How a Scanner Works - Haywood & Sullivan
Contents
1. in its Relationship to Output le Scanning Process Scanning for Output Method Bit Depth Scan Mode amp File Size Optimal Resolution Sizing Tone Curves amp Histograms Using Tone Controls Dot gain or Spot Variation Making Linear Adjustments File Size amp Storage published by HAYWOOD SULLIVAN The Scanner as an Input Device he scanner is simply a gadget for getting images into your computer That s it But as you probably know getting good images from your scanner requires more forethought and exper tise than simply scanning them into your computer For one thing getting good results requires knowing how to correct images with the tools in an image editing program However this book is not about image manipulation techniques per se If you want to become a Photoshop wizard please see the appendix for a list of supplementary literature Plus you ll find that not all images can be fixed in an image editing program The old saying garbage in garbage out applies to scanned images It s far better to fix images at scan time than to try to fix them later in Photoshop or your favorite image editing program To some degree great scanned images are also the result of good equip ment This explains why your images don t look as good as the profes sionally scanned images you see in magazines and brochures It s clearly due to the high end equipment tha2. Curve for Shadow Emphasis Before Tone Contrel Alier Tane Control Pot Lie signa gniiiy aks Fak West mae esei in the dekw mek Gimmispuz 13 BT iei mi Ciil pia ea eb eed bok ede ge ed Pee ce TE O PE abii NOTE tt won t be long before multitasking computers will be widely available They Il enable you to do several functions such as scan print save use a modem and lay out pages at the same time NOTE This work is time con suming but very valuable You should work at the high est resolution your equipment can handle Resample your image down to a more conve nient size later The Scanning Process Step 5 Adjust Tone Controls Your tone controls depend on your particular software hardware combi nation The names for the various tone controls and how they perform are not consistent from program to program For example some programs use the term lightness for gamma midtone lightness whereas others use gamma for overall lightness Unfortunately you can only determine how to adjust tone controls on your scanner by reading and understanding your user s manual or calling the manufacturer s technical help line Typical setting choices include DMIN DMAX These are density settings that use a logarithmic scale from O to 4 where 0 equals pure white and 4 equals pure black Most scanners limit you to about 0 1 Dmin to 3 2 Dmax Choose the maxi mum density Dmax and the minimum density Dmin for the dat
3. Cropping at the scanning stage reduces the amount of time needed to scan Cropping also deter mines the way your image will look by confining the area in which the scanner s tone controls sense the darkest and brightest areas of the image Step 3 Decide on Input Mode Line Art Halffone Grayscale or Color LINE ART also called 1 bit or bitmapped is the best mode for scanning pure black and white originals such as drawings logos signa tures etc Line art should be scanned at the same resolution as the final output device to obtain maximum detail HALFTONE or DITHERED is used for scanning black and white photos or color photos to output in tonal gradations from black to white This mode is a 1 bit format that simulates shades of gray by either creat ing halftone spots halftone lines or diffusion patterns This option is typically used for final output to low resolution black and white laser printers The high resolution imagesetters that are found at service bu reaus output grayscale images as described in Step 4 Halftone scans cannot be reverted to grayscale mode GRAYSCALE is the best mode for producing a black and white image with a realistic tonal range of grays Its 8 bit continuous tone configuration produces 256 variations of gray for each sample and pro vides the greatest flexibility when working with black and white images It s used primarily for producing black and white photographs and film negati
4. file is on tape Lowest cost per megabyte for media 100mb 199 20 29ms 1 40mb s Remarks Inexpensive solution for storage lgb 599 99 10 12ms 5 4mb s Remarks Inexpensive solution for storage Low cost per megabyte All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan
5. halftone spot when it is applied to paper a phenomenon known as dot gain The amount of this spread is affected by the absorbency of the paper and the surface area of each individual spot The porous surface of uncoated paper absorbs much more ink than the smooth surface of a coated paper causing the ink to spread more although even the shiniest of coated papers exhibits some dot gain On an uncoated sheet the darkest gray or shade of any color you can use without losing detail varies from an 88 percent to 95 percent tint Spots with greater surface area tend to exhibit more dot gain This is especially true in areas with adjacent square halftone spots ink tends to bridge the gap between spots creating even greater dot gain For this reason midtone spots generally spread more than either highlight or shadow spots An altogether different problem occurs when printing the lightest tints in an image You see a 5 percent spot at 150 lpi is extremely small On press anything smaller than this will probably not print The combination of factors involved in offset lithography water blankets and offsetting make it hard for very small dots to hold ink Med Ink Spread Max Surface Area Max Ink Spread Min Surface Area NOTE Some color separation software 1s now able to per form dot gain on the fly at imagesetting time check with your service bureau to see if they can provide this service for you The net effect is t
6. to describe a computer s screen resolution In this book I have chosen to use the term spi samples per inch to de scribe the scan data In essence spi is the input frequency used when scanning the original I also use the term to describe the frequency of the sample data when an image is scaled at output time For example al though an image may be scanned and saved at 300 spi when it is scaled to 50 percent of its original size in a page layout program its effective sample frequency becomes 600 sp1 Because samples are not the same as laser printer dots halftone spots or computer screen pixels all other such prepress and desktop publishing terms such as dpi lpi and ppi are used to describe output For more information see Choosing the Best Screen Frequency Definition of Terms Typical Term Meaning Description Measurement SPI Samples Per Inch Input Frequency of Scanned Image 300 600 spi flatbed 1850 3600 spi slide DPI Dots Per Inch Resolution of Imagesetter output 300 2400 dpi LPI Lines Per Inch Halftone Frequency for Printer output 133 40 lpi PPI Pixels Per Inch Resolution of Computer Screen output 65 96 ppi This chart defines the various terms and typical output frequency for each All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan Scanning for Output Method hen scanning you always have the choice of what kind of image you want to
7. 150 Ipi regardless of the output resolution of the laser printer or imagesetter should be scanned at 225 to 300 spi COLOR As for grayscale an spi setting of 1 5 to 2 times output fre quency is the maximum you should scan Because an RGB color file is three times the size of the same version of the image scanned in grayscale mode keep the image resolution as low as possible without compromising quality The right scanning resolution and scale for an image also depend on its ultimate destination HIGHEST RESOLUTION SAMPLING As mentioned before scanning at a high resolution is important when archiving an image You should also scan at a high resolution when outputting an image on a larger scale For example a 600 spi scan delivers 300 samples per inch when output at 200 percent of its original size Conversely a 300 sp1 scan delivers 600 samples per inch when output at 50 percent of its origi nal size Ultimately your computer s power and speed determine how large a file you can work with An 8 x 10 color image scanned at 300 spi a 2 to 1 sample rate for 150 Ipi output at 1 200 dpi results in a 21 6MB file Not every computer can handle files this big OPTIMAL RESOLUTION SAMPLING This sampling gives the best possible results for your image s output destination at a resolution that doesn t result in an enormously large file You can figure out what the optimum scan resolution should be when you know exactly what s
8. Step 9 Clean Up Image in an Image Editing Program At this stage you should remove dust eliminate scratches selectively brighten and or darken remove noise resize crop further color correct soften sharpen etc Step 10 Save Again Save this image with a different name so if necessary you can go back to the original scan at a later date Of course the downside of saving two images is that you have two versions taking up valuable storage space on your hard disk All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan Scanning and Its Relationship to Output alftone screens came about as a way to print in four color and in black and white the tone gradations that appear in photographs and illustrations Halftone screens consist of many spots in varying degrees of concentration In black and white or one color printing the concentration of spots is 100 percent for solid black or a solid area of color A white area or area of no color 1s repre sented by a spot concentration of 0 percent The range of spot concentra tion in between varies from 1 percent in the light est regions of an image to 99 percent in the darkest Four color reproduction is similar in that the four process colors cyan magenta yellow and black are combined in spots of various concentrations to yield an array of colors and tones Each color in a printed image is the result of a blend of
9. a in your image to map to This setting is similar to Set White Point Set Black Point described below except that it allows you to set the controls nu merically SET WHITE POINT SET BLACK POINT This option lets you choose precisely which samples or portions of the image will map to pure white and pure black All other colors will fall between these two extremes A nice feature that comes with some applications allows you to remove any color cast in the white black sample point as well EXPOSURE This option is especially useful when working with very dark originals where the scanner may not see details in shadowed areas However very few scanners actually support this feature Thus if you see this option be suspicious it might really be just a linearized lightness control LIGHTNESS You should probably avoid using this control if your scanning software offers it unless of course this is your scanner manufacturer s terminology for gamma or midtone brightness Techni cally lightness is a linearized curve that increases overall lightness by cropping out key data in a highlight region of the image I ll explain more about linear adjustments later in this chapter CONTRAST You should probably avoid using this control as well Technically contrast is a linearized curve that increases midtone con trast by cropping out key data in the highlight and shadow regions of the image GAMMA Gamma is
10. also known as midtone brightness With 1 0 as the median for an image s midtones adjusting the gamma to 1 5 lightens the midtones of an image while a gamma setting of 0 75 darkens them TONE CURVE Customized tone curve adjustments go beyond simple gamma controls When you adjust the tone curve of an image you re adjusting the concentration of tone detail See Using Tone Con trols IMPORT TONE CURVE This is probably the most powerful feature for adjusting tones if your scanner offers it Using this option allows you to import a tone curve from an image editing program that optimizes the way in which the scanner captures the data from an original HISTOGRAM This control shows a chart that depicts on a scale of 1 to 256 the number of instances within an image where one of 256 shades of gray appears It is a snapshot of the distribution of data within your image Thus dark low key images have most of the sample points skewed to the left while light high key images have most of the data samples skewed to the right Step 6 Choose Other Options Some scanner scanning software combinations offer Sharpening Descreening and other controls It is usually more efficient to take advan tage of these options at scan time rather than doing them later Step 7 Scan The time involved in scanning depends on the size of the scan deter mined by the overall resolution and bit depth the speed of your scanner SCSI bus and c
11. always get the best results if you adjust the tone curve setting on your scanner to a perfectly straight 45 angle which represents an exact one to one mapping Almost all images need some help Some images have important detail in the shadow areas some have important detail in the highlight region and some have midtone emphasis Some images are too contrasty some too flat some too light some too dark There are two reasons why it s far better to achieve an optimal tone curve at scan time than to manipulate the image later First it takes less time to scan an image correctly than to scan it poorly and adjust or manipulate it later Second every time you manipulate an image you lose some data even if just a bit The cumulative effect of too many post scan image manipulations is an image with less data To achieve the best results keep the contrast angle steep for the most important areas of the image There is no one formula that works best for all images Your understanding of this fact already makes you a much better scanner operator Linear Corrections A Major Post Scanning Mistake Fortunately most scanning applications don t provide brightness and contrast controls both of which are examples of linear correction A linear correction as the name implies adjusts the default tone curve a straight line up and down or changes the angle of the line In the post scanning environment using linear corrections is a no
12. ber is that tone curves and histograms do the same thing map input tones to output tones The difference is that histograms give you visual feedback providing information on tone distribution within the image Unfortunately histogram adjustment is rare in scanning software most programs just offer tone controls Tone Curves Made Easy In theory input should match output one to one But that s theory In reality most desktop scanners can t reproduce shadow detail correctly they simply don t have enough vision to see into the darker por tions of an image Thus most desktop scans lack shadow detail and are generally too dark Mastering nonlinear tonal corrections also known as gamma correction is really a matter of bringing out the detail in the dark areas of an image and adjusting the tone range to compensate for darkness The key to bringing out the detail in the dark portions of an image is in increasing the contrast in that area The more contrast you have in any given area of an image the more differentiation there is between shades of gray resulting in the perception of both more detail and emphasis in that area The degree of steepness in a tone curve corresponds to the degree of contrast But there s a catch you can t have a steep curve everywhere in an image Every steep or high contrast area needs to be offset by a shallow or flat area This is a zero sum game people Likewise you don t
13. capture For line art you would usually choose 1 bit mode although there are times when it 1s better to scan using 8 bit mode For B amp W photographs you generally would scan using 8 bit mode There are times however when you may want to scan a photograph in Halftone also known as Dither mode especially when outputting to low resolution laserprinters as the final artwork Scanning in Grayscale or 8 bit gives you the most flexibility since you can always later output using dithered or threshold methods 1f necessary Halftone or Dithered scanning refers to a 1 bit mode where the effect of continuous tone is achieved aE m Baro nak E using a dithered pattern of small black dots represent E i 7 s Seen ees i ing various shades of grey The advantage is smaller Ee uae file size and in some cases sharper looking images Threshold or Bitmapped scanning is useful when you want the unique look of a high contrast image Many designers use this effect for backgrounds and other graphic shapes in their designs All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan Bit Depth Scan Mode amp File Size T L See eciding on what Bit Depth mode to scan will affect the quality of the image as well as its file size Generally it is better to scan in more data than you need 1 e oversample then convert to another mode at a later date Howev
14. d from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan v What Not to Do Making Linear Adjustmants ever ever use linear controls that increase contrast darkness or lightness As clearly demonstrated in the images below you ll perma nently lose data besides the results are simply too harsh Instead utilize the curves command by gently adjusting the curve to achieve a smooth tonal range In that way you ll acheive much better scans See Using Tone Controls Original Scan Input Levels o Output Levels Input j vs a k tA y ae gt k Output en p a Input Levels o q fata Clipped Off b a Output Levels o ae Input i af i oe bs TERRE Output a oP Tr a S J Input Levels o 255 Input Output Input Levels fo bk Output Levels ile AJ Fa Input d Output Input Levels fo Lost Input Output All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan File Size amp Storage eedless to say scanning images takes lots of hard disk space And unless you have terabytes of free space sooner or later you have to deal with where to put all this neat stuff I prefer to scan at the highest resolution first manipulate the image to remove dust and scratches do some minor color correcting and archive the large file to mag
15. er this results in larger file sizes and 1s sometimes impractical The fact is scanning in color for subsequent B amp W conversion often produces better B amp W images than scanning in grayscale mode because of your ability to accurately control the process Sthultheise Wallner This 1 bit dithered image has a file size of 64K This 8 bit grayscale image with 256 shades of gray has a file size of 518K aa nae tele Ne sad ae gt a z i a i T Pr i F r gt i me La fi ead tiga rm re in 1 Itay i y k ia i al E 1 Ea i J So ae woes Yas Fn 3 E madai i E fi 4 ye coe P ti T Sth ilthieise Ey ae Sthuithciss 7 a re be Berliner 3 aE a Lemay es Betliner EE i x EFA Wei Wei This 8 bit color image with 256 colors has a file size of 518K This 24 bit color image with 26 million colors has a file size of 1 55MB All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan Optimal Resolution Sizing for print DOT E Equivalent to approximately 4 inches w cock This 72 spi image works fine as an FPO image and has a manage able file size c4mb This 150 spi image scanned at a scale of 100 percent from a5 x 7 chrome yields an image that doesn t hold as much detail as you generally want from output to print cJdmb This 225 spi image was scanned at 100 percent This co
16. eto Optical CD recordable DAT DAT Iomega Zip Drive Iomega Jazz Drive Storage Media Find the Best Option for Your Needs Size Drive Cost Media Cost Access Time Throughput 44mb 200 45 disk 30ms Imb sec Remarks Popular with service bureaus Low cost for drives high cost per megabyte for media 88mb 300 55 30ms Imb sec Remarks Compatible with Syquest 44 Low cost for drives high cost per megabyte for media 105mb 299 100 15ms 17mb sec Remarks Compatible with 3 5 media Not quite as universal of other Syquest types Low cost for drives high cost per megabyte for media 200mb 800 90 30ms Imb sec Remarks Compatible with Syquest 44 and 88 Low cost for drives high cost per megabyte for media 1 2gb 399 NA 9 8ms 4 3mb sec Remarks Needs backup to be safe SCSI 2 drives can be twice as fast as listed here 128mb 900 20 40ms 700kb sec Remarks Slow writes fast reads Compact 3 5 media High cost for drives low cost per megabyte for media 1 3gb 1700 85 30ms Imb sec Remarks Slow writes fast reads These drive are really two separate 650mb disks back to back 650mb 1000 10 200ms 300kb sec Remarks Very slow writes and throughput High cost for drives low cost per megabyte for media 1 2gb 700 20 Remarks Requires backup software to access files varies depends where file is on tape Lowest cost per megabyte for media 4 8gb 1 000 20 Remarks Requires backup software to access files varies depends where
17. four halftone screens one for each of the four process colors In the reproduction of digital images three factors need to be taken into consideration e The image s output resolution as defined by the number of dots per inch dpi 300 dpi is typical of laser printer output to plain paper whereas service bureau imagesetters typically output images on photo sensitive paper or film at a resolution of 1 200 or 2 400 dp1 e Spot concentration ranging from Q0 to 100 percent as explained earlier e Halftone or screen frequency as measured in lines per inch Ipi This term may seem confusing because it doesn t describe lines per se but rather the number of rows of spots that make up a half tone screen The lower the frequency or number of rows the coarser an image appears Conversely the higher the frequency the more refined the image is Clearing Up the Confusion Among spi dpi Ipi and ppi Basic to understanding scanning and printing images is a knowledge of the terminology involved As I researched other resources for this book it became clear to me that no one in the industry was consistent in using terms to describe the resolution of scanned images Many publications use the term dpi to describe the scan data only to use the same term to describe the resolution of the output device inconsistent and confus ing to say the least Others use the term ppi pixels per inch to describe scanned data a term that is also used
18. hat an image that looks right on your computer monitor unless your monitor is specially cali brated for offset printing does not print well on press Usu ally it exhibits loss of detail in shadow regions highlight regions drop out and midtones are muddy yuck Thus for printing you should prepare an image so that its maximum and minimum spots are as depicted in the Screen Guidelines for Optimum Halftone Output chart You may also want to set gamma or midtone lightness an additional 1 2 to lighten the midtone dot for most presses Screen Guidelines for Optimum Halffone Output Paper Min Tint Max Tint Min Output Level Max Output Level on a scale of 0 256 in Photoshop s levels dialog Coated 5 95 12 243 Uncoated 10 90 25 243 Newspaper 12 88 30 225 You can specify these tints when you scan if your scanning software supports it Or you can adjust these settings in an image editing program such as Photoshop after you scan by using the Levels command Simply open the Levels dialog box Command L in Photoshop type in the appropriate numbers for shadow output So highlight output Ho and midtone lightness B consult Screen Guidelines for Optimum Halftone Output Click OK and save as a new file name The image on your screen may look somewhat washed out but rest assured it will give you optimum results on press I prefer to scan at full tonal range since much of my scanning work is also used for m
19. ize your scanned image will be for final output For example a color 8 x 10 image scanned at 100 percent using a 1 5 to 1 sample rate for 133 lpi output 200 sp1 results in a more manageable 9 6MB file When printed at the same size the quality of this 200 spi image is more than adequate for most printing applications FPO SAMPLING These are low resolution images used for position only that will be replaced with a much higher resolution version of the same image at a later time They require a minimum amount of memory meaning they are faster to work with and easier to store than high resolution images A typical sample rate is 72 sp1 which matches most screen resolutions A color 8 x 10 image scanned at 72 spi results in a mere 1 2MB 24 bit color or 414K 8 bit color file size All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan NOTE For dark originals where there is the likelihood of noise generated by the scanner 1t 1s advisable to selectively sharpen light regions after scanning gt Ising Tone Controls ve pen whoo Ubersite i A fiherads only ona pape you remember i thes Whole bone this is il There ie mi oca pul cave that wii work for every scanned image In i will ran high hey Alter Tane Gentrol uree far Highlighi Empharis Defor Tene Coni rel Por psy anganis eel lighis you want mora sanira in iha Highlight asa Gamma equals OTS
20. mbination of scaling and resolu tion produces this best image for output to print S 44m This 300 spi image was scanned at 100 per cent This combina tion of scaling and resolution is best when archiving an image All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan Understanding Tone Curves amp Histograms HEY Si Shadow dot input H i Highlight dot input B Midtone brightness C Contrast angle of curve So Shadow dot output o Highlight dot output know it may seem intimidating but you need to understand tone curves to produce anything beyond mediocre scans Even professionals get confused by the complex interfaces found in pow erful scanning and image manipulation programs But don t fret It s really rather simple All image manipulation and scanning tools have one thing in common They all depict and thus provide an interface to the mapping of an input sample to a corresponding output sample The classic method is a tone curve as shown in Figure 1 An alternative and complementary tonal correction method is the histogram Histograms depict the distribution of tones within an image The legend depicts the relationship between each input tone and its corresponding output tone three quart Input Levels Si Output Levels Figure 1 Tone Curve Figure 2 Histogram The key thing to remem
21. nal using an analog to digital converter It is this digital information that makes up the data in your scan Since each individual scan line must be calculated separately scanners are not used to capture moving objects the scanner is not a camera But you can use a scanner much as you would a camera to capture 3 D artwork To A D converter and output processing CCD elements For more information see 3 D Objects Diagram courtesy of Agfa Division Bayer Corporation All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan The Scanning Process gt Hit Depth Scan Hode amp File Size NOTE For permanently archiving grayscale and color images scanning at the high est possible resolution 300 to 600 spi or more is advised You can resample the image to a lower resolution in an image editing program when the image is retrieved The following is a basic step by step overview of the scanning process Step Preview Previewing lets you see a quick small scale version of the image before committing to the final scan At this stage you also see a frame or mar quee around the image You can adjust this frame so that it crops in on the portion of your image that you wish to scan Step 2 Crop to Desired Size and Shape It s important to crop an image before scanning as opposed to after in your image editing or page layout program
22. neto optical or DAT backup I then make an opti mally sized image for my intended use by resizing down At this time I apply sharpening and any other image manipulation as necessary All this takes place on a mostly clean 1 2GB fast hard disk Nevertheless I often find myself swamped with hundreds of megabytes of images in no time flat For this reason I prefer 1 3GB magneto optical MO drives I have one disk for each of my clients and their respective projects Occasionally I need one MO drive for just one project Even then I make DAT backups of the whole enchilada in case I lose or dam age my MO drives It goes without saying that you also need a large hard disk to work on your active projects Photoshop not only works best with a lot of RAM but also needs a lot of free hard disk space Therefore it s important to keep your main hard disk clean most of the time Backing up your files on a daily basis weekly at the very least 1s of utmost importance I have outlined a few of the more popular large storage media available Unfortunately prices change monthly Use this chart as a rough compari son only But remember you should choose media that will be compat ible with others as well Call your service bureaus or other vendors and ask them what they use Most people will gladly give you their opinion Gust take it with a grain of salt Type Syquest Syquest Syquest Syquest Hard disks Magneto Optical Magn
23. no you lose important data More specifically adjusting brightness shifts the tone line straight up Data 1s lost in the highlights and simply disappears Contrast adjustments perform in a similar linear manner Although detail in the midtone grays is improved the original input shadow detail is mapped to pure black resulting in permanent data loss Film vs Paper Output Paper output halftones are typically limited to about 110 lpi This is because paper originals must be photographed by the printer in order to make film negatives and then plates for the printing press When photo graphing the original some detail is lost particularly the small dots in the lighter regions of the image Outputting to film eliminates a step because film negatives are used to make the plates Consequently very small dots can be used In general you should output to film negatives or film positives for processes such as silk screen whenever possible They give the best results All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan Using Tone Controls see also Understanding Tone Controls f there is only one page you remember in this whole book this is it There is no one input curve that will work for every scanned image In fact using the default GAMMA curve on every scan will ruin high key and midtone dominant images Beware of scanning software that won t let you make in
24. omputer Scanning used to be fairly time consuming however newer scanners and computers are more than twice as fast as yesterday s models Even 20MB images may now appear on your screen in a matter of seconds Step 8 Save Do this now before you make any changes You may want to revert back to this original later Here are your options e Save in proprietary Photoshop native format if you want to work with the image in this program While Photoshop native format can t be opened by desktop publishing programs it is run line encoded RLE for compact file size and it offers the ability to save alpha layers masks and clipping paths with the file Save as a PICT or BMP file if you want to use the image as is for computer display Many multimedia programs such as Macromedia Director Apple Media Kit and HyperCard only work with PICT BMP and PICT images can also be imported as templates for autotracing in illustration programs Save in TIFF or EPS the most popular file formats for desktop publishing TIFF images can be imported into page layout programs EPS files are used when a clipping path and or CMYK separations are used Personally I prefer to save the original scan in Photoshop format that way I know it s the original Only after I have scaled the image to its final size and made other adjustments for maximum printability do I save the image using a different name as a TIFF EPS or PICT file as appropriate
25. put curve adjustments In order to acheive the highest quality you as the scanner operator need to determine what data is 1m portant in the image BEFORE you scan Knowing this will make you a v much better scanner operator and yov ll get better scans too Curve for Highlight Emphasis Before Tone Control After Tone Control For High key originals mostly lights you want more contrast in the highlight area Gamma equals 0 75 Curve for Shadow Emphasis Before Tone Control After Tone Control For Low key originals mostly darks you want more contrast in the shadow area Gamma equals 1 5 Curve for Midtone Emphasis Before Tone Control After Tone Control For some images you want slightly more contrast in the midtones The curve to use is known as the classic S curve All text excerpted from Make Your Scanner a Great Design and Production Tool 1995 Michael J Sullivan Dot Gain or Spot Variation Factors Affecting Spot Gain Min Ink Spread Min Surface Area S canning for a computer monitor is easy A 24 bit monitor j can faithfully reproduce every sample in your scanned image However if your image looks just right on your computer monitor it will probably print too dark on press This is due in part to the inabil ity of the four process inks cyan magenta yellow and black to exactly reproduce every color in the spectrum It s also due to the fact that ink spreads beyond the perimeter of the
26. t publishers can afford to use Although great chefs can usually be found in great kitchens they can also perform their art under less than ideal conditions Even though you may only have an entry level flatbed scanner you can still achieve stunning results with it If you want to make the most of your equipment optimize your scanning time and deliver the best possible images then this book is for you Youll find your scanner can be a really valuable tool when you know how to get good results from it How Scanners Work The scanner is an electro optical device that captures image data one line or scan at a time hence the name Using optics much like a camera s a scanner moves over the imaging area called the bed which is defined by a grid of x y coordinates Each x y coordinate defines a sample area and the resulting captured image is described in samples per inch spi F The Basic Components of a Scanner A O l source of a known intensity The light either passes through in the case of a transparency or is re flected from in the case of artwork an original This light then makes its way through the optics of the scanner to a sensor which converts the light to an electrical charge The strength of the charge is in direct proportion to the intensity of the light which is a reflection no pun intended of how light or dark the original sample is The electri cal signal is then converted to a digital sig
27. ultimedia purposes Then when I go to print I modify the image file as appropriate for the intended output paper combination and dot gain I then save the modified image with a new file name This way I have two versions of the image the screen or master version that looks just the way I want it to look and the print version HOW Paper Affects an Image As mentioned earlier the quality of an image is affected by the type of paper on which it is printed Your paper choice dictates the maximum printing screen frequency lpi you can achieve and the lpi you select affects the spi setting to use for your image file In general use an spi setting from 1 5 to 2 times the printing screen frequency lpi Low resolution scans should be used for 300 dpi1 laser output where more data won t improve the resolution of an image On the other hand high resolution offset printing on coated stock demands a scan at a much higher spi setting resulting in a correspondingly larger file For more information on additional sources explaining dot gain prepress issues and printing standards see appendix The Best Screen Frequencies by Output Printing Method Paper Used Screen Frequency Scan 1 5 tol Xerographic copier Laser paper 50 75 lpi 75 125 spi Quick print Uncoated 50 75 Ipi 75 125 spi paper plates Coated 75 110 lpi 125 175 spi Offset Lithography Uncoated 85 133 Ipi 125 200 spi Coated 120 175 lpi 180 300 spi All text excerpte
28. ves COLOR is the mode to select when working with a color image or when colorizing a black and white image Its 24 bit continuous tone configuration produces 256 variations of each of three colors red green and blue RGB for a total of 16 777 216 possible colors per sample 256 x 256 x 256 It s used primarily for producing color photographs and slides Step 4 Decide on Input Resolution spi and Scale In Calculate the Right SPI you ll find tables to help you determine the proper calculations needed for optimal spi scale settings However here are some general guidelines you can follow for the various input modes LINE ART Output should be a multiple of the sample rate because the imaging data is sent directly to the laser printer or to the imagesetter s image engine A 300 dpi laser printer needs line art at a resolution of 300 spi or 150 spi a multiple of 300 For output to a 1 200 dp1 imagesetter scan at 300 600 or 1 200 spi You are limited in scaling this image in a page layout program if the resolution of the scaled image 1s not a multiple of the final dpi output But the obvious advantage of using line art mode is that it produces a much smaller file than an image scanned in grayscale or color mode For more information see Scanning Fine Line Art GRAYSCALE An spi setting of 1 5 to 2 times output frequency is usually the maximum you should scan For example a grayscale image that will be output to
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