Digital optical density for measuring the difference in optical density between an absorber of known density and a sample absorber

Abstract

Abstract An apparatus is provided for determining a difference in optical density between an absorber of known density and a sample absorber. An exponential decaying voltage is connected in series but of opposite polarity to a voltage source which is proportional to photodetector current flowing in a memory load. The current is generated by light of a specified wavelength passing through a standard optical absorber. The instant when the magnitude of the exponential voltage is equal to that generated by the photodetector, i.e., when the voltage across the combination is zero, the memory load is adjusted so that a specified number of clock pulses, No, (fixed by the clock frequency and the time constant of the exponential voltage) are counted by a ripple counter. A sample of unknown optical density at a specified wavelength is then measured, providing a voltage proportional to a selected calibrated memory standard. At the instant of time when this voltage, in combination with the exponential voltage is equal to zero, the difference in the optical density between the unknown and standard absorbers is equal to the absolute value of the difference between the reference count No and the count in the ripple counter. The density information is gated into a decade counter and read out digitally. Circuitry is provided for indicating positive or negative difference in density, and a reset automatically recycles the device at the end of a measurement. The density difference measurement is not affected by absolute light intensity or absolute photodetector sensitivity. This device is particularly useful in ''''White light'''' color printing methods.

Inventors

• 👤 P Bey
• 👤 M Bey

Bibliographic Details