What is the minimum UV intensity required by most codes at the surface during NDT?

• A. 100 microW/cm^2
• B. 5000 microW/cm^2
• C. 10000 microW/cm^2
• D. 1000 microW/cm^2

Answer: (D)

When performing LPI, the fluorescent penetrant must be excited enough by UV-A light to reveal any surface indications under normal viewing conditions. Codes set a practical minimum for this surface illumination to ensure detectability, not to exceed safety or equipment limits. The required threshold is 1000 μW/cm^2 at the surface. This level provides reliable fluorescence for typical penetrants and aids visibility of flaws while avoiding unnecessary glare or hazards. A value as low as 100 μW/cm^2 generally wouldn’t reliably excite the fluorescence, so it wouldn’t meet the standard. Values like 5000 or 10000 μW/cm^2 are higher than the minimum and may be used in certain setups, but they are not the required minimum specified by most codes.