PHOTOMETRY is the art of comparing the intensity of a source of light with that of another source which is taken as a standard. The possibility of making such comparisons de pends upon the power of determining by means of the eye when two neighboring fields of view, illuminated respectively by the two sources in question, are equally bright. The sensitiveness of the eye to inequalities of brightness does not greatly exceed 1 per cent, even under the best conditions; and since it frequently falls below that value from fatigue and from various other causes, numerous attempts have been made to find photometric methods which are independ ent of this organ, but thus far without much success.
Photometers.—Any instrument for the measurement of the intensity of a source of light is termed photometer. Since all attempts to substitute for the eye such instruments as the thermobile, the bolometer and the selenium cell have, for the ordinary purposes of photome try, led to unsatisfactory results, all existing photometers which have come into general use are based upon the above-mentioned power of the eye. The earliest form, which was orig inally described by Bouguer, was invented early in the 18th century. It consisted of a screen AB (Fig. 1), illuminated 'by the sources of light S and s, the intensities of which were to be compared. The partition PC prevented the light of S from falling upon BC and that of s from reaching AC. The distance of the two sources from the screen was adjusted until the illumination of AC appeared to the eye to be equal to that of BC.
Since the illumination produced by a source of light is inversely as the square of its dis tance: Ii b1 = Const.— (If bs= Const.— dj where di and d, are the distances of S and .r, respectively, from the screen, b, and b, the il lumination of the screen due to S and s re spectively and I, and I, the intensities of the screens. Since b, a b, we have d, An ingenious modification of this instru ment, devised by Lambert (1760) and subse quently used by Rumford (1794) is known as the Shadow Photometer or Rumford Photom eter.— In this apparatus an opaque body, usu
ally an upright rod (R, Fig. 2) is placed in front of the screen. The surface of the screen is illuminated by both sources but the two shadows AB, CD, are each illuminated by light from one of the sources exclusively. When these shadows are equally bright the distances of S and s from the shadows which they il luminate determine the relative intensity of the two sources.
Ritchie (1826) introduced a new principle into photometry. He placed the two lights to be compared at the ends of a track or bar along which a box containing two mirrors M,M' (Fig. 3) could be moved, until the rays from S, re flected at M to the left half of the screen AB gave an illumination equal tb that from the rays from s reflected by M' to the other half of the screen. This screen was of some translu cent material, usually paper.
Bunsen (1841) substituted for the Ritchie screen a sheet of unsized paper the central, usu ally circular, portion of which had been ren dered translucent by the application of oil or of melted paraffin. The paper when placed be tween two sources of light, the plane of the paper perpendicular to the incident rays on either side, affords a very simple and convenient means of determining when the illumination from the two sides is equal. When subjected to unequal illumination from the two sides the translucent portion of the face toward the brighter source appears dark, the unoiled por tion bright. On the other face the reverse is true (see Fig. 4). As the paper is moved away from the brighter source and toward the weaker an interchange in the appearance of the two sur faces occurs and there is a neutral position in which both appear alike and in which it is scarcely possible to distinguish the translucent portions. When this position has been found the relative intensities of the two sources may be calculated from the law of inverse squares. To facilitate the observations the bar or track upon which the paper screen is mounted is di vided into a convenient number of equal parts.