Patent GB-930610-A (GB)
Title: Electro-optical transducer
IPC: H04N1/40
Filed: 1962-03-27
Abstract: 930,610. Photo-electric photometers. EASTMAN KODAK CO. March 27, 1962 [March 27, 1961], No. 11696/62. Class 40 (3). The invention relates to an electro-optical transducer providing at its output a voltage which is inversely proportional to the logarithm of the intensity of its input light and comprises a reference light source, light control means for directing the input light and the reference light for alternate periods to an electro-optical device which provides an output signal whose voltage has a linear relationship to the logarithm of the intensity of the light directed thereon, means for setting the average level of the output signal from the electro-optical means to a predetermined reference value and signal gating means operating synchronously with the light control means for applying the said output signal whose average level has been set to the reference value to the transducer output only during those periods when the reference light is directed to the electro-optical means. In the embodiment shown the invention is associated with a photographic colour printer which consists of a light source 11 shining through a negative 12, through a lens 14 on to a printing paper 15. To obtain colour density information required to estimate and control the exposure of the paper 15, some of the light 17 from the negative 12 is directed by a mirror 18, through a lens 19 to form an image in the plane of a rotary disc mirror 20 having two opposite 90-degree sector openings (Fig. 2, not shown) driven by a constant speed motor 21. The mirror 20 when rotating alternately allows light from the negative and light from a reference source 30 to strike the input face 34 of a light integrating bar 35 consisting for example of a block of transparent material having highly polished walls. An output facet 36 of the bar 35 associated with a red filter 40R passes light to a photocell unit 41R which produces at its output 42R a voltage directly proportional to the logarithm of its input light. Fig. 3 (not shown) shows this output intensity waveform when the intensity of the red component of the light from the negative 12 is greater than that from the reference source 30, the portions 61 and 60 corresponding respectively to the light from the negative 12 and the source 30. This output is coupled to a cathode follower 44R where the average signal voltage level 62 is brought to a predetermined level 62<SP>1</SP>, Fig. 4, and then fed to the input of a signal gating means 46R whose output is the red signal output terminal 47R. The gating means 46R is switched in synchronism with the rotation of the disc 20 by a monostable multivibrator pulse generator 49 which is triggered by a magnetic sensing element 50 responsive to two magnetic elements 51 carried on the disc 20. The gating pulses 63, 63<SP>1</SP>, Fig. 4, fed to the gating means 46R are synchronised with the rotation of the disc so that the gating means is conductive only when light received by the bar 35 is from the reference source'30 so that the output at the gating means 46R is small as shown by 60<SP>11</SP>, Fig. 5. Corresponding waveforms for the case in which the intensity of the red component of the light from the negative 12 is less than that from the reference source 30 are shown in Figs. 6 (not shown), 7, 8, a high output voltage 160<SP>11</SP>, Fig. 8, being obtained. From this it can be seen that the output signal level varies inversely with the intensity of the input light. By similar means output signals corresponding to light in the blue and green spectral bands are obtained at terminals 47B and 47G respectively. U.S.A. Specification 2,413,706 is referred to.