Lighting · Explainer · 4 min ·
Why does a spot bulb make a pool of light? Narrow the beam and see
Why does one spot bulb light a picture like a small stage while another just brightens the whole wall? The answer is the beam angle, the width of the cone of light, printed on the box.
Slide the angle and the distance, watch the pool of light grow on the wall, and see how a narrow beam makes cosy pools while a wide one spreads thin. You get the one-line sum for working out the right angle for your own picture, shelf or plant, and what to measure before you buy.
One bulb, two very different rooms
Take two spot bulbs that use the same amount of power and give out the same amount of light. Screw one into a lamp pointing at a picture and it looks like a small, glowing stage. Screw the other one in and the whole wall turns evenly bright, but nothing in particular stands out. The difference is not the brightness of the bulb. It is the beam angle.
Beam angle is the width of the cone of light that leaves the bulb, in degrees. It is usually measured out to the point where the light has faded to about half of its strongest value, which means the edge of a real beam is soft, not a hard line. You will find it printed on the box of most spot bulbs (GU10 and similar), often next to the wattage, and it explains why some bulbs make a cosy pool of light and others just make a room brighter.
The maths is only one line
The width of the pool on a wall is 2 × distance × tan(angle ÷ 2). At 2 metres, a 24° beam makes a pool about 85 cm wide, a 36° beam about 1.3 m, a 60° beam about 2.3 m and a 120° beam about 7 m, which is wider than most walls. Notice that the last jump is huge: past 90° the pool grows very quickly, and very little light is left in the middle.
That is the trade-off. A bulb only has so much light to give. Spread it over a small circle and the centre is bright; spread it over a huge circle and every part is dim. Halve the width of the pool and the centre gets roughly four times brighter, because the area shrinks in both directions.
Match the angle to the job
As a rough guide, bulbs around 20–25° are made for accent light on a picture, a bookshelf or a plant. Around 35–40° is the all-round choice for a corner, a shelf or a reading spot. Around 60° washes a wall or a hallway more evenly, and 90–120° is for general light, much like an ordinary bulb.
For a cosy room, a mix works better than one type. A couple of narrow pools at different heights, with softer light around them, give the layered look that makes a space feel calm. If you have already read how many lamps a living room needs, this is the same idea seen through a single bulb: the best rooms are never lit evenly.
Measure before you buy
- Distance: measure from where the bulb sits to the thing you want to light, in a straight line, not along the floor.
- Target size: measure the picture, shelf or plant. You want the pool to be a bit larger than the target, not the same size, because the edge of a beam is soft.
- Do the sum: pool width = 2 × distance × tan(angle ÷ 2). If your answer is much bigger than the target, choose a narrower angle.
Light that is aimed straight at a wall from close by also shows every bump and flaw in the plaster. If your wall is far from perfect, step the lamp back or choose a wider angle, so the light is softer.
A few practical notes
Check the packaging for the beam angle, since it varies between makers. If you want to dim the bulb, check that it says dimmable and that your dimmer suits LED bulbs. Many spot bulbs run directly from the mains, so switch the lamp off and let it cool before you swap one. If the spot is part of a ceiling fitting, changing the fitting itself is a job for an electrician.
Cosy is not about buying more, either. Before you buy new spots, try moving an existing lamp closer to or further from the wall. It changes the size of the pool in exactly the way the sliders above do, and it costs nothing.