What is a beam angle?
The beam angle is the angle, in degrees, over which a light source spreads its light: the larger the angle, the wider the beam. On packaging it appears as, for example, 36°, 36D or 36 deg.
Light is brightest along the centre line of the beam and fades towards the edges. The beam angle is normally measured between the two directions where the luminous intensity has dropped to half of that central maximum. Light still falls outside that angle, only more weakly. The wider angle out to 10% of the maximum is called the field angle; some older definitions used the 10% point for the beam angle itself, which is one reason figures from different sources do not always match. The fitting also matters: a deep reflector or an anti-glare baffle can narrow the usable beam of the lamp inside it.
Recognising it
A rough classification, with boundaries that vary between manufacturers:
- Narrow, up to about 24°: accent lighting on a painting, sculpture or niche.
- Medium, about 25 to 40°: living rooms, kitchens, hospitality and reception areas.
- Wide, about 41 to 100°: even general lighting in offices, shops and under kitchen wall units.
- Very wide, over 100°: garages, storage areas and canteens.
In practice
- Work out the pool of light. Diameter ≈ 2 × distance × tan(half the beam angle). A 36° spotlight 2.5 m above the floor gives a pool of about 1.6 m; a 60° one about 2.9 m.
- If in doubt, choose 30 to 40° for downlights in living spaces. It is a safe middle ground between accent and general light.
- Narrow beams mean more fittings. For even coverage with narrow spots, you need more of them, closer together.
- Allow for mounting height. The further the lamp is from the surface, the larger the pool and the lower the brightness per square metre.
- Compare within one brand. Measuring and labelling practice differs, so the same number may not mean the same across brands.
Common mistake: choosing on lumens alone. Two lamps with the same lumen output can look completely different: a narrow beam gives a bright, small spot, a wide beam a soft wash.
Related terms
- Recessed spotlight
- LED lighting
- Colour temperature
- Reflection, absorption and transmission
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