is a non-compressed RGB image, which can be transferred to a computer's memory. The color output of different scanners is not the same due to the spectral response of their sensing elements, the nature of their light source and the correction applied by the scanning software. While most image sensors have a linear response, the output values are usually gamma compressed. Some scanners compress and clean up the image using embedded firmware. Once on the computer, the image can be processed with a raster graphics program (such as Adobe Photoshop or the GIMP) and saved on a storage device (such as a hard disk). Images are usually stored on a hard disk. Pictures are normally stored in image formats such as uncompressed Bitmap, "non-lossy" (lossless) compressed TIFF and PNG, and "lossy" compressed JPEG. Documents are best stored in TIFF or PDF format; JPEG is particularly unsuitable for text. Optical character recognition (OCR) software allows a scanned image of text to be converted into editable text with reasonable accuracy, so long as the text is cleanly printed and in a typeface and size that can be read by the software. OCR capability may be integrated into the scanning software, or the scanned image file can be processed with a separate OCR program. Document processing Document imaging requirements differ from those of image scanning. These requirements include scanning speed, automated paper feed, and the ability to automatically scan both the front and the back of a document. On the other hand, image scanning typically requires the ability to handle fragile and or three dimensional objects as well as scan at a much higher resolution. Document scanners have document feeders, usually larger than those sometimes found on copiers or all-purpose scanners. Scans are made at high speed, from 20 up to 280 or 420 pages per minute, often in grayscale, although many scanners support color. Many scanners can scan both sides of double-sided originals (duplex operation). Sophisticated document scanners have firmware or software that cleans up scans of text as they are produced, eliminating accidental marks and sharpening type; this would be unacceptable for photographic work, where marks cannot reliably be distinguished from desired fine detail. Files created are compressed as they are made. The resolution used is usually from 150 to 300 dpi, although the hardware may be capable of 600 or higher resolution; this produces images of text good enough to