GB504786A - Improvements in photo-electric exposure meters - Google Patents

Abstract

504,786. Photo - electric exposure meters. TONNIES, H. F. Oct. 25, 1937, No. 29126. [Class 40 (iii)] Relates to photographic exposure meters having a number of measuring ranges, for adapting the instruments for use in either strong or weak light. According to the invention, in order to vary the measuring angle (i.e. the. angle of the cone of incident light) at the same time as the measuring range is changed, a photo-electric meter having a tubular chamber disposed in front of the photocell to determine one measuring angle is provided with a stop for reducing the measuring angle, the stop being in the form of a lid, displaceably mounted on the front end of the chamber, remote from the cell, and having one or more light apertures of reduced area in com. parison with that of the mouth of the chamber. Alternatively, the stop consists of a separable disc slidable within the chamber and similarly provided with one or more apertures of reduced area. Fig. 2 shows diagrammatically a form of the invention in which the stop is a lid 6 hinged at 7 to a tube 3 at one end of which a photo-electric cell 1 is mounted, a small aperture 2 surrounded by a short tube being formed in the lid. In Fig. 1 is shown a construction in which the stop is a disc 4 removably inserted in the tube 3, the small aperture 2 in the disc being surrounded by an upstanding tube. Various modified forms of the device are described and shown. For example, in the construction shown in Figs. 5 and 6, the stop is formed as the apertured lid 11 of a casing 10 enclosing the galvanometer connected to the photo-cell 1, and the tubular chamber in front of the cell is divided into compartments 8<1>. The lid is closed under conditions of bright illumination, and when a comparatively small measuring angle is required. 28 Is a "finder" built into the instrument. Lenses may be arranged above the compartments 8<1>, and the latter may be omitted whilst the lenses are provided. A lens may be provided in the aperture in the stop member, and in one con. struction, Fig. 12, in addition to this lens 20<1> there is a large lens 19 in the chamber 3, covering the cell 1. A light-absorbing disc 25 may be arranged as shown. Where necessary, the wall of the light aperture in the stop may be ribbed or otherwise treated to prevent direct reflection of sunlight on to the photo-electric cell. A "finder" may be mounted on the instrument, to indicate, by means of circles &o. in the field of vision the measuring angles that are available.