Bulbs, Fittings and the Codes on the Box

Buying a replacement bulb goes wrong in four separate ways, and the packaging states all four. The codes are short and each answers a different question.

The cap code is the fit

The cap is the part that connects to the fitting, and it is described by a letter and a number, the number being a measurement in millimetres.

Bayonet caps push and twist; screw caps thread in. The two are not interchangeable, and the small versions of each are common in lamps and spotlights.

Photographing the old bulb's cap before going to buy a replacement is more reliable than remembering, since several sizes look similar out of context.

Brightness is measured in lumens now

Wattage described power consumption, which correlated with brightness only while all bulbs used the same technology. It no longer does.

Lumens measure light output directly, and it is the figure to compare between bulbs of different types.

Packaging often gives an equivalence to an old filament wattage, which is useful for judgement but is an approximation rather than a measurement.

Colour temperature changes the room

Colour temperature is given in kelvin, with lower numbers producing warmer, yellower light and higher numbers producing cooler, bluer light.

Mixing temperatures within one room is the most common cause of lighting that feels wrong without an obvious reason, since the eye notices the inconsistency.

Colour rendering is a separate figure describing how accurately colours appear under the light, and a low value makes a room look flat regardless of brightness.

Dimmers need compatible bulbs

Older dimmers were designed for filament loads and work by cutting part of the mains waveform, which many modern bulbs do not tolerate.

The symptoms are flickering, buzzing, a limited dimming range or the bulb failing early, and none of them indicates a faulty bulb as such.

Bulbs stating dimmable compatibility will usually work, though on an old dimmer the range may still be poor and replacing the dimmer is the real fix.

Enclosed fittings and orientation

Some bulbs are not rated for enclosed fittings, because the electronics in the base need airflow and overheat inside a sealed shade.

Fitting one anyway produces a bulb that fails in months rather than years, which reads as poor quality rather than as a mismatch.

Beam angle matters too: a narrow beam suits a spotlight over a work surface, while a wide one is needed for a bulb intended to light a whole room.