The desired wavelength range for use in nondestructive testing is between 3500 and 3800A with a peak wavelength at about 3650A. This wavelength range is used because it is in the UV-A range, which is the safest to work with. UV-B will do an effective job of causing substances to fluoresce, however, it should not be used because harmful effects such as skin burns and eye damage can occur. This wavelength of radiation is found in the arc created during the welding process. UV-C (1000 to 2800A) is even more dangerous to living cells and is used to kill bacteria in industrial and medical settings.
The desired wavelength range for use in NDT is obtained by filtering the ultraviolet light generated by the bulb. The output of a UV bulb spans a wide range of wavelengths.The short wavelength of 3120 to 3340A are produced in low levels. A peak wavelength of 3650A is produced at a very high intensity Wavelength in the visible violet range (4050A to 4350A),green- yellow (5460A), yellow (6770A) are also usually produced. The filter allows only radiation in the range of 3200 to 4000A and a little visible dark purple to pass.