Problem Solving Forum
May 1 - May 5, 2017
Is it practical to use black light to check for dirt or other foreign matter during surface preparation inspection?
Tom Swan of M-TEST on
May 4, 2017:
There is no one answer to this. It depends on wha ...read more There is no one answer to this. It depends on what is on the surface. It only works if what you are looking for will fluoresce at the frequency of light used. Most dirt will not fluoresce. Most hydrocarbons will fluoresce at UV or black light frequencies, but synthetic oils such as silicone oils and most cutting fluids will not. For oils, a light in the range of 365 nm to 395 nm (UV-A) generally works better than black light (400 nm to 450 nm). It is difficult or impossible to use this method in daylight or in a brightly lit area, so it should be done at night or with the lights off. The brightness of the UV light is also important. The little lights used to check photo IDs or money generally will not work. Generally, the light should be at least 4000 to 5000 W/cm2 to provide sufficient light. To get a UV light that will work will probably be $400 - $1,000 or higher. Black lights are much less expensive ($30 - $100) but may miss some hydrocarbons the UV would find. I always recommend using a second method to confirm cleanliness prior to painting.