Final answer:
To calculate the required new exposure time to maintain the radiographic density when changing the SID and film-screen combination, principles of the inverse square law and knowledge of film-screen speed are applied. A specific calculation cannot be provided without the formula or additional information.
Step-by-step explanation:
The student's question relates to the properties and calculation principles used in radiographic imaging, specifically the required exposure time adjustment when changing both the Source-to-Image Distance (SID) and the film-screen speed for maintaining the same radiographic density. To find the new exposure time, we must account for the changes in geometric factors due to the increased SID and the change in sensitivity of the film-screen combination.
When increasing the SID from 36 inches to 40 inches, the exposure is decreased according to the inverse square law. Additionally, moving from a 400-speed to a 200-speed film-screen combination means the receptor is less sensitive and thus requires double the exposure.
The original conditions used were: 300 mA, 30 ms exposure time at an 80 kV setting. To maintain the original density with the changes to SID and film-screen combination, we need to adjust the exposure time accordingly. However, without the necessary formula or additional information on how film-screen speed affects exposure, no specific new exposure time can be calculated here.