Colour modes: RGB (projector using light) + CMYK(used for print)
CMYK Printing
Cyan, magenta, yellow and black
CMYK is often called "process colour"
Each colour is laid down individually. Where the 2 colours overlap - this is where the additional colours are generated.
Varying the amount of ink (0 - 100%) creates the spectrum. K is the key colour.
Print from light to dark
When C M +Y are used all together at 100% a muddy brown is generated (K). This is why you need a true black ink as well when printing to create true black shadows where needed.
Screen Printing
Colours are pre-mixed in most cases when screen printing - unlike the CMYK process of layering.
"Spot Colour" is a pot of ink which is the colour you are going to print. This term is used in the commercial printing process industry. This is what is used in screen printing. The colour we are using is called spot colours - pre mixed pots for green etc.
Spot colours are used in the commercial print industry because:
-Printing with spot colours can be cheaper. CMYK printing needs 4 inks to create the required colours, whereas spot colour printing has the exact required colours already generated.
-Used for branding
-Spot colours allow the accurate reproduction of colour e.g Pantone. This is a spot colour reference system. Unique reference number is used in this case.
-Used to print colour that can't be reproduced with CMYK e.g fluorescent, vibrant, metallic etc.
In Photoshop
Separating a finished illustration
Magic wand > select > similar
This then selects all the other colour in that image that is the same as that first selection. E.g all the orange in the image.
Select > colour range.
-Like magic wand but better. Fuzziness slider - selects more or less of that selected colour depending on what you would like.
On the image preview > dropper the colour
To get a better preview, select the "section preview" dropdown whilst in the "colour range" window and select "white matte" and then your whole image (not just the tiny preview in the window) shows you exactly how much you will select when you click ok.
We now need to put this selection on a new layer. Obviously cut + paste works but this is more efficient:
Layer > new > layer via copy. The shortcut for this is: cmd J.
This creates the selection as a new layer for you to play with.
On the layers menu/panel:
-the lock means "lock everything"
-arrows prevents the image from moving - cant change their position.
-The checkerboard pattern square button locks the transparent pixels in that specific layer. This means the transparent pixels will always stay transparent.
To fill your selection with just 1 colour (in this case black for our screenprint positives) Short cut: shift and backspace or edit>fill and then fill contents with black.
The black on the original image may not be 100% black even after this photoshop separation process. So even with the separated "black" layer, do this process again for a solid black.
Photos (Jaguar)
Photographs are printed used CMYK
A3 or smaller screen print positives can be printed on laser printer in computer rooms
Larger than A3 positives requires a different method.
*******
Smaller than A3 process:
In Photoshop check the image size and check the "image>mode" that it is in CMYK.
Then save as a .tiff or as a .psd file. (Every time something is saved as .jpg you loose quality for printing).
Channels palette:
The main purpose of channels is to store info about the colour in the image. Every image has these channels.
Click "cyan" in the channels palette. This enables you to see this image with the Cyan isolated and where abouts this appears in the overall image.
This is the same for all the CMYK channels.
When the cyan channel is selected and the eyeball in selected for yellow - you can see how these channels work together to make up the image etc as if layered one on top of the other.
In Illustrator
We can now use illustrator to separate our CMYK inks and create the printing positives.
Make a new doc in Illustrator and use "file>place" to place the image in the document. (make sure the document is big enough for the image, e.g if the image is A5 create an A$ illustrator document etc)
To separate this image it is all done in the illustrator print dialogue box.
Set printer to "black and white". Select media/paper size
Go to "output" menu
"Composite" means whatever you see in the preview will be printed. In the drop down you have the option to choose "separations". This creates a positive for each ink needed.
Image area of the dialogue box now says "positive".
Document ink options has now split the printing options into each CMYK relevant inks.
In an image there is differing levels of C, M, Y and K in an image. We need to create these differing inks in our print to make up an image.
Halftones
When we create our positives we have to apply a halftone screen to the image. Halftones are the way we can print different tints of a colour using only 1 ink.
A half tone is used as it is made up of different sized dots. The more dots in an area - the illusion of more colour/darker colour is created. The less dots means more white space which means the illusion of a lighter colour is generated. This helps us create contrast with just 1 ink.
To make sure each half tone screen works along side each C, M, Y K separation, the halftone screens need to be at different angles from one another so that when they are printed they aren't all on top of each other, so they don't clash.
In illustrator, print dialogue box "frequency" means this half tone screen.
150-200 lines of dots per inch is used in commercial printing.
For screen printing we need a more course halftone screen. Use 50-65 lines per inch, any lower than this and you can use these dots as an aesthetic effect as they will become noticeable.
Angles for CMYK printing: 15, 75, 105 and 155 degree angles. If only using 2 colours use the 15 and 75 degree angles and 1 colour just 15 degrees etc etc.
Then once the angles are set and the frequency, you can print. Each 4 of these C, M, Y and K separations will be printed in black dots and can be used now as screen positives.
To label each of the print outs:
Make the page larger than the image/document size you have set up in illustrator (do this in the print dialogue box) and then go to: marks and bleed and select page information. This will then print the information about each separation on each page.
Larger than A3
Use the larger printer in digital printing office.
Only use photoshop for this next larger process.
In the layer palette, in the small right hand top corner there is a 3 line drop down select button which means "split channels".
If they aren't available - look at the layer and flatten image. This option will now be available.
Photoshop has now created a separate layer for each of our colour channels which will open in separate windows within Photoshop.
This option is only available if the image is greyscale. A bitmap image is whose pixels are only black or white. Not possible for tonal variation.
The setting for this bitmap needs to be the same for each separation.
For bitmap, work at a higher resolution: 1200dpi. This will help maintain the detail in the image. (Obviously keep it low for aesthetic in other circumstances.)
In this bitmap dialogue box, the method drop down gives you the option for a "halftone screen". Photoshop will now add this to this image separation.
Frequency and angle are the same applied earlier in illustrator (see previous notes).
One separation image will be 15 degrees, one will be 75 degrees and so on.
This dialogue box also gives you the option to work with differently shaped halftones.
Each separation needs a different angle so the dots don't clash. Frequency is also the same as the illustrator previously. Lower frequency means more space between dots therefore more noticeable dots, higher frequency means clearer image, more dots and less space.
*Note*: Laser printing already uses halftone dots in its printing techniques therefore if applying halftone in Photoshop using "Bitmap" and sending it to laser - this can cause interference in the final product. To get around this: use digital printing for photoshop bitmap techniques and use laser printing for previous illustrator process.
No comments:
Post a Comment