Which of the following factors will contribute to longer-scale radiographic contrast?

Long-scale radiographic contrast refers to images that display a wider range of grays, providing more detail in the transitional areas between light and dark areas. This is essential in medical imaging for better visualization of structures.

An increase in photon energy directly contributes to longer-scale contrast because it allows for a greater penetration through tissues. Higher energy photons result in a more consistent transmission of radiation across various tissue densities, which helps in producing more subtle gradations between different shades of gray on the radiographic image. Thus, higher photon energy effectively enhances the image's overall contrast range by allowing for finer distinctions in tissue composition, resulting in longer-scale contrast.

The other factors mentioned do not contribute to longer-scale contrast in the same way. While increasing kV can affect the overall exposure and contrast, it typically leads to shorter-scale contrast at lower levels of photon energy. Moreover, increasing the grid ratio mainly affects image receptor exposure and reduces scatter, which can influence contrast but does not inherently support longer-scale contrast. Therefore, photon energy is the primary factor among the options provided that leads to longer-scale radiographic contrast.