Photometry

Photometry (Gr. phōs, 'light,' and metron, 'measure'), the art of measuring the intensity of a source of light, by comparison with a standard of reference (see GAS-LIGHTING). The principles involved in the usual instruments are: (1) Lambert (1760), Rumford: equality of shadows cast by two sources at different distances; when the shadows are equal the intensities are proportional to the squares of the distances. (2) Equality of illumination through slits in screens; distances and intensities as before. (3) Bouguer, Ritchie, Leonhard Weber: reflection of light from two sources, so that they travel side by side to the eye, the distances being adjusted until they appear equal; calculation as before. (4) Wheatstone, the same; but the two reflections are from a polished sphere, which is set in motion, so that the comparison is between two looped luminous curves, produced through the persistence of vision. (5) Bunsen: a grease-spot on paper, equally illuminated on both sides, disappears. (6) Babinet: light from one is polarised by reflection, from the other by refraction; both pencils are sent through a double rotating quartz, and looked at through a double-refracting prism; they give coloured images, and the distances are adjusted until the images, on over-lapping, give a white field. All these methods are unsatisfactory when the sources of light are of different colours, as—e.g. a candle and an arc-lamp. Instruments have accordingly been devised for applying the above methods to each part of the spectrum of the light from each source. The degree of sensitiveness of the eye of the observer, or a difference of sensitiveness between his two eyes, affects the result. In other instruments used as photometers what is measured is not the luminous intensity so much as the radiation: among these we may mention the Radiometer (q.v.); Leslie's photometer, which is a differential thermometer; Bunsen and Roscoe's, which measures the quantity of hydrochloric acid formed in a given time from chlorine and hydrogen; Léon's, which measures the amount of nitrogen liberated from iodide of nitrogen; and various instruments based on photographic reactions, which truly measure not the luminous, but the actinic intensity. Stellar photometry is generally contrived by stopping off more or less of the surface of the object-glass, or by polarising apparatus, so as to bring the apparent brightness of a star down to that of a standard of comparison. The usual photometric standards are (1) the English standard candle (see GAS-LIGHTING); (2) the Hefner-Alteneck amyl-acetate lamp, which has now displaced the candle in Germany; (3) the carcel, a standard colza-oil lamp, used in France; (4) the Electrical Standards Committee's unit, the light given off by one square centimetre of platinum at its fusing-point; and (5), in scientific work, an incandescent electric-lamp under stated conditions of resistance and current, maintained constant.