UV-A Lamp Filters

The filter on the front of the UV-A lamp is extremely important for several reasons. First and foremost is that of safety. Although even with a functioning filter in place it is still hazardous to expose human tissues to the light for long periods, if the filter was damaged or removed the organic tissue would be exposed to wavelengths below 315nm (known as UV-B and UV-C) which are extremely dangerous.

Another function is to filter out wavelengths that would result in a loss of sensitivity in the test. For example, any white light and crucially wavelengths of approximately 550-570nm.
The filters on the front of the lamps must be checked regularly. If filters are found to be cracked, do not use the lamp. The equipment should be switched off immediately and reported to the appropriate supervisor.

The efficiency of the high pressure mercury vapour discharge lamp can deteriorate quickly without any obvious effect. Their intensity should be checked regularly with a radiometer, such as the one specified by BS EN ISO 3059. Only high pressure mercury vapour lamps are used for irradiating the fluorescent detection media.
Low pressure discharge lamps are only used for ambient lighting conditions. NDT equipment manufacturers now make LED (light emitting diode) UV-A inspection lamps but some aerospace primes do not allow use of LED UV-A lamps. LED UV-A lamps do not need to warm up before use.

Notes:

  1. The temperature alters the pressure inside the quartz crystal capsule which in turn alters the spectrum of emitted wavelengths.
  2. Handheld lamps have 100/125W pre-focussed bulbs with a medium screw base and a transformer to provide correct voltages.
  3. 400W flood lights are available, but more care is required with respect to operator protection.
  4. Line voltage fluctuations of as low as 8V will switch off the light source.
  5. Strong magnetic fields will extinguish the arc.
  6. Dust and contamination on the filter and bulb effects output.
  7. Switching on and off will take approximately 2 hours off the life expectancy of the bulb.
  8. The bulb can overheat and switch off.
  9. The output drops after approximately 3 months usage but should re-stabilise at a lower level.