Impulse (salt-and-pepper) noise
The PDF of (bipolar) impulse noise is given by
……(1)If b > a, gray-level b will appear as a light dot in the image. Conversely, level a will appear like a dark dot. If either or is zero, the impulse noise values woll resemble salt-and-pepper granules randomly distributed over the image. For this reason, bipolar impulse noise also is called salt-and-pepper noise. Shot and spike noise also are terms to refer to this type of noise.