Garden Lighting Help sheet  No. 6: - Choosing lamps for outdoor spotlights

Choosing lamps (bulbs) for any type of garden spotlight, outdoor uplight, or exterior downlight which uses a low voltage MR16 halogen spotlight lamp is partly a question of brightness and partly one of correct beam angle.

Beam angle

The principal 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 subjects 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 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° beam will give you between 2.3 and 3.5 metres of coverage, which gives a suggested spacing between spotlights in the border.

Comparison of MR16 tungsten halogen reflector lamps: beam spread, wattage ratings and typical applications

Brightness

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 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 75 watt garden spotlights for lighting dark 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 50 watt lamp to a 75 watt one.
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.

The table includes the light level 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!).