Optical densitometer

As a subcombination in an optical densitometer having a photomultiplier tube with constant anode current when measuring light intensities from optical density samples, apparatus for providing an analog reference anode signal, comprising:

• clock means for generating a pulse train,
• counter means for counting pulses from said pulse train,
• counter control means for counting up or counting down from a digital count output from said counter,
• means for obtaining an analog reference signal exponentially related to said digital count output from said counter, and
• comparator means for generating a control signal to said control means functionally related to the relative magnitudes between said analog reference signal and an anode signal produced by a reference sample light intensity,
• said control signal cooperating with said control means to establish an analog reference anode signal equal to said anode signal produced by said reference sample light intensity.

A method for setting the reference operating parameters for a high resolution optical densitometer using a photomultiplier tube, comprising the steps of:

• generating a first reference signal functionally related to a first reference sample light intensity,
• maintaining a voltage across a photomultiplier tube dynodes at a level functionally related to said first reference signal,
• exposing said photomultiplier tube to a second reference sample light intensity to obtain an anode signal,
• comparing said anode signal with a second analog reference anode signal to obtain a second control signal functionally related to the relative magnitudes of said anode signal and said second reference,
• adjusting said second analog reference anode signal to a value exponentially related to said second control signal to obtain substantially equal relative magnitudes for said anode signal and said second reference signal while maintaining said voltage across said photomultiplier tube dynodes, and thereafter
• retaining said adjusted second analog reference anode signal during subsequent optical density measurements.