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

Using a suitable filter over a black light bulb is crucial primarily to protect the human eye from damage. Black light bulbs emit ultraviolet (UV) light, which can be harmful with prolonged exposure. Direct exposure to UV radiation can cause various issues, including eye strain, skin burns, and a heightened risk of eye conditions like cataracts. Thus, filters are designed to block harmful wavelengths of UV light while allowing some visible light to pass through for better visibility without risking eye injury.

The other options reflect considerations related to visibility and the characteristics of light, but they do not address the primary health concern. For example, while enhancing the visibility of a penetrant can be a beneficial effect of certain filters, it is not the main reason for their use regarding eye safety. Similarly, reducing glare or improving the passage of ultraviolet light pertains to the quality of the viewing experience but does not directly prioritize health protection, which is the primary concern when using black light bulbs.