Double beam optical densitometer

Claim 2

An optical densitometer, comprising: a light source; a rotatable grey wedge disc; a rotatable chopper disc; means for rotating said grey wedge disc and said chopper disc; photo-sensitive means producing signals representative of light falling thereon; means for directing a measuring light beam from said light source through said grey wedge and chopper discs, and an object, the optical density of which is to be measured, onto said photo-sensitive means; means for directing a reference light beam from said light source through said chopper disc and onto said photo-sensitive means; said rotatable chopper disc alternately intercepting said light beams; a comparator; means for directing signals derived from said photo-sensitive means and representative of the intensities of said light beams to said comparator; a counting pulse transmitter; means for selectively varying the ratio between the rotational speed of the grey wedge disc and the pulse repetition rate of the counting pulse transmitter; a counter; and means responsive to the rotation of the grey wedge disc for connecting the counter to the counting pulse transmitter when said grey wedge disc passes a predetermined angular position, said comparator stopping said counter when the input signals to the comparator have a predetermined relationship.

Claim 3

An optical densitometer as claimed in claim 1, wherein said means for rotating comprises a motor and a variable transmission gear, said transmission gear drivingly connecting said motor to said grey wedge and chopper discs.

Claim 4

An optical densitometer as claimed in claim 2, wherein the variable transmission gear is infinitely variable.

Claim 5

An optical densitometer as claimed in claim 1, wherein the frequency of the counting pulse transmitter is determined bY the rotational speed of the chopper disc, each alternation between an open sector of the chopper disc and a following opaque sector in the direction of movement producing a counting pulse of the counting pulse transmitter.

Claim 6

An optical densitometer as claimed in claim 1, wherein the counting pulse transmitter comprises an independent pulse generator and wherein the ratio of the rotational speeds of the grey wedge and chopper discs is constant and wherein there is interposed between the pulse generator and the counter means for selectively varying the frequency of the pulse generator, said means comprising a frequency multiplier and a frequency mixer.

Claim 7

An optical densitometer as claimed in claim 1, wherein the counting pulse transmitter comprises an independent pulse generator and wherein the ratio of the rotational speeds of the grey wedge and chopper discs is constant and wherein the pulse frequency of the pulse generator is independently variable.

Claim 8

An optical densitometer as claimed in claim 1, comprising a light attenuator disposed in the path of the reference beam, said attenuator being so adjusted that the intensities of the measuring and reference beams, subsequent to passing through the maximum density zone of the grey wedge disc and attenuator respectively, are the same.

Claim 9

An optical densitometer as claimed in claim 1, wherein said grey wedge disc activates said photo-sensitive means when passing said angular position and wherein means responsive to the comparator deactivate the photo-sensitive means when the input signals thereto are equal.

Claim 10

An optical densitometer as claimed in claim 1, comprising a starting pick-off positioned adjacent to said grey wedge disc, an AND-gate connected between the starting pick-off and the counter and a variable delay stage connected between the starting pick-off and the AND-gate, said starting pick-off being responsive to the passage of the predetermined angular position on said grey wedge disc to actuate said AND-gate and connect the counting pulse transmitter to the counter.

Claim 11

An optical densitometer as claimed in claim 9, wherein said starting pick-off comprises a photo-electric detector, and wherein said grey wedge disc includes a mark mounted on said disc at the transition point between the zones of maximum and minimum density, said mark defining said angular position.

Claim 12

An optical densitometer as claimed in claim 1, comprising an indicating instrument and means responsive to the passage of said grey wedge disc past said predetermined angular position for transferring the count recorded by the counter to said indicating instrument.

Claim 13

An optical densitometer comprising: a rotatable grey wedge disc; a rotatable chopper disc; means for rotating said grey wedge disc and said chopper disc; at least one light source; a plurality of photoelectric elements; means for directing each of a same plurality of measuring light beams from a light source through said optical wedge and chopper discs and through one of a same plurality of objects to be subjected to density measurement onto one of said photo-electric elements; means for directing each of a same plurality of reference light beams from the same light source through said chopper disc and onto the corresponding photoelectric element; said rotatable chopper being adapted to alternately intercept each light beam in each pair of measuring and reference light beams; a same plurality of comparators; means associated with each photoelectric element for directing separate signals representative of the intensity of the corresponding light beams to one of said comparators; counting pulse transmitter means; means for selectively varying the ratio between the rotational speed of the grey wedge disc and the number of pulses generated by the counting pulse transmitter means; a plurality of counters, and; means responsive to the rotation of the grey wedge disc for connecting each countEr to the counting pulse transmitter means when said grey wedge disc passes a predetermined angular position, each comparator stopping the corresponding counter when the input signals thereto have a predetermined relationship.

Claim 14

An optical densitometer comprising: a rotatable grey wedge disc; a rotatable chopper disc; means for rotating said grey wedge disc and said chopper disc; at least one light source; a plurality of photoelectric elements; means for directing each of a same plurality of measuring light beams from a light source through said optical wedge and chopper discs and through one of a same plurality of objects to be subjected to density measurement onto one of said photo-electric elements; means for directing each of a same plurality of reference light beams from the same light source through said chopper disc and onto the corresponding photoelectric element; said rotatable chopper being adapted to alternately intercept each light beam in each pair of measuring and reference light beams; a same plurality of comparators; means associated with each photoelectric element for directing separate signals representative of the intensity of the corresponding light beams to one of said comparators; counting pulse transmitter means; means for selectively varying the ratio between the rotational speed of the grey wedge disc and the number of pulses generated by the counting pulse transmitter means; a plurality of counters, and; means responsive to the rotation of the grey wedge disc for connecting each countEr to the counting pulse transmitter means when said grey wedge disc passes a predetermined angular position, each comparator stopping the corresponding counter when the input signals thereto have a predetermined relationship.

Claim 15

An optical densitometer as claimed in claim 12, wherein said counting pulse transmitter means comprises a single pulse generator.

Claim 16

An optical densitometer as claimed in claim 13, comprising a same plurality of starting pick-offs positioned adjacent to said optical wedge disc, a same plurality of AND-gates connected between the starting pick-offs and the corresponding counters, and a same plurality of variable delay stages connected between the corresponding starting pick-offs and AND-gates; the variable delay stages being so set that each corresponding counter is connected to the counting pulse generator at the time coinciding with the activation of the corresponding pick-off by the passage of a predetermined position on the optical wedge disc.