Capturing Images

An image scanned by a flatbed scanner requires large amounts of disk space to be stored on the hard drive. The amount required increases with the size of the image and its resolution. Scanning an A4-size image with 256 grey levels at 300 dpi will require around 8 MB of disk space and RAM, which rises to more than 20 MB if you scan with 24 bit colour.

Bits Per Pixel	1	4	6	8	24
Colours or Grey Shades	1 Black or White	16 Greys	64 Greys	256 Greys or Colours	16.7 Million Greys or Colours
File Size of A6 Image	300kB	1MB	1.5MB	2MB	5.5MB
File Size of A4 Image	1.5MB	4.5MB	6.5MB	8.5MB	26MB

The Scanner

Scanners allow you to capture images and save them onto your computer. To scan an image place your original image face down onto the glass screen and the scan head passes across the image capturing it as it goes. Flatbeds are very useful for creative design as they are easy to use and are very reliable in scanning faithful colours and line artwork. You can work on the images just like with any other art medium. Scanners work by using a Charge-Coupled Device (CCD) which is a light sensitive scanning head. The CCD consists of a bunch of very small sensors that each respond to one of three colours, red, green or blue. The sensors respond to various intensities of light being reflected from an original by generating an electric charge. Black reflects little light and so generates a low charge, while white is very reflective and generates much higher voltages. The electric charges are converted into a series of 1s and 0s, each representing a single pixel (in 1 bit mode), which corresponds to the original image. 1 bit mode is another name for black and white, it reduces the image to pixels which are either on or off (on = black, off = white). 8 bit mode is a colour mode that can produce 256 greys/colours, each pixel can be produced in 256 combinations of 1s and 0s. 24 bit mode is a more advanced colour mode that can produce 16.7 million colours, each pixel can be produced in 16.7 million combinations of 1s and 0s.

Resolution

The quality of a scanned image depends on the resolution of the scanner. The more dots per inch, the sharper the image will be. The final appearance of an image depends entirely on how it is to be output. If the image is only going to be viewed on screen or printed by a 300 dpi printer there isn’t much point in scanning an image at a high resolution. It would be worth scanning at high resolution if you are planning on cropping the image.