Patent GB-530308-A (GB)
Title: Improved apparatus for testing eyes
IPC: A61B3/06
Filed: 1939-03-09
Abstract: 530,308. Sight-testing apparatus. TENNANT, W. J. (American Optical Co., and Feldman, J. B.). March 9, 1939, No. 7706. [Class 97 (i)] Apparatus for testing eyes for night blindness and for determining a patient's pathological condition comprises two separate compartments in a common housing or in two housings on a common support, a high-intensity light source in one compartment, a low-intensity light source in the other, means for turning on and off these light sources, means for varying the illumination from the low-intensity source, and a light-diffusing screen for each source, the compartments being so arranged that each light source is in, or can be brought into, the field of vision of the eye to be tested so that this may remain still during test. Colour filters may be interposed in the low-intensity beam to produce light simulating daylight or when colour tests are desired. One form of apparatus shown in Fig. 12 comprises a stand 48 adjustably supporting a vertical pillar 49 bearing a housing 60 enclosing, in an upper compartment high-intensity electric lamps 87 provided with a reflector 88, and in a lower compartment a low-intensity electric lamp 90, the lamps being arranged respectively behind a vizored window 62 fitted with a diffusing screen 63, and an opening 71 rotatably supporting a sleeve 72 fitted with translucent screens 74, 75, and a blue glass filter 84. Mounted on the end of the sleeve 72 is an opaque disc 78 having a diametral light slot 79 and a protecting glass disc 80, and behind the light source 90 is an inspection window 91. Connection with the mains is provided for both lamps through a plug 93, the supply cable 95 thereto including an on-and-off switch 96. The high-intensity lamps 87 are fed through a separate switch 108 and regulating resistance ; and in the circuit of the low-intensity lamp 90 is included a variable rheostat to adapt it for different mains voltages. The housing has vent openings at top and bottom for cooling air. The patient stands in front of the windows 62, 71, and the lamps 87 are switched on for a period sufficient to bleach out the visual purple of the eye. Lamps 87 are then switched off and lamp 90 switched on, and the time interval until the patient reports that he can just see the slot 79 is noted. Lamp 90 is now also switched off for a pre-determined time, say 3 minutes, after which it is switched on again and the timeinterval until the patient reports that he can see the slot 79 is noted again. This procedure is repeated at spaced intervals for half or one hour, and a curve is plotted showing the rate of recovery of the visual purple from the initial effect of the high-intensity light. Comparison of such curves with similar curves charted for a normal eye, or for the eyes of persons suffering from certain diseases, permits the condition of the patient to be determined. The rotatable light slot 79 can be set, unknown to the patient at any angle, and is a safeguard against mendacity on the patient's part. A similar form of apparatus is described for making a single recovery-time test. A further form for chart-testing is also described in which the low-intensity lamp is in a fixed housing opposite a head-rest for the patient, and the high-intensity lamp is in a separate housing carried by a swingable arm whereby it may be moved into or out of position between the low intensity lamp housing and the patient. In this form the low-intensity lamp is displaceable in its housing towards and away from the patient, and its intensity of illumination is also controllable by an iris diaphragm ; and in operation, at the end of each 3-minute interval, the intensity of this light is increased by either means until the patient reports that he can see the light slit. The value of the intensity at this instant is computable and is a measure for the eye's recovery. This form may also include a rotary colour-filter turret disc for use when colour tests of the eye are desired.