Laser Scanners - Optics of Scanners
As sources of pure and high intensity light, Lasers are the predominant light source used in reprographic scanners. The wavelength sensitivity of the recording medium (film, plate) normally determines the laser chosen. With the abundance of different media on the market, performance comparisons are not straightforward. Laser cost per watt, Versus expose speed, versus media unit cost, versus cycle time are major considerations. Another complication is that more expensive plates generally allow longer print runs . Design cannot begin until a cost/benefit analysis has been undertaken to resolve these trade-offs.
For silverhalide based films Argon ion and Helium neon are the main laser types. Both types have excellent beam uniformity which allows simple and diffraction limited optical designs. The more powerful argon-ion lasers are bulkier and need robust mounting and alignment fixtures. Low power HeNe's come in compact forms such as cylindrical and can be incoporated into systems with simple fixtures. Argon-ion lasers have been favoured because of their high photonic energy, high quality gaussian beams (TEMoo mode) and high resolution imaging.
Laser diodes emit at infrared wavelenghts of 780nm+, and have made a large impact on desktop printers. Their use in reprographics is more limited primarily because of poor beam quality, increased noise and restricted power levels. As a less mature technology there is scope for performance inprovement in the future, especially since they offer some major advantages; including direct intensity modulation, compactness and output at a desirable wavelength. Coupling a number of lasers on the same chip allows much higher output power, but has the disadvantage of larger beam size and unwanted radiation into sidelobes. Elements in this type of integrated array do not normally emit independently, unlike the distributed arrays discussed below.
Expose systems have also been realised using distributed laser sources, such as diode arrays. Each source in the array exposes a strip at the image (film) plane, and neighbouring strops precisely butt-up to each other. Full widths of film are exposed by a step and repeat method. This arrangement allows use of low power diodes or alternatively, enables improved beam quality by use of apertures to block unwanted sidelobes emitted from standard diodes.
Expose systems employing L.C.D. image bars work in a similar way, over much smaller dimensions. Each pixel of the L.C.D. matrix acts as an individual shutter. A typical image bar geometry spans several thousand pixels in its long axis (this is corresponds to the fast axis of an opto-mechanical scanner) and has a width of ten's of pixels in the short axis. In this translation (slow) axis, the pixels expose overlapping strips to allow high resolution imaging over the whole length of the image bar. The illumination source can be simply a bank of high power L.E.D.'s.
Array expose systems are attractive because of the reduced power requirement of the individual sources and because they reduce the number of mechanical components.
Diode Pumped Lasers
Frequency doubled and diode pumped Yag lasers are also expected to have a major impact on C.T.P. (computer to plate ) technology particularly for printing on plates at the near infrared and thermal end of the radiation spectrum. Such lasers have peak power and near diffraction-limited quality at wavelengths of 1064nm. These beam qualities also lend themselves to efficient frequency doubling,tripling and quadrupling using non-linear crystals which can provide other potentially useful output wavelengths. Yag lasers with powers of several watts are now readily available.
Optics of Gaussian Beams
Pixel placement linearity (in x,y coordinates) and pixel resolution are primary determinants of half-tone image quality. When available laser power exceeds the film/plate sensitivity requirements, the cycle time is limited by the video bandwidth. Cycle time is defined as the time taken to expose an image over a given format, typically 600 X 600mm.
Designing for a smaller spot size to get higher resolution is not a fundamental limit in itself, however, other aspects of system performance become increasingly critical with smaller spot size. Depth of focus reduces, making mechanical positioning and flatness of the expose medium much more critical. The numerical aperture and thus size of optics also increases. The reduction in raster line width by half for example, means that cycle time is approximately increased four-fold .
When the gaussian beam diameter (1/e2 points) illuminating the objective is less than 4 times the entrance pupil diameter, the beam is self limiting and the Spot diameter do, is given by :
do = (4/pi) * (W * f/di)
Where pi = 22/7, W = wavelength, di is the (1/e2) diameter and f is lens focal length.
When no image data is applied, the exposure profile on the medium is simply that obtained from overlapping raster lines having gaussian intensity profiles. A design compromise is required between minimum overlap (desirable for faster cycle time) and exposure uniformity. Practical results show that exposure "ripple" is minimised when adjacent lines overlap at the 50% intensity points. This is the Fwhm, Full Width Half Maximum point, which is related to the 1/e2 diameter(do), by:
Fwhm = 0.5887 * do
In this case, system resolution r, is defined as:
r = 1/fwhm
Exposure ripple is visible as "staircasing" when screen angle is oblique to the raster line direction.
Depth of focus z, is defined as:
z = pi * do2/4V
This relationship confirms the trade off between small spot size and depth of focus. All these parameters relate to an unmodulated beam, which cannot impart any information onto photosensitive medium. In the raster (fast scan) direction minimum pixel size is normally limited by the modulator bandwidth (Acousto-optical and Electro-optical modulators are typical devices used). However, as in all laser scanners, the linear velocity of the spot (relative to the medium) combined with the characteristics and non-linearities of the medium, increase the effective spot size.
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