COMPARISON DENSITOMETER INCLUDING MEANS TO MAINTAIN THE DETECTOR OUTPUT AT A MEANS VALVE

• US 3,623,817 A
• Filed: 02/24/1970
• Issued: 11/30/1971
• Est. Priority Date: 02/24/1970
• Status: Expired due to Term

• Current Assignee: Tino Celio
• Original Assignee: Tino Celio
• Inventors: Celio, Tino
• Primary Examiner(s): Wibert, Ronald L.
• Assistant Examiner(s): Chew, II, Orville B.
• Application Number:
  US05/013,650
• Time in Patent Office: 644 Days
• Field of Search: 356/179,205,212 250/204,22SD,233
• US Class Current: 356/408
• CPC Class Codes: G01J 1/16 using electric radiation detectors G01J1/20 takes precedence

Abstract

This invention relates to a densitometer for measuring small differences in density. The densitometer comprises a light source providing a reference light beam for imaging on a reference object and a measuring light beam for imaging on an object whose density relative to that of the reference object is to be measured. Light reflected from the two objects is transmitted to a photodetector via an optical chopper that sequentially interrupts the transmitted beams. The photodetector produces sequential pairs of voltages which are applied to a logarithmic element in the form of a diode which produces pairs of logarithmic voltages for application to a polarity reverser which changes the polarity of one logarithmic voltage of a pair relative to the other voltage of the same pair. The voltages of each pair are then applied to an indicating instrument where they are utilized to give an indication of the difference in density between the two objects. The pairs of logarithmic voltages produced by the logarithmic element are also applied to a control circuit which controls either the intensity of the source or the amplification of the photodetector so that the voltages produced by the photodetector and applied to the logarithmic element are always within a predetermined range of amplitudes.

Claims

1. A densitometer comprising a light source, means for directing a reference and a measuring light beam from said source alternately onto a reference object having a predetermined density and an object whose density is to be measured relative to the density of the reference object respectively, photodetector means receiving light transmitted from said reference and measuring objects and producing in response thereto output signals representative of the intensity of the transmitted light, a logarithmic element responsive to the signals provided by said photodetector means for providing signals whose magnitudes vary logarithmically with variations in intensity of the light transmitted to said photodetector means, and a control circuit responsive to the signals provided by said logarithmic element to maintain the signals produced by said photodetector means at substantially a mean value corresponding to the mean intensity of the light transmitted to said photodetector from said reference and measured objects so that the logarithmic element works substantially within a predetermined range of magnitudes of the signals applied thereto.

2. A densitometer according to claim 1 including means varying the intensity of said source and coupled to said control circuit to maintain the signals produced by said photodetector means substantially at said mean value.

3. A densitometer according to claim 1 including means varying the amplification of said photodetector means and coupled to said control circuit to maintain the signals produced by said photodetector means substantially at said mean value.

4. A densitometer according to claim 1 including an optical chopper sequentially interrupting said reference and measuring light beams causing said logarithmic element to produce pairs of sequential signals representing the intensity of the light transmitted to said photodetector means from said reference object and measuring object.

5. A densitometer according to claim 1 including an optical chopper sequentially interrupting said reference and measuring light beams, a polarity reverser coupled to the output from said logarithmic element and synchronously operable with said optical chopper so that the signal produced by said photodetector means on one of said beams being transmitted thereto is reversed in polarity relative to the signal produced by said photodetector means on the other of said beams being transmitted thereto, and an indicating instrument coupled to said polarity reverser to provide an indication of the difference between the density of said object and said reference object.

6. A densitometer according to claim 5 including a value correction circuit producing a value correction signal whose amplitude is adjustable, a gate circuit coupled to said value correction circuit, and an adder circuit having one input coupled to the output of said gate circuit, another input coupled to the output of said logarithmic element and an output coupled to said polarity reverser and said control circuit, and means operable in synchronism with said optical chopper and polarity reverser opening said gate circuit to allow a correction value signal to be added by said adder circuit to one of the signals produced by the logarithmic element whEn one of said light beams is transmitted to said photodetector means.

7. A densitometer according to claim 1 including a plurality of different color filters, and means for interposing said filters successively in the path of said light beams, and in which said control circuit includes a control voltage detector detecting the maximum voltage produced by said logarithmic element as each of said different color filters are interposed in both said paths, said densitometer further including automatic control means responsive to the operation of said control voltage detector to position in the paths of said light beams the filter producing said maximum voltage.

8. A densitometer according to claim 4 including a disc having a plurality of different color filters disposed radially thereon, a store, a distributor, drive means rotating said disc and driving said distributor, said distributor distributing to said store pairs of signals produced by said logarithmic element as each filter is interposed in both said light beam paths and both said light beams are interrupted by said optical chopper, and wherein said control circuit includes a control voltage detector detecting the maximum mean voltage of each pair of voltages produced by the logarithmic element as each disc is interposed in both said light beam paths and said light beams are interrupted by said optical chopper, and a gating circuit responsive to said maximum voltage for gating out of said store the pair of voltages producing said maximum means voltage.

9. A densitometer according to claim 4 including a disc having a plurality of different color filters disposed radially thereon, a store having n storage locations, a distributor, drive means rotating said disc and driving said distributor, said distributor distributing to the n storage locations pairs of signals produced by said logarithmic element as each filter is interposed in both said light beam paths and n indicating instruments each coupled to a different one of said n storage locations to indicate the difference in density between the reference object and the measuring object as represented by the difference in magnitude between the signals of a pair when a filter of said plurality is interposed in said light beam paths.

10. A densitometer comprising a light source, means for directing a reference and a measuring light beam from said source onto a reference object having a predetermined density and an object whose density is to be measured relative to the density of the reference object respectively, photodetector means receiving light transmitted from said reference and measuring objects and producing signals representing the density of the reference and measuring objects, an optical chopper for sequentially interrupting said beams so that said photodetector means produces pairs of sequential pulses representing the density of the reference and measuring objects, a logarithmic element responsive to the signals produced by said photodetector means for providing signals whose magnitudes vary logarithmically with variations in the intensity of the light transmitted to said photodetector means, a polarity reverser coupled to the output of said logarithmic element and synchronously operable with said optical chopper so that a signal of a pair produced by said photodetector means on one of said beams being transmitted thereto is reversed in polarity relative to the other signal of the same pair produced by said photodetector means on the other of said beams being transmitted thereto, and a control circuit responsive to the pairs of signals provided by said logarithmic element to maintain the pairs of signals produced by said photodetector means at substantially a mean value corresponding to the mean intensity of the light transmitted to said photodetector means so that the logarithmic element works substantially within a predetermined range of magnitude of the signals applied thereto.

11. A densitometer according to claim 10 including means varying the intensity of said source and coupled to said control circuit to maintain the signals produced by said photodetector means substantially at said mean value.

12. A densitometer according to claim 10 including means varying the amplification of said photodetector means and coupled to said control circuit to maintain the signals produced by said photodetector means substantially at said mean value.