What is the main reason for adding a filter to the primary beam?

Adding a filter to the primary beam primarily serves to decrease patient dose. In medical imaging, particularly in radiography, filters are utilized to selectively absorb low-energy photons that contribute little to the image quality but significantly increase the radiation exposure to the patient. These low-energy photons are often referred to as "soft" radiation, which can be easily absorbed by body tissues without contributing to useful diagnostic information.

By reducing the dose of these low-energy photons, the filter enhances the quality of the image while simultaneously protecting the patient from unnecessary radiation exposure. This is a critical aspect of optimizing radiographic techniques, as minimizing patient dose is a key principle in radiological practices, particularly in ensuring patient safety and adhering to the ALARA principle (As Low As Reasonably Achievable).

The other options do not correctly reflect the main purpose of using filters in the primary beam. For instance, while filters may influence contrast, that is not their primary purpose. Similarly, increasing signal strength or wavelength is not aligned with the function of filters in radiographic imaging.