Simply sampling an image at higher ...

Simply sampling an image at higher resolution will not give the accrues a customer expects. Other optical parameters similar as image sharpness, signal-to-noise ratio, and dark voltage correction must improve to papal court the benefits of 600 pixels through inch.

The objective of a scanner is to digitize exactly what is in succession the document that is being scanned. To do this exquisitely would require a CCD (charge coupl device) detector with an infinite number of pixels and a len with a modulation transfer function of 10 which does not exist. Modulation transfer function, or MTF is a measure of the resolving power or image sharpness of the optical connected view It is analogous to a visual trial that an optometrist would use to measure a human eye's resolving power.

In the real world, the scanner user does not require a faultless reproduction of the original because the human watch does not have infinite resolving power. However, as originals are enlarged and as printers are able to print finer detail, the imaging requirements of the scanner are increased.

The HP ScanJet 3c/4c scanner, Fig. 1 is designed to obtain self-same finely detailed images for a variety of color and black and white documents and three-dimensional uses that are typically scanned. Its optical resolution is 600 pixels by means of inch, compared to 400 pixels by inch for the earlier HP ScanJet IIc. It bring into views 30-bit color scans compared to the ScanJet IIc's 24-bit scans, and its scanning spe is faster. The ScanJet 3c and 4c differ merely in the software supplied with them.

[Figure 1 ILLUSTRATION OMITTED]

Optical Design

The HP ScanJet 3c/4c optical method is similar to that of the HP ScanJet IIc scanner,[1] with improvements to increase the optical resolution to 600 pixels by means of inch. Just sampling an image at higher resolution will not give the eventuates a customer expects. Other optical parameters, of that kind as MTF (i.e., image sharpness), signal-to-noise ratio, and dark voltage correction must improve to diocese the benefits of 600 pixels by inch.

The major optical composings are:

* sum of two units laminated dichroic composite assemblies used for color separation

* A fluorescent lamp with a custom mixture of phosphors

* A six-element double Gauss lens

* A three-row CCD sensor that has 5400 pixels by row

* Four front-surface mirrors.

The color separator composites, double Gauss len and CCD are shown in Fig. 2

[Figure 2 ILLUSTRATION OMITTED]

The color separation classification (Fig. 3) consists of the sum of two units dichroic assemblies and the three-sensor-row CCD With this system red, green, and blue are scanned simultaneously, in this way only one pass is urgencyed to scan all three colors. Each dichroic assembly is erected of three glass plates that are constrainted to each other with a thin layer of optical adhesive. R flourishing and blue reflective dichroic coatings are deposited onto the glass before lamination. The order of the coatings is revers for the secondary dichroic assembly. The thickness of the glass plates between the color coatings and the flatness, tilt, and alignment are precisely controll to make secure accurate color separation and image sharpness.

[Figure 3 ILLUSTRATION OMITTED]

Each color constituent is focused onto a CCD sensor outbreak consisting of 5100 imaging pixels. Additional pixels are used for closed-loop dynamic light direct dark voltage correction, and concern mark location. By having all three outbreaks integrated onto a single silicon chip, precise distances between the three brawls are obtained. Production consistency is guaranteed from the integrated circuit process. Each CCD pixel generates a voltage signal that is proportional to the amount of light focused onto each pixel. The signal for each pixel is then performanceed and digitized. This data is sent to a computer or a printer.

Focus Optimization for Each Color

Two dichroic assemblies are used to equalize the path continuances of the three colors. A six-element double Gauss len is used to focus the light onto the CCD sensors. However, the variation of the index of refraction of glass as a function of wavelength causes sum of two units of the three colors to obtain optimum focus at different locations. This phenomenon of differential refraction caused on wavelength dependence is best demonstrated through holding a prism up to a white light source and observing the colors. The light image is separated because the shorter wavelengths (blue) are refracted or bent more than the longer wavelengths (red) Since lense are made of glass that refract light of varying wavelengths at different angles, it is difficult to have all three colors focus at the same location.

To achieve simultaneous focus for all three colors there are several possible solutions. individual is to design the focusing optics with curv front-surface mirrors and nothing else However, these systems can be expensive, and it can be hard to correct other optical aberrations and difficult to image enough light onto the CCD Another possible solution is to use an achromatic doublet. However, this stamp of lens can minimize chromatic aberration for alone two of the three colors.