Use the Add Noise effect to reduce the detail in an image and add a grainy texture. This effect can also be used for image correction. By adding a small percentage of noise to an image, you can reduce the appearance of small imperfections and scratches.
To use the Add Noise effect:
Choose Adjust > Add/ Remove Noise > Add Noise to open the Add Noise dialog.
Choose the pattern of the noise: Random, Uniform, or Gaussian. With Uniform noise, the color of the noise resembles the original pixels. With gaussian noise, the majority of the noise values are even closer to the original.
In the Add Noise edit box, select the percentage of noise to add.
To use black and white noise pixels, mark the Monochrome check box. To use colored pixels, clear the check box.
Click OK.
Mathematically Combining Two Images