Spanish to English Medical Photography/Imaging (& Graphic Arts)

Hello everyone,

I'm translating the curriculum of a degree in Imaging/Radiography and when translating the syllabus of one of the courses I ran into the term "velos químicos". I looked it up and the definition I found in Spanish says that a velo is when "la radiografía aparece ennegrecida, con ausencia de imagen" and that it looks like a "cortina gris". I tried to find an equivalent in English but I couldn't. Does anyone have any idea? Is there a more specific term in English or should I just gloss it?

Thank you!!