Patent GB-582673-A (GB)
Title: Improvements in photometers and photometric exposure meters
IPC: G01J1/30
Filed: 1944-08-22
Abstract: 582,673. Photometers and photographic exposure meters. DUNN, J. F., and PLANT, G. S. Aug. 22, 1944, Nos. 15978 and 25416. [Classes 97 (i) and 98 (i)] [Also in Groups XXXVI and XL] A comparison photometer of the kind described in Specification 508,122, see Fig. 4 thereof, comprises a tubular head 16 having an eyepiece 20 at one end and an objective at the other. At the common focus of the lenses is a small silvered patch 82 carried on a glass plate 80 inclined at 45 degrees to the axis of head 16. Patch 82 is arranged to be illuminated by a lamp 24 energized by a dry cell 26 which is mounted in a removable sleeve 46 in the barrel 30 which depends at right angles to the head. A rheostat 38 mounted in a removable base 44 at the bottom of the sleeve, controls the brightness of the lamp. The lamp shines on a diffusing screen 28, light from which is collected by a lens 76 and passes through oppositely graduated gear wedges. These are movable by rotating sleeve 46 and gear 66 meshing with racks mounted in the barrel 30 and carrying the wedges. The brightness of any area of the image seen in the eyepiece is compared with that of patch 82 and the brightness of the patch adjusted to equality by rotating sleeve 46. The corresponding brightness may be read by noting the position of an index mark on a retaining ring 48 secured on sleeve 46 with reference to a scale engraved on barrel 30. If the instrument is to be used as an exposure meter, the wedges 22 are shaded so that equal angular movements of sleeve 46 cause variations of luminosity of patch 82 in equal ratio. Logarithmic scales are then employed and an independently rotatable ring 100 is provided to enable the scales to be set for different emulsion speeds or lens apertures. The brightness of the lamp 24 is kept constant as cell 26 ages by adjusting rheostat 38 to cause a predetermined indication of a galvanometer 34 mounted at the top of the barrel, the galvanometer indicating the output of a photo-electric element 32 illuminated by the lamp (see Group XL). Instead of the plate 80 carrying patch 82, two 90 degrees prisms having their hypotenuse surfaces cemented together may be employed, patch 82 being formed on one of the cemented surfaces. The two grey wedges 22 may be replaced by crossed polarising screens, one of which is fixed and the other, 84, Fig. 14, being rotatable with a gear 86 supported in barrel 30 and engaged by a rack 88 which is cammed across the barrel by engagement of arm 90 rigid with the rod with roller 92 on a pin 94 fixed in the top of sleeve 46. To'extend the range of the instrument, neutral filters. carried in rotatable carriers 118, 120 may be moved into the path of the beam, carrier 118 having a clear aperture and two neutral filters of different densities, while carrier 120 may carry a clear or blue filter and a neutral or blue-grey filter. Carrier 118 may carry an arcuate filter-holder 130 for inserting filters between the lenses 16 and 20 as shown in Fig. 10. An attachment is mounted on holder 130 for inserting filters between the lenses 18 and 20 is shown in Fig. 10. An attachment 140 may be provided on the end of head 16 for examining photographic negatives. This includes a collecting lens 138 and can be used to examine opaque objects by rotating sleeve 144 to open apertures 146, the object being applied flat across the end of sleeve 144. Specification 141,825, [Class 97 (i)], also is referred to.