Image resolution, the pain in my...brain!

I've been a Graphic Designer for around 20 years and this week I heard a term that I hadn't come across before (at least not that I remember). Samples per inch (SPI) the term came from a communication with a British publication I was sending a cover image for their magazine.

DPI, PPI, SPI; could image resolution be any more complicated. For those who don't know, DPI stands for Dots-per-inch, PPI, pixels-per-inch and SPI Samples-per-inch.

I'm not vain enough to think I know everything in my industry, but I know quite a bit and I had never heard of SPI before, so I did what anyone would do, I looked it up on the internet. I found the following definition on Wikipeadia.

Samples per inch (SPI) is a measurement of the resolution of an image scanner, in particular the number of individual samples that are taken in the space of one linear inch. It is sometimes misreferred to as dots per inch, though that term more accurately refers to printing resolution. Generally, the greater the SPI of a scanner, the more detailed its reproduction of the scanned object.
Typical consumer-level flatbed scanners are capable of optical resolution ranging from 100 to 2400 SPI; high-end scanners may have an optical resolution of 4800 SPI or more. Many scanners use interpolation techniques to achieve a higher effective SPI rating, with some manufacturers offering nearly one million SPI, though the quality is primarily limited by the optical resolution; interpolated SPI does not provide additional pixels in the scanned image.
Horizontal and vertical SPI ratings may differ for a given scanner; typical flatbed scanners use a horizontal array of sensors that are passed across the bed using an electric stepper motor. The density of the array of sensors determines the horizontal scanning resolution, while the minimum step size of the motor determines the vertical resolution. Similar characteristics are present in drum scanners, which continuously spin the item being scanned past the sensor array for numerous imaging passes.
To calculate the number of raw data bytes that a scanned image will take up, you can use the follow formula :
Where : vSPI is the vertical SPI hSPI is the horizontal SPI (can be considered the same as vSPI if not specified specifically) area is the squared area of the scanned document in inches²
color depth encoding is the number of bits used to encode a given amount of color information (256 colors=8; 65'536 colors=16; 16million colors=24)
Remember that this will give raw data bytes, images are almost always compressed when saved to disk using lossless (like PNG, TIFF) or lossy image formats (like JPEG).
That is understandable to me, but not to most people, and most of the scanners that I've owned don't have the SPI measurement at all. Most have DPI but that isn't the same as SPI.

DPI stands for dots-per-inch. This is primarily a printing term standing for the number of halftone dots within an inch. The more DPI you have, such as 300 dpi, the better quality of image you have on the printed page. Most scanners have this measurement when you scan in an image.

PPI stands for Pixels-Per-Inch. PPI is the screen resolution or resolution you see on your monitor. In general PPI and DPI are used interchangeably although they don't mean the same thing. 300 ppi is close enough 300 dpi so it makes little difference when scanning. If you notice your scanner probably says dpi and when looking at it in Photoshop it shows ppi. When creating an image for the internet 72 ppi is generally the accepted measurement.

I explain all this to my students and they look at me like I just grew horns out of the top of my head. To make it more difficult for them to understand, the resolution you are using needs to be the dpi, ppi. spi at the size you will be using the image. For example, if you have a space in your brochure that is 4 inches wide, you need your image to be 300 dpi, ppi at 4 inches wide. If you are putting an image on the web at 600 pixels wide then you need your image to be around 72 ppi, dpi at 600 pixels wide.

Onscreen graphics don't need to be as hi-resolution as graphics that are going to print. This is the reason you can't get a great quality print from an image that is optimized for the web.

The rule-of-thumb I teach is, you can always scale something smaller but you shouldn't scale bigger than the original resolution, if you do you will lose image quality.

As far as Samples per inch goes, I'm doing what I always done. Scan or shoot my digital images at the ppi or dpi that I need, and not worry about SPI until someone brings it up again.

©2010 Michael Burke