PHOTOMETRY (front Gk. (pals, ph6s, light, measurement, front )serpov, metron, measure). The science of comparing the intensities of sources of light. It is impossible at the present time to measure the intensity of a source of in terms of any absolute standard. i.e. in terms of watts; and therefore it is customary to compare the intensity with that. of a stand ard source of light, and for this purpose various standards of white light have been proposed and used. but none have lwen completely satisfactory. These will be found discussed below. The 'inten sity' of any source of light is &tined to be the amount of light emitted through a cone whose solid angle is unity. The 'flux of light' is the amount of light emitted through any sized solid angle. The 'illumination' of any illuminated sur face is the flux of light falling upon it divided by its area. The 'brightness' of a source of light, if it is a surface. is the intensity divided by the area of the surface. See an article by Crew, Astrophysical Journal, vol, vii., p. 295, 159s1.
If a surface is illuminated by a small source of light. its illumination varies directly as the intensity of the source and inversely as the square of its distance from the surface. (See LICIT.) The intensities of two sources may be compared, therefore. by allowing two portions of the same surface to he illuminated by them. If the illtni niti over these two portion% is the same parently to the eye. the intensities of the two source• are in the same ratio as the squares of their distances front the surface. There have been several devices invented to SeeIlre the illumi nation of two portions of the same surface by the two sources. Thcsr instruments are known as photometers and are described below. The prob lem of compari”g two colored sout•ees of light, or of comparing the intensities of the colors pro duced by two source's of white light when dis persed b?• a prism, is practically the same as that just discussed, but requires a photomet(r adapted for this purpose. See SPECTROPHOTOMM:TRY.
PlorromrrErts, Most photometers are based upon the law' of the inverse squares and enable us to eomp:n•e the relative intensities of two lights by finding at what distance they fur nish equal illumination. There are also other photometers, hut those depending on the equality of illumination are by far the most extensively employed. The eaPliest photometer was devised by niniguer and consisted of a vertical screen at right angles to a partition. On either she of the partition was placed a light so that it would illuminate one side of the screen. Those lights were then adjusted until the two halves of the screen appeared of equal brightness. The dis tances of the lamps or lights from the screen Were and a proportion formed according to the law of the inverse squares. The screen, which was at first opaque. in later instruments was sup planted by one of ground glass, oiled paper, or other translucent material. An instrument based on the same principle which is in some respects modified is shown in the accompanying diagram I Fig. '2). here. and B arc the two illuminants, as before, and the rays of light fall on the sur faces of the prism C. The two lights are moved so that the surfaces of the prism appear equally illuminated to eye of the observer at E. .1 simple and effective photometer was devised liv Lambert in 1760, but as it was used extensively by Nuniford some years later. it is known by his name. It consists of a vertical screen with a white surface, in front of which stands a cy 'Marie:II stick. The lights to be eompared are placed in front of this stick. so that each will east a shadow on the screen. shadows rep resent the amount of light furnished by hut one of the sources, consequently by varying the dis tances of the lights from the screen a position can be (Maine() where they furnish shadows of equal intensity. Measuring the distance of the lights from the screen, squaring these numbers, and forming a proportion, we can again compare the intensity of the two illuminants. A second screen with an aperture cut in it may be placed parallel to the first with the lights in front of it and some little distance apart. so that each il luminates the screen by means of rays sent through the aperture. The two separate spots of light on the screen can be made equal by adjust ing the lights and their comparative intensity computed as before. The photometer in most gen
eral use is that of Bunsen, where the two lights to be compared, A and B, are placed at. the opposite ends of the apparatus and a screen, C, whose face is perpendicular to the incident rays can be moved to and fro between them. This screen is made of a sheet of white paper with a central spot, d, produced by oil or grease, which appears bright on a dark ground by transmitted light, and dark on a light ground by reflected light. When the transmitted and reflected lights are equal in intensity the spot should disappear and the surface of the paper appear uniform, but in actual practice this condition is not realized. The sheet of paper is usually mounted in a box or carriage with mirrors o• prisms enabling the observer to see simultaneously both sides of the paper, and is moved along a scale between the two lights, which is either graduated in units of length o• indicates candle power direct. The reading is taken when the spot disappears as nearly as possible, and the candle power or rela tive intensity is determined as in the other cases. The arrangement of the lights, of course, varies in different forms of the instrument. as does also the construction of the box carrying the screen, but in general the results do not possess a greater accuracy than 4 or 5 per cent. The Ron.ford photometer cannot be depended upon closer than S or 10 per cent., but in all classes of photometric work much depends upon the observer, The most modern and accurate photometer is that of Lmniner and Brodhun, in which the e'en• tral spot disappears entirely when the lights are of equal intensity. This is accomplished by the use of two right-angled prisms, one, F. with its hypothenusal face plane, the other. F, with this face ground spherical except in the centre, where a circular spot is perfectly plane. The two prisms are then placed with their hypothenusal sides adjacent, but separated except at the centre, where the plane ground surface of the second prism is in contact with that of the other. The lights are placed at opposite ends of the appa ratus, as in the ease of the Bunsen photometer, and the rays fall on a central opaque screen, C, from which they are reflected to mirrors, D and D', and then to the faces of the prism. The rays from B entering the prism F, which has a spher ical surface, undergo total reflection, except at other. If the illuminant to the left of the oh se•ver, for example, is of greater intensity, then we have a bright spot. on a dark ground, and reversing the conditions the (lark spot sur rounded by light. When the lights are of equal intensity the spot entirely disappears and the field is uniform. The Lummer-Brodhun screen is usually mounted on a carriage which can be moved along a scale and which is easily reversed as a check on the observations. It also has a telescope, H, to the surface of the prism, which is a distinct advantage, as it permits the use of one eye and enables the observer to decide more accurately when the spot disappears.
The main difficulty in photometry is the com parison of lights of different colors, as then the determination of equally illuminated surfaces is an extremely difficult matter. For this purpose llood, employing a principle first made use of by Whitman, has devised a. 'flicker photometer' in which the light from two different sources is pre the central portion (d), where the glasses are in contact. Therefore, only the rays passing through this central portion enter the second prism, E, and produce a spot. of light of elliptical shape on its opposite surface. The rays from the other source of light, A, are reflected into the prism E, and by total reflection at the hypothenuse are brought to the third face of the prism, except those falling on the surface of con tact. which enter the prism F. Consequently we have the surface of the prism E nearest the ob server illuminated in part by the rays from one source of light and in part by rays from the sented alternately to the eye in rapid succession. When the flicker disappears, it is shown that the light coming from one source is equal in intensity to that from the other.