| achromatic threshold | Threshold of visual sensation, the minimal light intensity evoking a visual sensation. Synonym: achromatic threshold, minimum light threshold. (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 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) |
| 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 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) |
| absolute intensity threshold acuity | The minimal light that can be seen. (05 Mar 2000) |
| absolute threshold | The lowest limit of any perception whatever. Compare: differential threshold. Synonym: stimulus threshold. (05 Mar 2000) |
| anaerobic threshold | The oxygen consumption level above which aerobic energy production is supplemented by anaerobic mechanisms during exercise, resulting in a sustained increase in lactate concentration and metabolic acidosis. The anaerobic threshold is affected by factors that modify oxygen delivery to the tissues; it is low in patients with heart disease. Methods of measurement include direct measure of lactate concentration, direct measurement of bicarbonate concentration, and gas exchange measurements. (12 Dec 1998) |
| auditory threshold | The audibility limit of discriminating sound intensity and pitch. (12 Dec 1998) |
| brightness difference threshold | The smallest difference that can be perceived as a difference in brightness. Synonym: light difference. (05 Mar 2000) |
| pain threshold | Amount of stimulation required before the sensation of pain is experienced. (12 Dec 1998) |
| galvanic threshold | The minimal strength of an electrical stimulus of indefinite duration that is able to cause excitation of a tissue, e.g., muscle or nerve. See: chronaxie. Synonym: galvanic threshold. Origin: rheo-+ G. Basis, a base (05 Mar 2000) |
| relational threshold | The smallest degree of difference between two stimuli that permits them to be perceived as different. (05 Mar 2000) |
| visual threshold | Threshold of visual sensation, the minimal light intensity evoking a visual sensation. Synonym: achromatic threshold, minimum light threshold. (05 Mar 2000) |