In the radiographic histogram, what does the Y-axis represent?

• A. Specific signal intensity values from low to high
• B. Frequency of specific signal intensity values
• C. Number of lines-pairs recorded by the receptor
• D. Range of x-ray energies absorbed by the receptor

Answer: (B)

In a radiographic histogram, the focus is on how often each gray level occurs in the image. The vertical axis shows the frequency — the number of pixels that have a given signal intensity. So the Y-axis represents how many pixels share each gray value, while the horizontal axis represents the gray levels themselves.

This helps you understand the exposure distribution: a histogram with most pixels clustered in mid-gray indicates a well-balanced exposure, while shifts toward dark or light regions signal under- or overexposure. The other descriptions don’t fit because they refer to the horizontal axis (specific gray values), a different concept (lines-pairs or resolution), or beam energy, none of which describe what the Y-axis measures.