Reflection densitometer

A densitometer for measuring the reflectance of a surface, said densitometer comprising:

• a light source positioned to project light rays toward a surface whose reflectance is to be measured;
• a photodetector positioned to receive light rays reflected from the surface, said photodector having an output which has a signal component characteristic of the amount of reflected light received and a noise component;
• a logarithmic amplifier for amplifying the output of said photodetector;
• means for switching said light source off and on so as to create alternating first and second time periods in which the output of said photodetector is characteristic of only a noise component and in which the output of said photodetector is characteristic of both said signal component plus a noise component, respectively;
• and
• means for subtracting a logarithmically amplified output of said photodetector detected during said second period from an amplified output of said logarithmic amplifier during said first period to obtain a signal substantially characteristic of reflected light received by said photodetector.

A densitometer for measuring relative optical densities of a developed toner image on the surface of an image receiver, said densitometer comprising:

• a light emitting diode positioned to project light rays toward a developed toner image on the image on the surface of an image receiver;
• a photodiode positioned to receive light rays reflected from the toner image, said photodiode having an output which has a signal component characteristic of the amount of reflected light received and a noise component;
• a logarithmic amplifier for amplifying the output of said photodiode;
• means for switching said light emitting diode off and on so as to create alternating first and second time periods in which the output of said photodiode is characteristic of only the noise component and in which the output of said photodiode is characteristic of both said signal component plus a noise component, respectively;
• and
• means for subtracting a logarithmically amplified output of said photodiode detected during said second period from a logarithmically amplified output of said photodiode detected during said first period to obtain a signal substantially characteristic of reflected light received by said photodiode.