloidosis in which the fibrillar amyloid deposits are derived from the amino terminal variable region of the light chains of immunoglobulin; seen in B-lymphocyte and plasma-cells dyscrasias. (05 Mar 2000) |