| neutral-density filter | <equipment> A light-absorbing filter whose absorption spectrum is moderately flat. Depending on the type, the absorption curve is flat primarily in the visible spectral range, or may extend to varying degrees beyond the visible range. For video microscopy, this is an important point since the absorbance may or may not extend into the near-infrared region where the sensitivity of many video image pickup devices is very high. (05 Aug 1998) |
|---|---|
| density | 1. <radiology> The amount of darkness or light in an area of a scan reflects the compactness and density of tissue. Differences in tissue density are the basis for CT and MR scans. 2. <microscopy> Logarithm to the base 10 of the reciprocal of transmittance. (12 Jan 1998) |
| density-dependent | <epidemiology> 1. Effects whose intensity changes with increasing population density. 2. Effects whose intensity increases with increasing population density, for example a mortality which becomes very high near a carrying capacity. Effects which become less pronounced at higher population sizes (perhaps the reduction in fecundity caused by the difficulty of mating) are sometimes called inverse density dependent. Density dependent effects are often a cause of nonlinearity in a model. (05 Dec 1998) |
| density dependent inhibition of growth | <cell culture> The phenomenon exhibited by most normal (anchorage dependent) animal cells in culture that stop dividing once a critical cell density is reached. The critical density is considerably higher for most cells than the density at which a monolayer is formed, for this reason, most cell behaviourists prefer the term density dependent inhibition of growth as this avoids any confusion with contact inhibition of locomotion, a totally different phenomenon that is contact dependent. (12 Jan 1998) |
| density gradient | <chemistry> A column of liquid in which the density varies continually with position, usually as a consequence of variation of concentration of a solute. Such gradients may be established by progressive mixing of solutions of different density as for example: sucrose gradients) or by centrifuge induced redistribution of solute (as for caesium chloride gradients). Density gradients are widely used for centrifugal and gravity induced separations of cells, organelles and macromolecules. The separations may exploit density differences between particles or primarily differences in size, in which latter case the function of the gradient is chiefly to stabilise the liquid column against mixing. (12 Jan 1998) |
| density gradient centrifugation | <technique> High-speed centrifugation inwhich molecules float at a point wheretheir density equals that in a gradient of cesium chloride or sucrose. (12 Jan 1998) |
| incidence density | The person-time incidence rate. (05 Mar 2000) |
| intermediate density lipoprotein | Class of lipoproteins formed in degradation of very-low-density lipoproteins; about half are cleared quickly from the plasma into the liver by receptor-mediated endocytosis; the other half are degraded into low density lipoproteins. (05 Mar 2000) |
| inverse density dependence | <epidemiology> See density dependence. (05 Dec 1998) |
| optical density | 1. <chemistry, investigation> Absorbance is defined as a logarithmic function of the percent transmission of a wavelength of light through a liquid. 2. <microbiology> This can be used as a measure of the amount of light absorbed by a suspension of bacterial cells or a solution of an organic molecule, it is measured by a colourimeter or spectrophotometer. Absorbance values are used to plot the growth of bacteria in suspension cultures and to gauge the purity and concentration of molecules (such as proteins) in solution. See: absorption. (15 Jan 1998) |
| organismal density | <microbiology> Number of organisms or items per unit area or volume. (09 Oct 1997) |
| either particle flux density | The particle fluence rate, or energy flux density, the energy fluence rate of intensity. Compare: fluence. (05 Mar 2000) |
| familial high density lipoprotein deficiency | Familial high {density lipoprotein deficiency}; a heritable disorder of lipid metabolism characterised by almost complete absence from plasma of high density lipoproteins, and by storage of cholesterol esters in foam cells, tonsillar enlargement, an orange or yellow-gray colour of the pharyngeal and rectal mucosa, hepatosplenomegaly, lymph node enlargement, corneal opacity, and peripheral neuropathy; autosomal recessive inheritance. Synonym: familial high {density lipoprotein deficiency}, Tangier disease. Origin: G. An-, priv., + alpha, a, + lipoprotein + -aemia, blood (05 Mar 2000) |
| flux density | <radiobiology> Total amount of a quantity passing through a unit surface area in unit time See: flux. (09 Oct 1997) |
| low density lipoprotein | <biochemistry> A lipoprotein substances (combination of a fat and a protein) which acts as a carrier for cholesterol and fats in the bloodstream. High levels of low density lipoprotein are considered a positive risk factor for the development of coronary artery disease. Less than 130 mg/dl is desirable, 130 to 159 mg/dl is borderline high, over 160 is considered high. Acronym: LDL (10 Jan 1998) |