Optics: measuring and testing – For light transmission or absorption
Reexamination Certificate
2000-04-03
2003-05-20
Dang, Hung Xuan (Department: 2873)
Optics: measuring and testing
For light transmission or absorption
C355S117000
Reexamination Certificate
active
06567171
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 toner-covered test patches or other areas for controlling process parameters in electrostatographic apparatus such as copiers and printers.
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 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.
Whether of the transmissive or reflective type, the densitometer photodetector signal is input to signal processing circuitry, either analog or digital. The modem trend is toward digital circuitry, such as disclosed in U.S. Pat. No. 5,117,119 to Schubert et al. The Schubert et al densitometer is auto-ranging, where one of several available ranges is utilized according to the density of the test sample. The individual ranges span a factor of 10 in transmittance or reflectance, equivalent to 1.0 density units.
Photoconductors tend to be fatigued, i.e., degraded in their photoconductive characteristics for subsequent imaging cycles, after long or repeated exposure to light, resulting in degraded image quality. Improvements in photoconductor formulation, disclosed for example in U.S. Pat. No. 4,397,932 to Young, have been helpful in reducing the fatigue problem, but fatigue remains an issue for many photoconductors in use today. For this reason, copiers and printers are typically designed to minimize the exposure of the photoconductive member to unwanted non-imaging light, such as room light. Light-tight machine enclosures, careful service procedures, and careful photoconductor belt or drum packaging and installation are typically used to minimize the room light exposure. However, light sources within the machine enclosure remain a potential cause of photoconductor fatigue.
In some instances, light sources within the machine enclosure can be shielded from the photoconductor. This can be effective with electro-optical devices commonly used to sense the position and motion of image receiver sheets, for example.
In many copier/printer configurations, an on-board densitometer employs a light source directed at a spot on the photoconductor, for the aforementioned purpose of process control. Shielding would defeat the function and purpose of the densitometer. After prolonged exposure of the photoconductor by the densitometer light emitter, fatigue and image defects can result. The severity of the problem depends on the spectral sensitivity of the photoconductor and the spectral emission of the densitometer light source, as well as the intensity and duration of the exposure.
In some applications the spectral sensitivity of the photoconductor does not match the spectral emission of the densitometer light source. For example, a visible red-sensitive photoconductor may not be significantly affected by the emissions of a densitometer having an infrared light-emitting diode (LED) light source. In that case the emitter may be left fully energized indefinitely, even when the photoconductor is motionless between print jobs, without causing image defects. Continuous operation in this manner is not only convenient, but also avoids warm-up effects except when first turned on.
However, many preferred photoconductors have spectral sensitivity extending through the visible range into the near infrared. In these cases the infrared light emitted from the densitometer and incident on the photoconductor can cause significant fatigue, leading to defective images. The problem is most acute in any spot on the photoconductor that is parked motionless opposite the energized densitometer light emitter when the machine is idle between jobs.
Another problem with on-board densitometers is that their typical LED's or other emitter types do not have the ideal constant light intensity for density measurement. Short-term instability results from temperature sensitivity during warm-up. In continuous-mode operation at a fairly high LED current, unstable warm-up periods of a minute or more are commonly observed. Additional longer-term instability results from gradual degradation of the LED with age. Complicated and expensive approaches may be required to avoid inaccurate measurements due to these instabilities. Such approaches include extended warm-up periods, temperature compensation, intensity feedback control, and periodic recalibration.
Operating an LED in a pulsed-mode is a well-known approach to reducing average power dissipation and reducing PN junction temperature rise. When a current pulse is applied, the typical LED PN junction temperature response includes a component with a fast time constant of about 10 to 40 milliseconds. Pulsed-mode operation with a pulse-width much less than this, say a few hundred microseconds or less, along with a low duty-cycle, minimizes temperature rise, improves light emission stability during warm-up, and prolongs LED useful life.
In densitometer applications, pulsed-mode operation has been used to isolate a density signal from an ambient light or noise signal, as in U.S. Pat. No. 5,173,750 to Laukaitis, and U.S. Pat. No. 5,900,960 to Reime. Benwood et al (U.S. Pat. No. 3,830,401) use pulsed-mode LED operation to monitor the reflectivity of a developer mixture of relatively reflective carrier particles and light-absorbing toner particles. Butler (U.S. Pat. No. 5,119,132) discloses a pulsed-mode reflection densitometer suitable for both black and colored toner over a wide density range. Pulsed-mode operation has also been used to enable higher intensity for measurement of higher-density samples, as in U.S. Pat. No. 4,068,956 to Taboada. For densitometry of toner images on a photoconductor, pulsed-mode operation obviously reduces the exposure of the photoconductor, compared to continuous operation at the same intensity. The reduced exposure of pulsed-mode operation can be beneficial in reducing the aforementioned problem of photoconductor fatigue.
In the case of a moving photoconductor, a problem with pulsed-mode densitometer operation is that too low a pulse frequency can yield m
Abutayeh Mohammad
Dang Hung Xuan
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