The degree of protection of a luminaire indicates whether it is actually suitable for the required lighting application and can be operated safely. An example: luminaires installed in a dry interior do not need to be protected against moisture; in a bathroom, or outdoors, that protection is absolutely imperative.

As a matter of principle, luminaires need to be designed to prevent penetration by solid foreign bodies and moisture. The degree of protection afforded is indicated on the luminaire using the IP (Ingress Protection) numbering system.

The first numeral after the IP indicates the degree of protection against solid foreign bodies, the second denotes the degree of waterproofness (see table). For example, an IP 20 luminaire is protected against the ingress of solid foreign bodies > 12 mm but not against penetration by moisture. A IP65 luminaire for damp interiors is dustproof and protected against jets of water.