Why must a suitable filter be used over a black light bulb?

The necessity of using a suitable filter over a black light bulb primarily relates to the protection of the human eye. Black light bulbs produce ultraviolet (UV) light, which can be harmful if exposure is prolonged or largely unprotected. The filter serves to block potentially damaging UV radiation, ensuring that only the safe wavelengths of light, primarily in the visible spectrum, reach the viewer. This becomes particularly relevant during liquid penetrant testing, where visibility is crucial for detecting flaws, but maintaining the safety of personnel is paramount.

While factors such as excessive heating, fluorescent visibility enhancement, and energy consumption may be relevant in other contexts, the core reason for employing a filter in this situation is to safeguard vision from UV exposure.