Photometer

“ Thus, all the photometer can do is to give us the means of increasing or diminishing the light from one star, so that we can make it equal by estimation to that from some other star or source of light. ”

“ Light is bought and sold almost as readily as are the products of farm and dairy; many factories, churches, and apartments pay a definite sum for [107] electric light of a standard strength, and naturally full value is desired. An instrument for measuring the strength of a light is called a photometer, and there are many different varieties, just as there are varieties of scales which measure household articles. ”

“ In the second, star magnitudes obtained by extinction with the wedge will agree better with those obtained by photography than those obtained with other visual photometers, since photographic action is chiefly produced by rays from E to G in the spectrum, and the E light of ultimate importance with the wedge photometer is nearer this light in character than the D light with which other photometers are chiefly concerned. It would also appear that results obtained with the wedge photometer are independent of the aperture of telescope employed, which is not the case with other photometers. ”

“ Having settled upon a standard candle or lamp the measurement of light demands extreme care, and, at the best, can never approach the accuracy of other laboratory measurements. Many photometers have been invented, some of them highly elaborate, but the type oftenest used remains in essence the simple instrument long ago devised by Bunsen. ”

“ A specially constructed instrument or piece of apparatus for comparing light intensities or illuminations is called a photometer The earlier forms of photometers were very simple and not capable of giving very precise results. The principles of construction are, Photometers.however, the same in all the recognized forms down to the most elaborate of recent inventions Two of the earliest forms were described by P. Bouguer and W. Ritchie. The Ritchie wedge constitutes the basis of many varieties of type. ”

“ The instrument employed was the "wedge photometer," which measures brightness by resistance to extinction. A wedge of neutral-tint glass, accurately divided to scale, is placed in the path of the stellar rays, when the thickness of it they have power to traverse furnishes a criterion of their intensity. Professor Pickering's "meridian photometer," on the other hand, is based upon Zöllner's principle of equalization effected by a polarising apparatus. After all, however, as Professor Pritchard observed, "the eye is the real photometer," and its judgment can only be valid over a limited range. ”

“ However bad the images may be, they are always exactly alike, and may, therefore, be compared with accuracy. Both stars are affected equally by passing clouds, so that this photometer may be used whenever the stars are visible and at times when other photometric work is impossible. The diminution in light also follows a simple geometrical law, and is readily computed with great accuracy. ”

“ In almost all accurate methods of photometry the aim is to bring the illuminating powers of the two sources to equality. This may be effected by altering the distance of either light from the illuminated surface. Or we may use polarized light and diminish the intensity of the stronger beam by suitable rotation of a Nicol prism, a method particularly useful in spectrophotometer’s. The same result may also be effected by interposing absorbent disks, the precise absorbing powers of which must, however, be known with great accuracy. ”

“ The photometry of arc lamps presents particular difficulties, owing to the great difference in quality between the light radiated by the arc and that given by any of the ordinarily used light standards. (For standards of light and Photometry of arc. photometers, see Photometer.) All photometry depends on the principle that if we illuminate two white surfaces respectively and exclusively by two separate sources of light, we can by moving the lights bring the two surfaces into such a condition that their illumination or brightness is the same without regard to any small colour difference. ”

“ The illuminations of the two pieces of ground glass as viewed through the photometer double prism are brought to equality, either by shift of the ground glass to or from the standard light, or by means of two Nicol prisms placed in the cross tube. One advantage of the instrument is its portability.
The photometry of incandescent electric lamps has led to several special modifications and devices. The candle power varies distinctly in different horizontal directions, and one measurement in any particular direction is not sufficient. ”

“ The inventors of this instrument claim that their instrument can gauge accurately and easily the relative intensities of two lights, whether of the same or of different colour (Phil. Mag., 1904) . There is no doubt that results obtained by different observers with a flicker photometer are in better agreement than with any other form of photometer. The comparative ease with which the balance is obtained even when the tints are markedly different shows that its action depends upon a visual distinction which the eye can readily appreciate, and this distinction is mainly one of brightness. ”

“ Agreeably to the announcement of the annual report of Harvard College Observatory for 1877, as to photometry, a beginning was made by constructing a photometer suitable to be attached to the great telescope. Other photometers have been devised at different times for use independently. One of the earliest was applied during the year beginning Oct. 12, 1877, in measuring the light of all known satellites excepting the two inner ones of Uranus, which are too faint to be discerned, even by the great telescope. ”

“ This allows the instrument to operate unattended while it monitors work processes, filter penetration breakthrough, etc. In some cases, the photometer and the data acquisition system together are sufficiently small that they can be worn by a worker. These photometers can be used to provide a time-dependent profile of exposure over a work period as well as an integrated time-weighted average exposure. In addition, the worker can be made aware of aerosol concentrations while carrying out his tasks so that he can modify work practices to reduce exposure. ”

“ Examination with the photometer has revealed that many popular patterns of reflectors and shades are most ineffective, while those of the Holophane make, when kept scrupulously clean, send the light just where it does most good and at the lowest possible expenditure of energy. ”

“ Journ. of Science, 1893) that, when a disk whose halves differ in tint but not in luminosity is rotated rather slowly, the eye of the observer sees no flickering such as is at once apparent when the halves differ slightly in luminosity. Rood himself suggested various forms of photometer based on this principle. In his latest form (Amer. Journ. of Science, Sept. 1899) the differently coloured beams of light which are to be compared photometrically are made to illuminate the two surfaces of a Ritchie wedge set facing the eye. ”

“ Alongside or between these the real star image can be brought for a comparison, and by turning the polarizer through an angle [alpha] the images can be equalized with the real image. Then a similar comparison is made with a reference star. If A be the brightness of the former and B of the latter then
A/B = sin2α/sin2 [Pg 197] ββ
The Zöllner photometer was at first set up as an alt-azimuth instrument with a small objective and rotation in altitude about the axis C. Since the general use of electric lamps instead of the inconvenient flame it is often fitted to the eye end of an equatorial. ”

“ The celebrated mathematician and astronomer, Pierre Bouguer (1698–1758) , who had published, in 1729, his “Essai d’Optique,” etc., which was greatly enlarged in his “Traité,” etc., brought out by La Caille in 1760, may be considered the founder of this branch of the science of optics, to which the name photometry has been given by English writers. The photometer designed by Sir Benjamin Thompson, Count Rumford (entered at A.D. 1802) , has been described in Phil. Trans. ”

“ Again the apparatus is somewhat intricate and delicate, but gives a precision of working if anything a little better than that of the selenium cell, quite certainly below 1 per cent.
Neither instrument constitutes an attachment to the ordinary telescope of modest size which can be successfully used for ordinary photometry, and both require reduction of results to the basis of visual effect. [27] But both offer great promise in detecting [Pg 200] minute variations of light and have done admirable work. For a good fundamental description of the selenium cell photometer see Stebbins, Ap. ”

“ The power possessed by ether waves of inducing chemical change, either
of decomposition or of combination. The violet and ultra-violet end of
the spectrum of white light, generally speaking, represent the most
highly actinic rays.
Actinometer, Electric.
Properly an apparatus for measuring the intensity of light by its action
upon the resistance of selenium. A current produced by fixed
electro-motive force passing through the selenium affects a galvanometer
more or less according to the intensity of the light. It is more
properly an electric photometer. ”

“ Some substances have the property of transforming luminous radiations, presenting the phenomena of Calorescence, Fluorescence and Phosphorescence. An optical system is composed of any number of Mirrors or Lenses, or of both. If light falling on a system be not brought to a focus, i.e. if all the emergent rays be not concurrent, we are presented with a Caustic and an Aberration. An optical instrument is simply the setting up of an optical system, the Telescope, Microscope, Objective, optical Lantern, Camera Lucida, Camera Obscura and the Kaleidoscope are examples ”

“ And, with the spectroscope, it is hardly too much to say that no investigator ever observes visually with that instrument any more: practically every spectrum is made a matter of photographic record first. The observing, or nowadays the measuring, is all done afterward.
All telescopes and cameras are alike, in that each must form or have formed within it an image by means of a lens or mirror. In the telescope the eye sees the fleeting image, in the camera the process of registering the image on a plate or film is known as photography. ”

“ In all photometric methods a standard light is employed, which should not vary in brightness, and with which the light from the source is compared. In optical pyrometers no attempt is made to measure the illumination in terms of candle-power; all that is necessary is to bring the standard and the source to the same degree of brightness by suitable adjustments. ”

“ Accurate determination of the indices of refraction becomes important when lenses have to be constructed for rendering the electric beam parallel. The index for electric radiation is often very different from the optical index, and the focal distance of a glass lens for light gives no clue to its focal distance for electric radiation. I found, for example, the index of refraction of a specimen of glass to be 2·04, whereas the index of the same specimen for sodium light is only 1·53. ”

“ The total intensity of a star is, however, essentially dependent on the instrument used in measuring the intensity. Besides the eye, the astronomers use a photographic plate, bolometer, a photo-electric cell, and other instruments. The [Pg 12] difference in the results obtained with these instruments is due to the circumstance that different parts of the radiation are taken into account.
The usual photographic plates, which have their principal sensibility in the violet parts of the spectrum, give us the photographic magnitudes of the stars. ”

“ The camera is equipped with a special lens ground out of rock crystal. This lens lets in ultra-violet light which the ordinary lens shuts out, and X-ray film is especially sensitive to ultra-violet light. In order to be sure that we get enough illumination, I will set up these two ultra-violet floodlights to illumine the cage. The teller will have to wear glasses to protect his eyes and he'll get well sunburned, but something has to be sacrificed to science, as Dr. Bird is always telling me. ”

“ In the measuring machines in general use the field of view, as in the case of the glass-scale micrometer, is sufficiently large to include the image of the 5 mm. square. The microscope or viewing telescope is fitted with a spider-line micrometer having two screws at right angles to each other, by means of which readings can be made first on one réseau-line, then on the star, and finally on the opposite réseau-line in both co-ordinates. ”

“ The lens, carried in a short tube, has its axis directed to this image, just as it would be pointed at the luminary itself. In solar photography, the exposure is made through a very narrow slit in an opaque screen, which is caused to move rapidly in front of the image. Very fair photographic images of the sun, of several inches diameter, can, however, be obtained with an ordinary telescope of five feet or so focal length, by substituting a small photographic lens and camera in the eye-piece, and by enlarging the image in printing. ”

“ The most important optical instrument with which we have to deal is the projecting arc lamp and its optical system. The passage of light through this system of lenses is altogether different from that passing through a camera lens for instance. In a camera lens the picture is formed upon the ground glass screen by the light reflected from the object. From any single point of the object, rays of light strike all parts of the lens, pass through it, and are recombined or focused at some point behind it. ”