title: “If they only knew…” date: “2000-06-07 14:00:00” layout: post tags: [ historical, fonts, typography ] —
This post was originally published on TheMacMind.com
This one’s for the pixel hungry crowd who just can’t get enough digital images onto their already stuffed hard drives. Whether you’re scanning magazines to add to your collection of cool Apple advertisements or preparing a file for the local printing bureau, there’s a few things to remember that will make your life a whole lot easier. And save you time and money too…
A small bite of terminology for your platter
Even the best designers can get confused when it comes to conversations about images and resolutions so first let’s cut through the mess and set the record straight on some basic terminology.
- Dots Per Inch. In advertisements if you hear the word resolution you hear DPI. Your monitor for example is generally considered to have 72dpi (however this isn’t exactly true for multiscan monitors with variable resolutions). DPI is very vague so forget everything you’ve heard about it and ignore it. We’re going to use terms that fit better with what we’re talking about.
- Pixels Per Inch. This term is used to describe digital images. It simply represents the number of pixels of information per inch in the file.
- Spots Per Inch. This term is used to describe imaging devices such as an ink jet printer, a laser printer or the image setter at your local service bureau. Think of SPI as the number of ink drops or laser spots per inch. Your inkjet may have 1440 little drops of ink per inch and your laser printer may have 1200 laser spots per inch (Bare with me here, I know the manual says DPI, but think SPI)
- Lines Per Inch. Lines per inch is a common term among the printing industry. It refers to the line screen that is applied to an image when printed. If you’re not sure what I’m referring to grab a newspaper and look at an image closely. You’ll notice there are rows of dots, this is the screen of the image. LPI is the number of rows of these dots per inch. A newspaper is typically around 85lpi while the image in a glossy book is about 150-180 lpi.
Now on to the main course
If your laser printer prints at 1200 spi, your image should be 1200 ppi, right? Wrong.
This is a common assumption but it is far from true. Most types of printers are binary devices. This means that in the 1200 laser spots per inch, each can only be white or black. So what happens if you’re printing a grayscale image? How is the black and white printer going to print gray? Well it simply uses a bunch of black spots and spaces them further apart for light gray and closer together for dark gray. Unless your looking really, really close, your eye perceives gray but really it’s just black and white.
It’s here where the LPI term comes into the mix. If your laser printer puts down 1200 spots per inch and your output device is using an LPI of 120, the device will use 10 laser spots for every one dot in the line screen. In reality your image file only needs to contain enough information to tell the output device what the image looks like on each of the 100 dots in the line screen. If you’re using a 120 lpi screen when printing, your file only needs to have 120 pixels of information per inch.
The same idea applies for your color ink jet however with color, there are four printing heads (cyan, magenta, yellow and black) that overlap to create the many colors you see. Each of the individual colors are the same as a laser printer in that light yellow is spaced out spots and solid yellow is spots that are close together.
For best results, determine the LPI your image will be printed in and multiply it by 1.5 to determine your required image resolution. The extra .5 gives the printer a little extra when going from one shade to another and results in smoother transitions through shades.
If printing from home, you can set the LPI for laser devices using the Page Setup or the Print commands. Look for LPI or possibly Line Frequency or Line Screen and set it to around 100 lpi. If your file will be professionally printed elsewhere, consult with a technician to determine the proper LPI for their process. For ink jet devices use a file that’s about 300 pixels per inch and if the file is for use on a web site, make sure it’s no more than 72 dpi (which is the same as your screen).
Your sweet dessert
A file with 120 dots per inch takes up about 1% of the disk space for the same file at 1200 dpi, but on most printers they will look exactly the same when printed with a 100 line screen. If you apply the same idea to color images, you will be saving more than three times the disk space (color image are composed of multiple greyscale channels such as RGB or CMYK, each of with will be reducded in size). Now with smaller files you won’t need to purchase a mammoth hard drive and files will be quicker to scan and transfer over networks.
Now before you start gorging again to fill up your newly acquired disk space remember these few simple things:
- All files should be saved at final size, the size they will be presented when printed.
- Multiply the Line Screen (LPI) you will be using on the output device by 1.5 to determine what image resolution you will need.
- When someone says an image is 170 dpi, think PPI
- When someone says their printer is 1200 dpi think SPI
And when someone tells you to scan your image at 1200dpi to output on their 1200dpi printer look at them, smile and then think: “If they only knew.”