"DPI" and the misuse of graphic arts terminology

The prepress and press worlds are some of the worse misusers of terminology with the all too frequent resulting confusion in sales, marketing, specification, and production. Here is one of the most misused: "DPI" (or as it is spoken of in the rest of the world: DPCM).

"DPI" - Dots Per Inch is a term used for a variety of things that properly speaking it shouldn't.

DPI - when used to describe the resolution of a computer to plate imaging device or filmsetter. E.g. "This is a 2400 dpi CtP device."

"Dots" in this case refers to the laser "Spots" of energy that expose the printing plate or film. However, while DPI, identifies the number of dots per inch - it doesn't actually describe the resolution of the device or size of the spot of energy. Instead it defines the device's "addressability." In other words, dpi tells you how many locations per inch a spot of energy can be focussed on – not the actual size of the spot of energy.

This graphic shows plate media being exposed at 2,400 dpi by six different CtP devices:Note that they are all 2,400 dpi - that is that they all can hit the target location (address) with their beam of energy - however the exposing spots of energy are all different sizes, in this example ranging from about 2 microns on the left to about 30 microns on the right.

Resolution vs addressability is explained in more detail by clicking HERE.

DPI - when used to describe the resolution of an inkjet printer. E.g. "This inkjet proofer prints at 2880 x 1440 dpi."
In the case of an inkjet printer, the clue to this misuse of dpi to wrongly mean resolution is revealed with asymmetrical dpi specifications. So, an inkjet proofer that has the specification that says it prints at 2880 x 1440 dpi does not mean that the resolution is finer, or that the droplets of ink are finer in one direction. Instead it simply means that the paper is moved more slowly in one direction - i.e. the addressability is changed - while the physical size of the droplet of ink, and hence its resolution remains the same.
On the left a symmetrical inkjet addressability grid (600 x 600 dpi). On the right the same printer set at 1200 x 600 dpi. The addressability has changed but not the size of the cyan droplet of ink and therefore the actual resolution of the device remains the same.
In any case, the actual size of the mark the droplet of ink makes on the paper is unknown. For a 600 or 1440 "dpi" ink jet printer it most certainly is not 1/600ths or 1/1440th of an inch in size. As a result, with some inkjet printers, reference is sometimes made to "picoliters" in addition to dpi when the resolution of the device is described in the specifications. A picoliter is a unit of fluid volume. A lower minimum ink volume tends to yield a smaller minimum droplet size of ink allowing more dots of ink to be in the same area thereby yielding higher actual resolution. While picoliter is a better indicator of the relative size of the splat of ink on the paper it is still a unit of volume and not area. So it suggests a difference in resolution but doesn't actually specify it.

DPI - when used to describe the resolution of an image scanner. E.g. "This is a 600 x 2400 dpi scanner."
An image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. The resolution of Digital images is usually expressed as dots per inch or pixels per inch. As a result the resolution of scanners is often expressed in terms of dpi (and sometimes "ppi" pixels per inch). The more accurate description is "spi" which stands for "samples per inch" since scanners sample the document they are scanning.

A related issue with defining scanner resolution is that manufacturers typically refer to the scanner's interpolated resolution - which is a software upsampling algorithm method to increase the pixel density - instead of using the scanner's true optical resolution. If the scanner's dpi is asymmetrical (e.g. 600 x 2400 dpi) then the smaller number usually indicates the particular number of individual samples that are taken in the space of one linear inch while the larger number is the interpolated samples.

DPI - when used to describe the resolution of an image. E.g. "This is a 300 dpi image."

Once an image has been digitized, either via scanning or captured with a digital camera, it is in the form of a raster image made up of pixels (picture elements). In graphic arts usage the pixels are typically square in shape and 8-bits (256 grey levels) in depth per channel (greyscale = one channel, RGB = three channels, CMYK = four channels).

Because pixels are generally thought of as the smallest single component of a digital image, the more pixels that are used to represent an image, the closer the result can resemble the original.As ppi, a.k.a. "dpi", increases so does the amount of image detail that can be rendered creating the impression of greater apparent resolution.Pixel counts can be expressed as a single number, e.g. an image at 100% reproduction size being 300 "dpi", or as in a "three-megapixel" digital camera, which has a nominal three million pixels, or as a pair of numbers, as in a "640 by 480 display", which has 640 pixels from side to side and 480 from top to bottom (as in a VGA display), and therefore has a total number of 640 × 480 = 307,200 pixels or 0.3 megapixels.
Again, the measures dots per inch (dpi) and pixels per inch (ppi) are sometimes used interchangeably, but have distinct meanings, and although dpi is often used to refer to digital image resolution the proper term is "ppi" - pixels per inch.

Image resolution for printing - LPI vs DPI a.k.a. LPI vs PPI a.k.a. LPI vs SPI

Background - pixels make the original image

A digital "raster" image acquired from a scanner, a digital camera, or created directly in a "paint" application like Adobe Photoshop is made up of a mosaic of "pixels" (picture elements)."
Here is an original image at actual size:

Here is a close up view showing the actual pixels that form the image:

The physical size of the image is described by two numbers which can be expressed two ways:

1) The number of pixels per inch/centimeter.
and
2) The number of pixels in both horizontal and vertical dimensions.

Or:

1) The number of pixels per inch/centimeter.
and
2) The horizontal and vertical dimensions expressed in inches/centimeters.

Those are just two ways of saying the same thing.
Here is the original image with a dialog box showing its dimensions:

Note that the dimensions have a "lock" icon beside them. This is because the relationship of pixels per inch (ppi) and vertical/horizontal size are "locked" together. Changing one changes the other as you can see in the below dialog boxes (click on image to enlarge):

Note that as the resolution is changed (from 600 to 300 and 300 to 150 pixels per inch) only the density of the pixels changes, not the number of total pixels in the image, in this case 1412 pixels x 2028 pixels, therefore the file size remains the same. Put another way, each time the resolution in ppi is increased, or lowered, the physical image size changes but the total number of pixels forming the image (and hence the detail) remains the same.

Note that I use the term "pixels per inch" - ppi. Very often the term that is used is "dots per inch" or dpi. Technically the terms are not interchangeable - however, in daily usage, when speaking about digital images the terms are considered as meaning the same thing. You may sometimes hear the term "spi" - samples per inch. This refers to a scanner's resolution - i.e. it ability to acquire an image at so many samples per inch (e.g. 300 spi). Again, in practical usage, when speaking about digital images - ppi, dpi, and spi can be understood as meaning the same thing.

Interestingly, digital cameras typically do not have a resolution assigned to them.

Instead a digital camera captures data based on the "megapixel" ability of its CCD sensor. For example, a 14.2 megapixel camera might capture an image that's 4592 pixels by 3056 pixels, which equals 14,033,152 total pixels. When you open the file into an image-editing program a resolution must be assigned to the file. Most programs, including Photoshop, use 72 ppi as the default resolution.

Background - halftone dots make the image reproduction

Because printing presses can only lay down 100% ink or 0% ink, digital images acquired from scanners, digital cameras, or created directly in "paint" applications need to be converted into a binary (on/off) format. This is done through a process called halftone screening. The result is that the image will be converted to dots of either 100% or 0% ink with the original tones being simulated, in this case, by the size of the dots. Bigger dots represent darker tones - smaller dots represent lighter tones:

The fineness of the screen, and hence the level of detail in the original that can be preserved, is determined by how densely packed the dots are and is indirectly described by how many rows - or lines of dots are used per inch (or centimeter) to create the image. These virtual lines are highlighted in red below:
In this example the image is made up of 85 lines of dots per inch – expressed more commonly as an 85 lines per inch halftone - or more simply stated: an 85 lpi halftone image.

The key thing to remember is that although the halftone image is made up of dots - the level of detail that it can reproduce is described in terms of lpi NOT dpi.
So, original image pixel density/detail = ppi, spi, or dpi. Halftone reproduction dot density/detail = lpi.

Of course, in order to pack more lines of dots into an inch - the smaller the dots become and hence the greater amount of image detail that is preserved.

40 lpi halftone:

100 lpi halftone:

200 lpi halftone:

It is the relationship of how densely packed the original pixels are (see part 1) compared to the frequency of lines per inch of the halftone screen dots that determines what image resolution is appropriate for its reproduction in print.

The relationship between dpi/ppi and lpi for
grayscale images

The guiding principle for understanding what original image resolution (ppi/dpi) is needed compared to the halftone screen (lpi) that will be used is that the image pixels should always be more densely packed (ppi/dpi) than the detail resolving ability (lpi) of the halftone screen that is used.

To illustrate this principle I'll take a section of the same image at different resolutions (ppi/dpi) and reproduce it using the same 150 lpi halftone screen:

Original 75 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is one half of the halftone screen resolution (lpi). As a result the halftone reproduces the individual pixels of the original. This visible artifact is termed "staircasing," the "jaggies," or "pixelation."

Original 100 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is two thirds of the halftone screen resolution (lpi). As a result the halftone still reproduces the individual pixels of the original - but they are less visible.

Original 150 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is equal to the halftone screen resolution (lpi). As a result the halftone still reproduces the individual pixels of the original - but they are much less visible.

Original 225 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is 1.5 times greater than the halftone screen resolution (lpi). Although some original image pixels may still be visible, in general, the halftone no longer resolves the individual pixels of the original - just the tones they represent.
This minimum required original resolution can be represented by the formula: 1.5 X lpi = ppi @ 100% reproduction.

Original 300 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is twice the halftone screen resolution (lpi). As a result the halftone no longer resolves the individual pixels of the original - just the tones they represent.
This ideal required original resolution can be represented by the formula: 2 X lpi = ppi @ 100% reproduction.

Original 600 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is four times the halftone screen resolution (lpi). The image file size is about 7 times larger than the 225 ppi/dpi image but provides effectively no difference in the final reproduction.

The relationship between dpi/ppi and lpi for CMYK
images

As with grayscale images, the guiding principle for understanding what original image resolution (ppi/dpi) is needed compared to the halftone screen (lpi) that will be used is that the halftone screen should not reproduce the image pixels themselves but instead the tones the pixels represent. It is worth comparing these images to their grayscale equivalents in part 3.

To illustrate this principle, I'll take a section of an image rendered at different resolutions (ppi/dpi) that has been converted from grayscale to CMYK and reproduce it using the same 150 lpi halftone screen:

Original 75 ppi/dpi - halftone screen 150 lpi:Here the image ppi/dpi is one half of the halftone screen resolution (lpi). As a result the halftone reproduces the individual pixels of the original. This visible artifact is termed "staircasing," the "jaggies," or "pixelation." That being said, the jaggies are less severe than we saw in the grayscale image at the same ppi/dpi. Also the numbers on the sail appear clearer. This suggests that it might be possible to use a lower image resolution for reproducing a CMYK image than can be used for a grayscale image.

Original 100 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is two thirds of the halftone screen resolution (lpi). As a result the halftone still reproduces the individual pixels of the original - but they are less visible.

Original 150 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is equal to the halftone screen resolution (lpi). Because the CMYK image is a composite of four individual halftone images it tends to lessen the visibility of the individual pixels of the original.
This minimum required original resolution for a CMYK image can be represented by the formula: lpi = ppi @ 100% reproduction.

Original 225 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is 1.5 times greater than the halftone screen resolution (lpi). The halftone no longer resolves the individual pixels of the original - just the tones they represent.
This ideal original resolution can be represented by the formula: 1.5 X lpi = ppi @ 100% reproduction.

Original 300 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is twice the halftone screen resolution (lpi). As a result the halftone no longer resolves the individual pixels of the original - just the tones they represent.
This maximum required original resolution can be represented by the formula: 2 X lpi = ppi @ 100% reproduction.

Original 600 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is four times the halftone screen resolution (lpi). The image file size is about 7 times larger than the 225 ppi/dpi image but provides effectively no difference in the final reproduction.

The below table provides image resolution requirements for a variety of typical print applications:Note that this table refers to conventional "AM" halftone screening where the lpi signifies the dot density and hence the resolution of the halftone screen. However, there is another type of halftone screen in use which does not have a traditional lpi. Instead, this type of screening organizes the halftone dots in random appearing patterns. Below are three different vendor's offerings (click on images to enlarge):
This type of halftone is called "FM" or "Stochastic" screening (covered in other posts in this blog). Rather than indicating resolution according to "lpi" - the average actual dot size, specified in microns, of the screen pattern is used instead. Typical dot sizes are: 10 - 20 micron for commercial work, 20 - 25 micron for magazine work, and 35 micron for newspaper work. Because this type of screening has a higher average resolution than conventional AM screening - it's a good idea to use images at a higher resolution to take advantage of this screening's detail rendering capability. Typically 400 ppi/dpi for 10-20 micron FM, 300 ppi for 25 micron, and 200 ppi/dpi for 35 micron.

Image resolution "gotchas" – where things can go wrong

Whether you are targeting your images for AM or FM screening, there are at least three places where the resolution of the images may be accidently altered:

1) If the image is resized/scaled in the page layout application – it may no longer have an appropriate resolution:
2) If the image is resized/scaled when the file is converted to the PDF format – it may no longer have an appropriate resolution:
3) If the printshop's workflow is setup to resample incoming documents – they may no longer have an appropriate resolution. Most prepress RIPs are set, by default, to downsample incoming files to 300 ppi/dpi.