ASTM E3022 requires all LED UV-A lamps include a UV-A pass filter, but why is this important?
The quick answer is that UV-A pass filters reduce glare and increase contrast of fluorescent indications.
From a technical point of view, ASTM defines UV-A as 320-400 nm, and visible light as 400-760 nm. The emission spectrum of an UV-A LED is a single peak in the 360-370 nm range. But this is not a tight, sharp peak, and at the tail end you get measurable emission above 400 nm.
Tail emission above 400 nm is a very small amount of light compared to the bright UV-A peak, but it is visible as a deep violet glow on the surface. Using a calibrated laboratory spectroradiometer, the tail emission can be measured and characterized by the lamp manufacturer. However, regular visible light meters normally used to check ambient light in an inspection area are not capable of measuring those tail emissions accurately. Abnormally high readings may be seen even when the actual visible light levels are below 2 ft-candles or 20 lux.
Instead of creating an all-new method for monitoring LED UV-A lamps for inspection, ASTM E3022 requires the lamp manufacturer to include a UV-A pass filter in each lamp. The filter acts to remove tail emissions above 400 nm, which eliminates the accuracy issue with visible light meters. Since tail emission is visible as deep violet light, adding the UV-A pass filter eliminates the glare and makes the background of the part surface darker. This increases contrast between fluorescent indications and the darkened part surface when using fluorescent magnetic particle or penetrant techniques.
The Magnaflux EV6000 LED UV-A lamp includes a high-performance UV-A pass filter to completely eliminate glare. Learn more about the EV6000, or contact us for more details.