VueScan reads raw CCD sensor data from scanners and can write this to a raw TIFF file for later reprocessing. The final cropped data can be stored in any combination of TIFF, JPEG, PDF and OCR text files. Index prints are stored as a Windows BMP file.
The raw and cropped TIFF files can have six different formats, each with a different number of samples per pixel and bits per sample. A grayscale image has one sample per pixel, a normal color image has three (red, green, blue), and scans from a scanner with an infrared channel have four samples per pixel (red, green, blue, infrared).
VueScan internally keeps all samples in 16-bit linear format, even when a scanner only supports 10-bit samples, but to minimize the disk usage, various TIFF file formats are supported:
1 bit B/W 1 bit per pixel 1 sample per pixel 1 bit per sample
8 bit Gray 1 byte per pixel 1 sample per pixel 8 bits per sample
16 bit Gray 2 bytes per pixel 1 sample per pixel 16 bits per sample
24 bit RGB 3 bytes per pixel 3 samples per pixel 8 bits per sample
48 bit RGB 6 bytes per pixel 3 samples per pixel 16 bits per sample
64 bit RGBI 8 bytes per pixel 4 samples per pixel 16 bits per sample
16 bit Infrared 2 bytes per pixel 1 sample per pixel 16 bits per sample
If you want to process the full bit depth of an image in Photoshop(TM), use the 48 bit RGB setting for the Crop TIFF file. Note that some other image editing tools cannot process 48 bit TIFF files; in this case use 24 bit which is more widely compatible.
Note that the raw scan files are stored in linear format when using more than 8 bits per sample, and stored in gamma 2.2 format when using only 8 bits per sample. The saved TIFF files are always gamma corrected according to the Color | Output color space used (1.8 for Apple RGB, ColorMatch RGB, ProPhoto RGB and ECI RGB and 2.2 for all other color spaces). Note that the raw scan files stored in linear format will look dark when viewed - this is normal.
Note that both the raw TIFF file and the crop TIFF file can be compressed. VueScan uses CCITT Group-IV compression for 1-bit files, and LZW compression otherwise. This may be slower to write, but takes around 40% less disk space. The size of JPEG files can be controlled with the Output | JPEG quality option, with useful values ranging from 75 (very compressed, medium quality) to 95 (less compression, high quality).