Choosing a Bulb

Electrical Terms.

All lamps are labelled with their relevant electrical operating characteristics, which are voltage and wattage.   Light fittings are also labelled with a maximum wattage rating that should not be exceeded.  This is because General Lighting Service (GLS) lamps convert only 10% of the electricity they use as light and convert the remaining 90% to heat.   The maximum wattage  rating of light fitting is to prevent damage by excessive heat.

Voltage is measured in volts and can be considered as the pressure of the electricity.   The higher the voltage the more dangerous it is in terms of electric shock.

Current is measured in Amperes (or amps for short) and can be considered as the amount or quantity of electricity used.   The Higher the current the thicker the wire needed to carry it.

Wattage is the power used and equates to the voltage (in volts) multiplied by the current (in amps)

For instance, a 12 volt, 50 watt lamp uses 4.16amps (4.16 x 12 = 50) yet a 230 volt, 50 watt lamp uses only 0.217 amps (0.217 x 230 = 50).   As a general rule, all 50 watt lamps will emit the same amount of light, regardless of their voltage, providing they are the same type of lamp.

Lamp voltage

The voltage measures the electrical pressure in the same way the pounds per square inch measures water pressure.  The mains in most countries are 220 -240 volts while in North America they are 110 volts.

Incandescent Lamps

These are often called General Lighting Service (GLS) lamps and are the least efficient.  They will produce about 15 lumens of light for every watt of electricity used.
Incandescent or GLS lamps can be dimmed (ie run on a lower voltage).   When the rated voltage of a lamp is reduced (by dimming), the light out put and colour temperature are lowered.   Running a lamp on lower than its rated voltage will also increase its life.   Conversely, running a GLS lamp on a higher than rated voltage (over-voltage) will shorten its life.   Over-voltage and over-current are the most common reasons for GLS lamps failing.   This often occurs when lamps are first switched on and the current rises rapidly (over a few milliseconds) or when variations in the supply voltage causes it to rise above 230volts.
These lamps have a life of about 1000 hours

CFL  Compact Fluorescent Lamps

These lamps will produce the same light as an incandescent lamp but require only 20% of the power.    They can do this because less energy is wasted in heat.   A CFL can be used to replace either bayonet or Edison screw bulbs.

This table shows the equivalent wattages of Osram Duluxstar Twist and incandescent lamps.

For comparison a CFL lamp produces about 60 lumens of light per watt of electricity used

Compact    Fluorescent Wattage	Incandescent  Wattage
5 Watt	25 Watt
8 Watt	40 Watt
11 Watt	60 Watt
13 Watt	75 Watt
16 Watt	100 Watt
23 Watt	120 Watt

The twist bulbs are more efficient and smaller than the u shaped bulbs and will fit into any light that uses an incandescent bulb.
The average life of an Osram Duluxstar bulb is about 5000 hours but lower quality bulbs can be much less.

Halogen Bulbs

12 volt halogen bulbs are usually of the MR16 type.  The MR stands for multi faceted reflector and the 16 for the diameter of the bulb in one eighths of an inch i.e. two inches or 50mm.    Halogen bulbs that operate on 240 volts or 110 volts are the same shape but the pins on the base are of the twist and lock type and cannot be inserted into a 12 volt fitting.
A halogen lamp will produce about 35 lumens per watt.

Osram Decostar bulbs are available in four beam widths.  The ten and 24 degrees are used to give a narrow spot beam, the 36 degrees is most common domestically  used for floodlighting and the sixty degrees is becoming more popular for general floodlighting.

Osram Infra Red Coating Lamps (IRC) are about 30% more efficient than normal halogen lamps.  They achieve this by making better use of the heat generated.
A low voltage halogen lamp has a life of between 2000 and 5000 hours depending on the quality.  A mains voltage lamp will last 2000 hours

Fluorescent Tubes

Fluorescent tubes are specified by their wattage and diameter.   The diameter is given as T and a number.  The number is the diameter of the tube in one eighths of an inch.
ie a T4 tube is half an inch in diameter.  Common tubr diameters are T4, T5 and T8

The wattage of the tube determines its length, so if you buy a tube of the same wattage it will fit into the light.

The tubes are available in a range of colour temperatures which range from warm white to cool white and daylight.

These lamps are very efficient and produce about 70 lumens per watt, they have a life of about 10,000 hours.

LED Lighting.

The light is given off by a light emmitting diode and it promises to be a very efficient light source.  At the moment the 3 and 5 watt lamps produce about 70 lumens per watt which is only marginally better than fluorescent.  Future development could see the output of 200 watts per lumen.

LED lighting can do what no other source can do in creating special effects and mood lighting.  For domestic purposes it can give low level lighting and night lighting at very low operating cost.

Types of Bases

The base of the lamp makes the electrical connection with the  with the light fitting and there are a wide variety of different bases.
Theses are the more common bases:

Incandescent and Compact Fluorescent.



Bayonet fittings are the push and lock type and come in two sizes.  The most common is B22d which is 22mm in diameter and the small bayonet B15d which is 15mm in diameter.


The screw bases are called Edison Screw and there are two sizes in common use.
The E27 is the most common and has a diameter of 27mm.  The Small Edison Screw (SES) or E14 has a diameter of 14mm.

Halogen Lamps 12 Volt

Halogen capsules have pin bases.  The number refers to the distance the pins are apart,IE the G4 has pins 4mm apart.  These are the common sizes in use in NZ


The MR16 12 volt reflector lamp base has  two pins and the number 16 refers to their distance apart in millimeters. 

Halogen Lamps 220 to 240 Volts

These lamps have two pins with a larger diameter end which allows them to twist and lock into the fitting.
The most common type is the GU10 which has the pins 10mm apart.