If you have a beam of light that doesn’t spread at all (i.e. a laser), the candelas will essentially be the same as the lumens. But as soon as any beam angle is introduced, the intensity will start to drop for every metre the light travels. The wider the beam, the more those lumens are shared over a larger area, and the candelas will get smaller and smaller the further it travels.

In domestic terms, you can think of a naked bulb hanging from the ceiling as an extra-wide angle light. It will have a beam of 360° in all directions, apart from a small area occupied by the bulb fitting itself. As downlights are mounted in the ceiling (or in a shielded surface mounted fitting), it’s impossible for the beam to be more than 180°, as no light can escape upwards. In practice, even 180° would be impossible to achieve as the mount itself would cause some shadow.

If you’re not sure about intensity, you can always opt for dimmable bulbs with a dimmer switch. That way, you can tailor the intensity to your needs at any given moment.

Janet Raloff is the Editor, Digital of Science News Explores. Prior to this, she was an environmental reporter for Science News, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

frequency     The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.

However, if you are working with a high ceiling, it’s probably best to choose narrow angle bulbs. By the time the light reaches head height, it will have spread enough to cover the whole room. Conversely, if you have a low ceiling, the widest beam angle available will ensure consistent lighting throughout, without excessive shadows and hotspots.

Parts of the electromagnetic spectrum also can be described in terms of their frequency. A radiation’s frequency will be the inverse of its wavelength. So the shorter the wavelength, the higher its frequency. That frequency is typically measured in hertz, a unit which stands for cycles per second.

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When you’re working with an existing installation, you can still use your choice of beam angles to flood the room or focus it in smaller areas, depending on how you want the room to feel. You can even mix it up a little if you want, with both narrow and wide beams dotted around.

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Energy travels throughout the universe at the speed of light as radiation. What that radiation is called depends on its energy level.

At the really high-energy end of the spectrum are gamma rays. They’re close cousins to  the X-rays that doctors and dentists use to probe for unusual structures in  your body. Radio waves fall at the extreme other end of the spectrum. They’re used to (among other things) deliver music and news broadcasts to your home radios.

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spectrum     (plural: spectra) A range of related things that appear in some order. (in light and energy) The range of electromagnetic radiation types; they span from gamma rays to X rays, ultraviolet light, visible light, infrared energy, microwaves and radio waves.

For example, you might want plenty of lights in the ceiling, but with narrow beams, or you might want as few as possible by spacing them out more. Alternatively, you might want to focus the downlights in a certain zone, in which case you can carefully work out what certain beam angles will look like, and how the light will interact with objects, floors, walls, furniture and people.

Founded in 2003, Science News Explores is a free, award-winning online publication dedicated to providing age-appropriate science news to learners, parents and educators. The publication, as well as Science News magazine, are published by the Society for Science, a nonprofit 501(c)(3) membership organization dedicated to public engagement in scientific research and education.

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When you’re installing LED downlights, the beam angle is a very important factor. It influences the way the room is illuminated, and therefore the overall visual effect of the lighting. If you have a good idea of the precise lighting effect you’re going for, you can work out exactly what beam angle you need in your bulbs, as well as the number of bulbs you need to install in the ceiling. But if you’re stuck with an existing setup, you can still use bulbs with different beam angles to achieve a lighting effect that’s more to your satisfaction. So let’s find out all about beam angles.

It’s more difficult to do, but scientists measure electromagnetic waves the same way—from crest to crest or from trough to trough. In fact, each segment of the energy spectrum is defined by this wavelength. Even what we refer to as the heat given off by radiators is a type of radiation — infrared rays.

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microwaves     An electromagnetic wave with a wavelength shorter than that of normal radio waves but longer than those of infrared radiation (heat) and of visible light.

gamma rays     High-energy radiation often generated by processes in and around exploding stars. Gamma rays are the most energetic form of light.

For most domestic purposes, downlights are spaced evenly around the ceiling of a room, however. This ensures uniform lighting throughout. For regular, comfortable illumination, bulbs with average beam angles (around 45°) are perfect.

radiation     (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

universe     The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

So if you’re starting from scratch, with a lovely blank ceiling ready to be drilled, think about what effect you’d like. With a little trigonometry you can work out how the light beams will interact with each other, and design your fitting layout accordingly.

hertz     The frequency with which something (such as a wavelength) occurs, measured in the number of times the cycle repeats during each second of time.

What separates one type of this radiation from another is its wavelength. That’s the length of a wave that makes up each type of radiation. To identify the length of a wave  in the sea, you would measure the distance from the crest (upper part) of one wave to the crest of another. Or you could measure from one trough (bottom part of a wave) to another.

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radio waves     Waves in a part of the electromagnetic spectrum. They are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals. They also are used in radar.

electromagnetic spectrum     The range of radiation that spans from gamma- and X-rays through visible light and on to radio waves. Each type of radiation within the spectrum typically is classified by its wavelength.

Ultraviolet rays, visible light, infrared radiation, and microwaves fall at energy levels in between. Together, all of these make up one long, continuous electromagnetic spectrum of light. Its energy travels in what are usually referred to as waves.

broadcast     To cast — or send out — something over a relatively large distance. A farmer may broadcast seeds by flinging them by hand over a large area. A loudspeaker may send sounds out over a great distance. An electronic transmitter may emit electromagnetic signals over the air to a distant radio, television or other receiving device. And a newscaster can broadcast details of events to listeners across a large area, even the world.

Professionals use software such as Dialux or Relux to model the shape of the room and to play with different lighting concepts. If you’re a lighting designer, they are well worth investing in.

As you can imagine, a single downlight with a narrow beam angle, right in the middle of a room, would only illuminate a small circle underneath it. For most purposes, that would be uncomfortable and unacceptable. However, if there’s other lighting in the room, and that one light is there to illuminate a single object, such as a table or an ornament, it could be exactly what you want.

Downlights are usually parabolic, and set in a ceiling which means light can only escape in one direction – the back is either mirrored, or the cluster of LEDs only points downwards. The beam of light forms a cone emanating from the bulb, the angle of which is part of the bulb’s specification.

If you have narrow beams with high lumens, the light might be very intense when it hits a surface, in which case you might prefer to opt for a less intense source. Again, this should all factor into your choice of bulbs, whether you’re designing from scratch or working with an existing arrangement of fittings.

inverse     Something that is the opposite or reverse of another thing, or that moves in the opposite direction to something.

Beam angle can perhaps be best explained with reference to the theatre. There, you’ll find all sorts of lighting that have been set up to maximise the visual and emotional effect of the performance.

wavelength     The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

Bulb brightness is measured in watts for consumers, but a more scientific measurement is the lumen. You’ll also see this on most bulbs’ packaging. This represents the total amount of light emitted by the bulb. Another measure is candela, which is the amount of light that lands on an object, or the amount measured at a certain point.

Remember, you will never find a downlight that spreads more than about 120°, as they are always recessed into the ceiling (or the fitting) to a certain degree, so no light can escape upwards. Any illumination of the ceiling will be incidental, reflected off surfaces below.