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110 Transactions of S.M.P.E., July 1927
some means of carrying the light from this source into the set. The prevaihng method, up to this time, has been to use parabohc mirrors of 24 and 36 inch diameter, with a 150 ampere arc. These mirrors are very expensive, heavy, and easily breakable; a mirror frequently breaks in the first week of use. Consequently, the studios have welcomed this polygonal mirror from the economic, as well as the practical angle.
Floodlight Beam from Parabolic Mirror.
A floodlight beam from a parabolic mirror has certain peculiarities that seriously interfere with its usefulness as a source of general illumination. On the other hand it has certain excellent features and it has been successfully used in many studios. The mirror is usually mounted in a barrel to support it and to cut off the radiation of arc light to the sides and rear. This barrel enters into the optics of the beam and in every case where the barrel is of normal proportions it leads to a loss of light as can be illustrated by the aid of Figs. 1 and 2.
If the arc is moved from the focal point along the axis of projection so as to increase its distance from the mirror the beam may be made to converge to any desired degree. After the beam passes through the point of convergence it becomes divergent and spreads to the desired width. This bteam when formed by the use of a searchlight mirror of the precision type has great smoothness of texture, being generally free from images or dark spots, but there are two features about it that must be ranked as defects of the first order. The edges of the beam are much brighter than the centralparts. This change in brightness is gradual and therefore often not strikingly apparent to the eye, but it nevertheless reduces the effectiveness of the central parts and wastes light around the edges where it is not useful. The second defect is the presence of a dark spot or shadow of the lamp mechanism that appears in the exact center and along a radial zone to the upper edge of the beam. This is illustrated in Fig. 3 where the curve and sketches are from physical measurements made on an experimental floodlight beam. This mechanism shadow can be reduced in harmfulness but not entirely eliminated by moving the heads several inches below the axis. This moves the spot towards the top of the beam, but does not entirely remove it from the field. In Fig. 7, curve A, the distribution is seen to be unsuitable for general floQdhghting, although in the particular floodhght of which this is an actual test, the lamp was lowered several inches in order to avoid the central dark spot.