Four-Color Process Separations in Photoshop


The following is based on having predone a full system calibration including:
* Adjusting the monitor
* Scanner calibration
* Output device calibration and Transfer Function adjustment
(Refer to the Photoshop manual for an excellent section on system calibration.)

  1. Scan image on full-color scanner or acquire image from Photo CD. (For PhotoCD in PS 5.0 use Source: PCD4050 Output: Generic Monitor

    a. Scan resolution (ppi, dpi or spi – depending on who’s manual or article you are reading!) should be 150 to 250 dpi. Use 200 dpi if the image has hard edge text or graphic elements.

    b. For better control, place a grayscale next to the image when scanning.

  2. Bring scan into Photoshop. If scan was done in-house adjust scan to compensate for low-end flat-bed scanner deficiencies.

    a. Use the Tone Curve and lighten the shadows and midtones. If scan is from a service bureau, these steps may be necessary but use lower settings.

    b. Apply Unsharp Masking. A setting of 150% Amount, 1.0 Pixel Radius and Threshold of 6 works. If not enough use Unsharp Masking again.

  3. Adjust Color Balance. Use the Info Palette (Window/Show Info) to check for hue error in the grayscale. Compensate for this by adjusting specific tone curves for each color. Solid white areas should read as close to R 255, G 255, B 255 and dead black areas should read as close to R 0, G 0, B 0 as possible. If the image isn’t evenly balanced in color (white areas reading too red, too blue, etc.), use a tone curve on just a specific channel (RGB) to correct these errors. Increase the saturation if the design is flat. Certain colors may need to be boosted (bright red) by using Color Range to select the color and then the Tone Curve or Hue-Saturation to increase the color level.

  4. Adjust Preferences for Inks and Separations. These settings must be done BEFORE converting to a CMYK because Photoshop makes the separations based on the new Ink Separation tables. In Photoshop 5.0 & 5.5 these settings are combined under File/Color Settings/CMYK Setup.

    Ink Options

    a. Use SWOP Newsprint as a default. (You can try to make your own “Yxy” coordinates by comparing a solid print of each color and adjusting the Hue, Chroma and Value.) Certain manufacturers supply a free plug-in for Photoshop that has the Yxy coordinates for their ink and the appropriate dot gain for various mesh counts.

    b. Set Dot Gain to 35%. In PS 5.0 & 5.5 you can build your own custom dot gain curve using a device like a Colortron from Lightsource.

    Separation Options
    Use GCR with settings of:

    Black Generation: Light if not much black in the image. Medium if a lot of black.
    Black Ink Limit: 85%
    Total Ink Limit: 250%
    UCA 0

  5. Add additional dot gain of 5% to 10% using the Tone Curve. Reduce the 50% section of the curve to 40% or 45% (less dot gain if printing on an automatic press).

  6. Convert image to CMYK in Mode Menu (Image/Mode/CMYK).

  7. In CMYK mode open Show Channels and display for editing individual channels. Each channel can be edited, lightened, sharpened, etc.

  8. If the design has a lot of black outlines and detail, increase the black plate sharpness. In CMYK mode display just the Black channel for editing.

    a. Apply additional Unsharp Masking to enhance detail and if necessary adjust Tone Curve to darken the 3/4 tone and 90% shadow areas. The “skeleton black” may need boosting in these areas.

  9. You many need to decrease the cyan in the highlight areas. Select the cyan channel and with Tone Curves, move the slider in 7% on the highlight end. Sometimes the ink values you use from industry ink manufacturers boost the cyan too much. If you are having a problem with too much cyan at the press, use the SWOP Newsprint numbers (with 35% dot gain).

  10. The cyan will often contaminate areas that have solid yellow. Remove cyan from the highlight areas of yellow.

    a. Use Color Range and select yellow from the image. Do Not Invert.

    b. Use Fuzziness to pull the lightest areas of yellow.

    c. Select just the Cyan channel. Use a tone curve (Image/Adjust/Tone Curve) and move the highlight curve from 0% to 5%.

  11. Print the file to the laser printer or dry film imaging system. Adjust settings under Page Layout.

    Screen: Set the halftone line count to 55 lpi for manual printing and 65 lpi for automatic presses. Use an elliptical dot for each color.

    Angle: Change to the angles of: C/15° M/45° Y/75° K/75°

    Optional Angles: C/22.5 M/52.5 Y/82.5 K/82.5

    Border: Put a border around just the black if necessary.

    Captions: It is helpful to put captions on test files.

    Transfer Function: Based on dot gain and output device calibration, use the default setting or use a custom Transfer Curve. The following curve allows for problems with printing to laser printers and/or dry film imaging systems where there is a smaller highlight dot than normal and a closing in in the shadow dots. This curve also allows for additional dot gain – over and above the settings in Printing Inks Setup.

    0%=0%, 5%=7%, 10%=10%, 20%=20%, 30%=30%, 40%=40%, 50%=50%, 60%=60%, 70%=65%, 80%=75%, 90%=85%, 95%=90%, 100%=100%

    Print Registration Marks, Labels and Calibration Bars.

  12. Print to a high quality vellum paper or to white paper and then do a camera shot to enlarge to the correct size. If you are going to enlarge the image, the line count needs to be higher so when it is enlarged the line count becomes 55 lpi.

  13. Option: Send the file to a Service Bureau to have it printed on an Image Setter on film. The file can be sent as either a TIF or EPS. If the file size is too big, it will need to be put on a Syquest or Zip drive cartridge. Specify: Final Film Size, Line Count (lpi), Film Positive, Right Reading Emulsion Side Up, Registration targets and file information, Angles.

  14. Use properly tensioned screens with 305 (120 cm) mesh for manual printing and 355 (140 cm) for automatic. Take care to hold the dots in the 5% and 10% areas. Print in good register with process colors.

  15. If the design needs text and graphics, do all of the above manipulations in Photoshop before exporting into Illustrator, CorelDraw or other drawing program. Export as a TIF or EPS file. Add text and graphics and print the file out of the drawing program as a CMYK file. This will print the text and graphics as line work with hard edges. If the file is exported to Photoshop the text and graphics will print as halftones.

  16. If a design needs a specific spot color you can pull just the color needed by using the Color Range selection. After you have selected the color with Color Range, you must Save the Selection as a Channel. The halftone angle is not as critical in a spot color. To make it simple, use the same angle as the similar color (red spot same as magenta). Generally the spot prints after the similar color.

  17. To make a white printer for a design, mask around the design with black and then use Color Range to pull the white (as in step 16). Use Fuzziness to increase the amount of white. Place more white in a design when going on medium colored shirts.

  18. An optional way to make a white underbase is to mask around the design, convert the image to a grayscale, invert the image and print the file.

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