Exposure rate decreases with an increase in which factor?

The exposure rate is influenced by several factors in radiography, and one of the most significant is the Source-to-Image Distance (SID). As SID increases, the distance between the X-ray tube (the source) and the image receptor increases, which results in a decrease in exposure rate. This is due to the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source. Therefore, as the distance increases, the exposure rate significantly decreases because the X-ray beam spreads out over a larger area.

In contrast, increasing kilovoltage generally increases the penetration power of the X-ray beam, leading to a higher exposure rate because more suitable photons reach the image receptor. Additionally, altering the focal-spot size primarily affects image sharpness and resolution rather than directly impacting the exposure rate. Hence, the only factor among the provided options that decreases the exposure rate when increased is the SID.