Karin Lau Photography

Capturing Pixels CCD Versus PMT

The overwhelming majority of today's scanners share a common technology, illustrated in Figure 2.31- The film image is recorded onto a CCD (Charge Coupled Device) chip. Drum scanners, by contrast, rely on the older but optically superior method of PMT (Photo Multiplier Tube) capture. At the most basic level, both CCD and PMT scanners work by illuminating the original image, recording analog color values, and then converting the information to ones and zeros. Virtually everything else about their operation is different.

A CCD scanner captures image data for an entire row of pixels at a time. Both the CCD and light source are designed to cover a fixed width and simply scroll line by line down the length of the original image to complete the scan. Light passing through the original hits the CCD, which has a color filter array to separate red, green, and

Figure 2.31 In a CCD

scanner, the light source passes through the transparency (1). It is then directed through a lens, which focuses the light onto a fixed-width CCD sensor (2). The sensor reads one row of pixels at a time and records them onto red, green, and blue photosites on its surface element (3).

Figure 2.31 In a CCD

scanner, the light source passes through the transparency (1). It is then directed through a lens, which focuses the light onto a fixed-width CCD sensor (2). The sensor reads one row of pixels at a time and records them onto red, green, and blue photosites on its surface element (3).

blue light waves. The CCD measures the light's intensity and converts it into electrons. An analog-to-digital (A/D) converter then translates the information to the ones and zeros that your computer understands.

Compact scanner design allows for a small footprint but packs a great number of heat generating components, as well as a power supply, in close proximity to the sensor chip. As a CCD's sensitivity is amplified to read lower light levels, electrical heat from these various components, including the CCD itself, is often recorded, showing up as random noise in shadow areas.

PMT scanners capture data one pixel at a time. In this scenario, shown in Figure 2.32, it is the original image that moves, wrapped around a rapidly spinning cylindrical drum. A highly concentrated

Figure 2.32 A highly concentrated light source illuminates the transparency as it rotates atop the spinning drum (1). Each spot of captured light is then split into individual wavelengths (2) that are directed to a set of red, green, and blue photo multiplier tubes (3). The intensities of light are converted into an electrical signal and sent to an A/D converter (not shown), which converts these signals to digital data.

Figure 2.32 A highly concentrated light source illuminates the transparency as it rotates atop the spinning drum (1). Each spot of captured light is then split into individual wavelengths (2) that are directed to a set of red, green, and blue photo multiplier tubes (3). The intensities of light are converted into an electrical signal and sent to an A/D converter (not shown), which converts these signals to digital data.

beam of light travels perpendicular to the drum's rotation, illuminating a single spot at its apex on the drum, where the film lies perfectly flat, ensuring sharp, edge-to-edge focus. This spot on the image is split and directed to three very precise photomultiplier tubes (one red, green, and blue) based on wavelength. In these tubes, the light intensity is measured and converted to electrons. The information is passed through an analog to digital (A/D) converter on its way to the host computer. Native resolutions of 4000 ppi and higher are the norm. For even greater precision, PMT scanners allow manual adjustment of the aperture or spot size of incoming light. An aperture matched to the size of a particular film grain lets you capture faithful image detail right down to the film's grain structure. With some scanners, the A/D converter can be optimized on the fly for each and every film/exposure/development combination that is scanned. The large physical size of drum scanners has inherent cooling advantages. Electrical heat is kept well away from the optical components. The analog tubes also have a much greater ability to read low light values without signal amplification.

Hasselblad—after acquiring Imacon—markets virtual drum scanners. The Flextight models borrow from traditional drum scanners the idea of wrapping the original around an arc for increased sharpness. The film image is captured with a high-resolution CCD sensor that collects light in much the same way as those in film and flatbed scanners. Hasselblad's scanner optics remain stationary as the film passes between the light source and lens. Their vertical case design provides for a glass-free optical path. Signal noise is reduced through the use of an active cooling device mounted right on the CCD chip. Mounting film is a simple affair using any number of holders matched to specific film sizes.

Drum scanners were primarily conceived for production environments rather than home use. Improvements in consumer level image capture devices took a heavy toll on this market, and only a handful of models are still in production. There is a learning curve involved in wet mounting film to a drum, and even minor repairs can be costly. Small footprint, ease of use, and maintenance-free operation clearly favor flatbed and dedicated film scanners. The Hasselblad scanners are very easy to use, maintain film flatness without wet mounting, and provide resolutions of up to 8000 ppi. But they are not priced for the faint of heart. You could take out a car loan for less than the cost of their Flextight X5 model.

In terms of comparative quality, a drum scanner run by an experienced operator will produce sharper images over a wider dynamic range and resolve noise-free shadow detail right down to the grain structure of the emulsion. For difficult-to-scan negatives destined for large output, a drum scan can be the best way to faithfully replicate all of the detail in the film. One approach taken by many fine art photographers is to scan in-house for evaluation purposes or small print sizes and send select images requiring a higher degree of quality or enlargement out for drum scanning.

See Figure 2.33. With the HP DesignJet Z3100 (top left), I can output long-lasting color and black-and-white images up to 44 inches wide. The Epson Stylus Pro 9600 (top right) is a 44-inch printer outfitted with PiezoTone Selenium and Sepia monochrome ink sets from third-party manufacturer InkjetMall. The image PROGRAF IPF 5000 (bottom left) is Canon's 17-inch wide, pigment-based printer and includes a cassette tray for unattended printing of multiple sheets. The Epson Stylus Photo R2400 (bottom right) is loaded with UltraChrome K3 inks, while the smaller but similarly styled Epson Stylus Photo R800 prints with a K7 monochrome ink set from InkjetMall.

Figure 2.33 With the variety of fine art printers now on the market, I have found it difficult to settle on just one. Instead, I prefer to leverage the unique capabilities of different models for specific output purposes.

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