How does the collimator affect scatter radiation ?

Collimation affect decreases the  scatter radiation.  

Explanation:

As we know that collimation in radiography reduces the size of x-ray beam which results in low amount of scatter radiation in the patient’s body. This less amount of radiation extends to the IR.  The decreased radiation increases the contrast and decreases the brightness of radiographic images and vice versa.

Its increased amount also reduces the tissue volume that is irradiated. While increasing the amount of collimation the technique factors were needed to be increased to maintain the exposing factor to the IR.