Coded data generation or conversion – Analog to or from digital conversion – Nonlinear
Reexamination Certificate
2002-03-11
2004-09-14
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Nonlinear
C250S2140DC, C331S010000
Reexamination Certificate
active
06791485
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to densitometers for measuring optical density. In particular, the invention relates to optical density measurement of ink-covered or toner-covered surfaces produced by apparatus such as printers, and to photographically printed areas.
BACKGROUND OF THE INVENTION
In electrostatographic apparatus such as copiers and printers, automatic adjustment of process control parameters is used to produce images having well regulated darkness or optical density. Copier and printer process control strategies typically involve measuring the transmissive or reflective optical density of a toner image on an exposed and developed area (called a “test patch”) of an image receiver. Optical density has the advantage, compared to transmittance or reflectance measures, of matching more closely to human visual perception. A further advantage, especially for transmission density, is that density is approximately proportional to the thickness of the marking material layer, over a substantial range.
Typically, toned process control test patches are formed on the photoconductor in interframe regions of the photoconductor, i.e., between image frame areas. An “on-board” densitometer measures the test patch density, either on the photoconductor or after transfer of the patches to another support member. From these measurements, the machine microprocessor can determine adjustments to the known operating process control parameters such as primary charger setpoint, exposure setpoint, toner concentration, and development bias.
A transmission type of densitometer is particularly well suited to transmissive supports. In this type, a light source projects light, visible or infrared, through an object onto a photodetector such as a photodiode. In a copier/printer, the photoconductor passes between the light source and the photodetector. When the photoconductor has toner on the surface, the amount of light reaching the photodetector is decreased, causing the output of the densitometer to change. Based on this output, the amount of toner applied to the photoconductor can be varied as required in order to obtain consistent image quality. Another type of densitometer such as described in U.S. Pat. No. 4,553,033 to Hubble, III et al uses reflected light flux rather than transmitted light flux to determine density, and is particularly suited to opaque reflective supports.
One well-known approach to converting to a density measure uses an analog logarithmic amplifier, as suggested by the mathematical logarithm function in the definition of optical density:
D
=−log
10
(
T
) Equation (1)
where D is optical density, and T is transmittance or reflectance (for transmission density or reflection density, respectively). The subscript “10” indicates that the logarithm is to the base
10
. Since T must be between 0 and 1, the logarithm of T is negative, and the minus sign (−) in equation (1) provides positive values for density, D.
The following U.S. Patents, for example, all teach the use of an analog logarithmic amplifier in a densitometer: U.S. Pat. No. 3,918,815 to Gadbois, U.S. Pat. No. 5,148,217 to Almeter et al, U.S. Pat. No. 5,173,750 to Laukaitis, and U.S. Pat. No. 5,903,800 to Stern et al. The high cost of precision analog logarithmic amplifiers does not seriously deter their use in expensive laboratory instruments. However, the high cost of analog logarithmic amplifiers has been an obstacle to the wide use of densitometers as built-in components within moderately priced copiers, printers, and other products.
Digital approaches to densitometer design have been developed, as digital electronics improve in performance and decline in price, relative to analog logarithmic amplifiers. One digital approach in the prior art is to obtain a photodetector 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 lookup table (LUT) of and density values. The digital density value corresponding to the digital intensity value is read from the LUT. To cover a reasonably large range of density with the required resolution, an amplifier with selectable gain has been used.
U.S. Pat. No. 5,117,119 to Schubert et al discloses an automatic gain selection, i.e., an “auto-ranging” 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 (A/D) 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 density increment associated with the gain selected. The two LUT outputs are summed to obtain the actual density measurement. LUT approaches are also disclosed by Rushing et al in U.S. Pat. Nos. 6,222,176 and 6,225,618, and by Rushing in U.S. Pat. No. 6,331,832.
Prior art digital densitometers, such as those in the aforementioned disclosures, typically have an analog amplifier stage near the photodetector end of the circuit, before converting the measurement signal to digital form. While the signal is in analog form, it is especially vulnerable to corruption by electrical noise and small voltage offsets.
Moreover, the analog amplifiers in prior art densitometers have a selectable gain, requiring the switching of the analog measurement signal. Even small noise levels and error introduced in switching can have a relatively large effect on a small measurement signal. For example, integrated circuit analog switches typically have an “ON” resistance of several 10's to a few 100's of ohms, which varies with the voltage being switched and other factors. This resistance alters the amplifier gain, introducing a variable error, which is difficult to compensate. Unwanted switching transients during gain changes compound the problem.
Another digital approach to digital densitometry is suggested, without design specifics, by Edwards in the November, 1996 issue of “Nuts and Volts” magazine. Edwards suggests using a light-to-frequency (L-to-F) converter integrated circuit as a photodetector in a densitometer. In U.S. Pat. No. 6,188,471, Jung et al disclose the use of such L-to-F converters for the measurement of optical properties such as color, gloss, and translucence. Jung et al do not address the logarithmic conversion underlying optical density measurement.
In a L-to-F converter, the output frequency is proportional to the incident light intensity. Reflected or transmitted light incident on the converter can be measured in intensity by one of two methods, as suggested in the “TSL230 Evaluation Module Users Guide,” by Texas Instruments, Inc. One method is to count the converter output pulses during a fixed time period, yielding a frequency count Optical density can then be determined from the known relationship of intensity, frequency and optical density. The other method is to measure the period or pulse width by counting clock pulses during a single output pulse from the converter, yielding a period count. Optical density can then be determined from the known relationship of intensity, converter output period and optical density.
In the frequency count method, updated density measurements are obtained only as often as the fixed counting time period. This counting time must be long enough to provide a large frequency count to obtain good density resolution, even for low frequencies (high optical density).
In the period count method, updated density measurements are obtained with every measured period For very high incident light intensities (low optical density) the frequency is very high, and the period so short that the count of clock pulses is small, and the conversion to density has insufficient resolution. For very low incident light intensities (high optical density), the frequency is very low, and the period so long that the count of clock pulses may cover a ver
Nguyen Linh V
Tokar Michael
LandOfFree
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