Photoshop RAW File Format

A Raw File normally refers to an image that is an unaltered file from either a scanner or a digital camera; an image that has not had any colour correction or levels settings done.

FIGURE 1

While scanners usually create TIFF or JPEG files, Camera Raw files can come in many different formats depending on the camera make and model.  But there is also a quite separate Photoshop RAW format.

FIGURE 2

Most formats, such as TIFF and JPEG, have a section at the beginning of the file called a header to hold extra information. Here is held the type and order of the pixels, how many pixels wide and high the image is, how many bits per pixel, etc. All this is necessary information that is saved automatically with the file so that it can be viewed correctly when opened. But in a RAW file all header information is left out and only the pixels themselves are saved. RAW files were used in closed work areas where all images had the same details. For example, a science lab may take images through a microscope to be used for their own research. All the images would be the same dimensions, colour, etc, and so there would be no need to store that information within the files. Any equipment would have these details and so have no trouble understanding them. It is only when images need to be exchanged externally that the extra header information has to be saved with the file, thus allowing others to use them correctly.

FIGURE 3

The image from the previous page had the following details and was saved as a RAW file:
• 1800x1200 pixels
• 3 Channels
• Interleaved pixels
• 8 Bits per pixel
• No header
Since this information was not saved into the file header then Photoshop will prompt for the data when the file is opened.But if the correct information is not known then it will have to be guessed at.

Most images have three colour channels with 8 bits per channel, so it is usually just the dimensions that need to be guessed at.  The Open Options has a Guess button, which will force Photoshop to try the most likely figures.  But the important point is that should you ever use the Photoshop RAW format, then do remember to write down the image details first.