Digital densitometer with auto-ranging

Details Digital densitometer with auto-ranging Digital densitometer with auto-ranging

1998-11-04

2001-05-01

Le, Que T. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C347S184000

Reexamination Certificate

active

06225618

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to density measurement devices and methods, and is particularly useful in imaging devices such as for example in electrostatographic reproduction apparatus.
BACKGROUND OF THE INVENTION
In the field of light measurement and densitometry, the need to convert from an electrical voltage proportional to light intensity to another signal proportional to optical density has long been recognized. One approach in the prior art has been to obtain a sensor voltage signal representing intensity of transmitted or reflected light and convert this analog signal to digital form. The digital value is then used to enter a stored LUT of intensity and density values. The digital density value corresponding to the digital intensity value is read from the LUT.
U.S. Pat. No. 5,117,119 discloses an automatic gain selection electronic circuit, along with a second LUT to obtain high accuracy and resolution over an increased range of large densities. The first (or “base”) LUT contains density values corresponding to an analog-to-digital converter output for the lowest gain. The second (or “range”) LUT is much smaller than the first LUT and contains the relative density corresponding to each available gain. It provides the additional density output associated with the gain selected. The two LUT outputs are summed to obtain the actual density measurement.
The three ranges illustrated in U.S. Pat. No. 5,117,119 are divided by two threshold values in a 10:1 ratio. Thus two ranges have 10:1 max-to-min light or voltage input ratios, and the third range (used for lowest light intensities or highest density) may have arbitrarily small input light or voltage level. The illustration in U.S. Pat. No. 5,117,119 shows accuracy of 0.01 density units, but requires a 10-bit analog-to-digital converter to do so. In turn, the 10-bit analog-to-digital converter requires a large “base” LUT of 2
10
=1024 entries. A major limiting factor in accuracy is the analog-to-digital converter resolution. Worst-case density resolution for each range comes at the high-density (low light intensity) end of the range, where the analog-to-digital converter resolution, i.e., one count, corresponds to the largest density increment. U.S. Pat. No. 5,117,119 uses multiple analog threshold voltage levels for comparison to the light sensor voltage signal. Low levels of electrical noise and circuit variability could degrade the comparator accuracy and reliability for the low-voltage thresholds.
SUMMARY OF THE INVENTION
It is an object of the present invention to use more but smaller gain ranges (roughly 2:1 ranges in a preferred embodiment) so that the same accuracy as in the device of U.S. Pat. No. 5,117,119 of 0.01 density units or better can be obtained with a less-expensive 8-bit analog to digital converter. The 8-bit analog to digital converter is also easier to interface to conventional LUT memories and digital microprocessors, which operate on 8-bit digital bytes. Furthermore, the 8-bit analog to digital converter requires a smaller “base” LUT of only 2
8
=256 entries, rather than 2
10
=1024 entries.
It is another object of the present invention to provide an improved automatic gain selection circuitry. The present invention uses relatively noise-resistant digital logic on the output of the analog to digital converter to select the amplifier gain such that the analog to digital converter output is toward the high end of the analog to digital converter output range, but not saturated at the maximum analog to digital converter count.
Accordingly, it is a feature of the present invention to provide an auto-ranging densitometer for determining the density of a test sample, wherein the densitometer includes a photodetector and an amplifier circuit adapted to produce an output signal proportional to intensity of light on the photodetector. The amplifier circuit has multiple gains that successively increase in an approximately 2:1 ratio increment. An analog to digital converter is adapted to convert the output signal of the amplifier circuit to digital format. A density signal generator circuit is adapted to receive the output of the analog to digital converter, whereby density resolution non-uniformity is minimized due to small gain increments such that the analog to digital converter needs deal with less extreme variations in density resolution.


REFERENCES:
patent: 4473029 (1984-09-01), Fritz et al.
patent: 4546060 (1985-10-01), Miskinis et al.
patent: 4550254 (1985-10-01), Zomorrodi et al.
patent: 4673807 (1987-06-01), Kobayashi et al.
patent: 5117119 (1992-05-01), Schubert et al.
patent: 5649266 (1997-07-01), Rushing
patent: 5678132 (1997-10-01), Shiba et al.

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