ACT (Adobe Color Table File)
The ACT Settings file format
ACT files are associated with Adobe Photoshop, a graphic and image editing software developed by Adobe Systems. They are color table files that store information about pre-defined colors. ACT files are also known as CLUT (Color Lookup Table) files. Using an ACT file allows you to optimize your image for publishing it on the web. In Photoshop's "Save for Web" dialog window as well as under "Image Adjustments", one can choose and change the CLUT applied to the image. The colors contained in the ACT files can be reduced or increased which directly effects the quality of the file.
Technical details of ACT files
As stated above, ACT files are available to be applied to the edited image in the "Save for Web" dialog window in Adobe Photoshop. In newer versions, a color lookup table can also be applied to the image while editing using the "Image Adjustments" function. They can be loaded and saved by clicking on the Color Table menu and choosing the according action. Adobe Photoshop comes with a set of predefined ACT files already. They are part of the installation. For all image formats to be able to make use of the CLUT, the ACT file has to be saved in the right folder. When saving a file in the GIF format, the Color Table is part of the "Save for Web" dialog window. It doesn't appear when saving images and graphics as PNG or JPEG. To use them on PNG and JPEG files, they have to be saved under /Presets/Optimized Colors