CHOOSING HALOGEN LAMPS FOR 12V GARDEN LIGHTS

12 volt MR16 reflector lamps are 50mm in diameter and fit most low voltage outdoor spotlights, recessed uplights, downlights, recessed steplights and underwater lights except miniature garden lighting fittings such as the Microspot miniature spotlights and downlights which use the smaller MR11 35mm lamp. Claimed average lamp life is 3000-4000 hours, which should be halved for conditions of use in outdoor lighting; 1000 hours is about the same as having your garden lighting on every night of the year for about 3 hours, so lamp life in most domestic gardens means you won’t be changing quality lamps more often than every 1-2 years on average. Make sure you use a quality branded lamp such as BLV or GE as many lamps imported from the far east, especially "retail own brand" lamps won’t last anywhere near as long.

Choosing lamps (bulbs) for any type of exterior spotlight, uplight or downlight which uses a low voltage MR16 halogen spotlight lamp is partly a question of brightness and partly one of correct beam angle. The beam angles are 12, 24, 36 and 60 degrees. Sometimes you will want a small pool of light from a narrow beam to accent a feature without lighting the surroundings; other garden lighting subjects, such as lighting diagonally across shrub borders, will require the widest coverage available, such as a 60° beam. For example, a 60° beam will provide wide coverage from a pergola beam only 1.5 metres above a table below, but if you are uplighting a slender tree the coverage of a 60° beam will be wasted in the night sky; a narrower beam will “fit” the shape of the tree more closely. The widest beam angle available in MR11 lamps is 36°.

Energy consumption is increasingly a factor in choosing garden lighting: The new generation of Powersaver 12 volt MR16 halogen lamps now provides increased efficiency compared to standard MR16 halogen reflector lamps. A 50w Powersaver now does the same job as a 75w standard MR16 halogen lamp and a 35w Powersaver now does the same job as a 50w standard MR16 halogen lamp: this means 50w spotlights can now be used instead of 75w MR16 spotlights for tree uplighting applications.

Brightness isn’t just about the wattage of the lamp. The brightness of a 20 watt lamp squeezed into a narrow spot beam can produce a brighter pool of light than that of a 50 watt lamp projected in a wide flood beam. So, first “fit” the beam angle to your subject, then think about brightness. The diameter of the beam can be read from the table below to select the correct beam angle lamp if you want to fit a circle of light precisely to a subject, such as a wall plaque or wall fountain. For example, if you are using spike mount spotlights to illuminate a border between 2 and 3 metres deep and containing shrubs up to 2 metres high, a 60^o beam will give you between 2.3 and 3.5 metres of coverage, which gives a suggested spacing between spotlights in the border.

The applications table above gives some general guidelines on application of various beam angles in a garden lighting scheme. The result will vary for a number of reasons;

1. Individual perception of brightness varies, and this may be affected partly by ambient light from nearby streetlights or “cityglow”. Use exterior spotlights with a range of lamp wattage choices to allow for changes.
2. Darker, textured subjects reflect less light back to the eye than smooth, light coloured ones and will therefore require brighter lamps to make them stand out. Use 50 watt garden spotlights for lighting most features, at least to give you flexibility in changing lamps to suit darker subjects, distance from viewpoint or just the client’s taste in how bright these subjects should be in the view as a whole or in relation to other objects in the garden lighting scene. Make sure your transformers are rated to match this flexibility by having enough spare capacity, to allow for upgrading from, say, a 35 watt lamp to a 50 watt one. Upgrading from a standard 50w halogen lamp to a Powersaver lamp will boost brightness by 50% without having to upgrade the transformer to a larger size.
3. When the pool of light strikes the surface or subject at an angle, the circle of light is “stretched” into an ellipse, which means the output from the lamp is being spread over a larger area, producing a lower lighting intensity. You may need to compensate by increasing the lamp wattage.

Comparison of MR16 50w tungsten halogen reflector lamps: beam spread coverage and brightness
Lamp:	12 Degrees			24 Degrees			36 Degrees			60 Degrees
Beam:	Narrow spot beam			Spot beam			Flood beam			Very wide flood beam
	Beam Diameter (Metres)	Distance from lamp (Metres)	Light level (lux) for 50w lamp	Beam Diameter (Metres)	Distance from lamp (Metres)	Light level (lux) for 50w lamp	Beam Diameter (Metres)	Distance from lamp (Metres)	Light level (lux) for 50w lamp	Beam Diameter (Metres)	Distance from lamp (Metres)	Light level (lux) for 50w lamp
	0.21	1	11000	0.43	1	3600	0.65	1	2000	1.15	1	850
	0.42	2	2750	0.85	2	900	1.31	2	500	2.31	2	212
	0.64	3	1222	1.28	3	400	1.95	3	222	3.46	3	94
	0.85	4	688	1.71	4	225	2.61	4	125	4.62	4	53
	1.06	5	440	2.13	5	144	3.25	5	80	5.76	5	34
	1.28	6	305	2.56	6	100	3.91	6	55	6.92	6	23
	1.47	7	224	2.98	7	73	4.55	7	40	8.08	7	17
	1.71	8	171	3.41	8	56	5.21	8	31	9.24	8	13

The comparison table above includes the light level (lux) achieved by 50 watt lamps at each beam angle, so you can see how brightness reduces with distance from the lamp and as the lamp beam is spread wider. Be prepared to experiment to get the right result. For example, when calculating transformer ratings, allow 50 watts per fitting even if you think 35 watt lamps will suit your newly planted shrub border best to begin with; if you think a 20 watt lamp over the table will be bright enough, allow a little spare capacity in the transformer in case your partner thinks you should have fitted a 35 watt lamp instead (or maybe fit a narrower beam to get a smaller but brighter pool of light!).

Copyright 2010; Information based on products supplied by Lighting for Gardens Limited and is advisory only. The company accepts no responsibility for incorrect use or application of information given. Light fittings can become hot in use; exercise appropriate precautions. Exterior electrical installations should be undertaken by a qualified electrician.