An element of an instrument approach system (e.g. an Instrument Landing System (ILS) localiser) is often used by pilots to assist in alignment for a visual approach.
A visual approach is not permitted when the Runway Visual Range (RVR) is less than 800 metres. (Appendix 1 to JAR-OPS 1.430 (g)).
Although a visual approach is the first type of approach normally taught to student pilots, this type of approach may be hazardous and careful consideration should be given before flying a visual approach in preference to an instrument approach.