| achromatic lens | A compound lens made of two or more lens's having different indices of refraction, so correlated as to minimise chromatic aberration. (05 Mar 2000) |
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| achromatic | <microscopy> Literally, colour-free. A lens or prism is said to be achromatic when corrected for two colours. The remaining colour seen in an image formed by such a lens is said to be secondary chromatic aberration. A truly achromatic lens would transmit light without decomposing it spectrally and there would be no colour fringing in the image. A doublet, composed of a positive and negative lens element, can be made achromatic for two colours which much improves the performance for most of the others. (05 Aug 1998) |
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| achromatic aplanatic condenser | <microscopy> A well-corrected microscope condenser lens, corrected for chromatic and spherical aberrations and satisfying the sine condition. (05 Aug 1998) |
| achromatic apparatus | The nonstaining asters and spindle fibres in a dividing cell. (05 Mar 2000) |
| achromatic objective | <microscopy> An objective that is corrected chromatic for two colours, and spherically for one, usually in the yellow-green part of the spectrum. (05 Aug 1998) |
| achromatic threshold | Threshold of visual sensation, the minimal light intensity evoking a visual sensation. Synonym: achromatic threshold, minimum light threshold. (05 Mar 2000) |
| achromatic vision | A severe congenital deficiency in colour perception, often associated with nystagmus and reduced visual acuity. Synonym: achromatic vision, monochromasia, monochromasy, monochromatism. Origin: G. A-priv. + chroma, colour, + opsis, vision (05 Mar 2000) |
| acoustic lens | In ultrasonography, a lens used to focus or diverge a sound beam; may be simulated by electronic manipulation of signals. (05 Mar 2000) |
| anterior pole of lens | The central point on the anterior surface of the lens of the eye. Synonym: polus anterior lentis. (05 Mar 2000) |
| anterior surface of lens | The anterior surface of the lens of the eye. Synonym: facies anterior lentis. (05 Mar 2000) |
| aplanatic lens | A lens designed to correct spherical aberration and coma (q.v.). Synonym: periscopic meniscus. (05 Mar 2000) |
| apochromatic lens | A compound lens designed to correct both spherical and chromatic aberrations. (05 Mar 2000) |
| asphenc lens | <physics> A lens made aplanatic by grinding the outer zones to a greater radius than the inner zones. Aplanatic condensers can be made in this way. A simple aspheric lens is widely used as a lamp condenser because of its efficiency in converging light rays to one focal plane. (05 Aug 1998) |
| aspheric lens | A lens with a paraboloidal surface that eliminates spherical aberration. (05 Mar 2000) |
| astigmatic lens | A lens in which one of the surfaces is curved in one meridian and less curved in the opposite meridian; e.g., a teaspoon or a football. Synonym: astigmatic lens. (05 Mar 2000) |
| axis of lens | A line connecting the anterior and posterior poles of the lens of the eye. Synonym: axis lentis. (05 Mar 2000) |