Why there is no single number
A sun tunnel passes on the daylight that reaches its dome, minus what the journey down the tube costs, so its output changes with every hour, every season and every cloud.
A ceiling lamp gives the same light whenever you switch it on. A tubular skylight does not. At any moment its output is the product of two things: how much light is falling on the dome, and how much of that survives the trip through the tube and diffuser into the room. The first is set by the sky over Cambridge that day. The second is set by the installation: the width of the tube, how long it is, how many bends it takes and the quality of its reflective lining.
That is why we do not quote a light output as our own figure. Any single number would only be true for one product, one run and one sky. What we can do is explain which of these variables you can control, which you cannot, and how they combine in your room. The parent page on sun tunnel installation in Cambridge sets out how we plan a tube around them.
What the bulb comparisons mean
Manufacturers often describe their tubes in terms of the old incandescent bulbs people remember, and those comparisons give a useful sense of scale as long as you read them as a best case.
Makers compare a small tube on a bright summer day to a traditional 100-watt bulb or more, and the wider sizes to several such bulbs together. Treat those figures as theirs, measured under their own test conditions: usually a short, straight run under strong sun. They are not a promise of what any particular ceiling will give, and they say nothing about a grey afternoon in February.
There is also a quirk in the comparison itself. A watt measures how much electricity a bulb uses, not how much light it gives. The proper unit for light is the lumen, and a daylight tube uses no electricity at all, so "equivalent to a 100-watt bulb" really means "gives roughly the light an old 100-watt bulb gave". The light itself is different too. Daylight through a tube is cooler and more natural in colour than a filament bulb, which is why a room can feel brighter than a lumen figure suggests. If you want to weigh daylight against electric light directly, our answer on whether a sun tunnel is better than LED lighting covers that.
Diameter: the variable that matters most
The width of the tube sets how much sky the dome can collect, and the collecting area grows with the square of the diameter, so a modest step up in size gives a large step up in light.
This is simple geometry rather than a product claim. A 350mm tube has roughly twice the cross-section of a 250mm tube, and a 530mm tube roughly four and a half times. A wider tube also loses proportionally less light on the way down, because light bounces off its walls fewer times over the same length. Both effects push the same way, which is why a wider tube is usually the better choice whenever the ceiling and roof allow it.
| Diameter | Area vs 250mm | Typical use |
|---|---|---|
| 250mm | 1 times | Bathroom, WC, wardrobe |
| 350mm | About 2 times | Hall, landing, stairwell |
| 450mm | About 3 times | Larger landing, small kitchen |
| 530mm | About 4.5 times | Kitchen area, open space |
The typical uses are a guide, not a rule. Matching a size to a particular room is covered in choosing a sun tunnel diameter, and how big a room a sun tunnel can light looks at floor area.
Length, bends and the tube lining
Every metre of tube and every bend costs some light, so the shortest, straightest run with the most reflective lining delivers the most to the room.
Light travels down a sun tunnel by bouncing off a mirror-finish lining. Each bounce keeps most of the light but not all of it, and a longer run means more bounces. Bends add more, because they turn a clean downward beam into a scatter that strikes the walls at awkward angles. A short, straight rigid tube through a flat roof into the room below is close to the ideal. A long run through a pitched loft with two elbows will give noticeably less from the same dome.
The lining matters as much as the geometry. Rigid tubes with a highly reflective finish lose far less per bounce than flexible, ribbed ducting, which is why the comparison in rigid vs flexible sun tunnels is really a comparison of light. The practical limits on run length are set out in sun tunnel length and bends.
Season and sky over Cambridge
The same tube can give many times more light at noon in June than at noon in December, because the height of the sun changes both the strength of the light and the way it enters the tube.
Cambridge sits at about 52 degrees north. At midday on the shortest day the sun climbs to only about 14 degrees above the horizon, low enough to skim the rooftops of a Romsey terrace. At midday in midsummer it reaches about 61 degrees. A high sun delivers stronger light and drops it steeply into the dome, where it travels down the tube with few bounces. A low winter sun is weaker, strikes the dome at a shallow angle and needs many more bounces to reach the bottom. Modern domes are shaped to catch low-angle light, which helps, but they cannot make December into June.
The sky matters as much as the sun. On an overcast day the tube runs on diffuse light from the whole sky, which gives an even, soft glow with no bright spot, at a lower level than in sunshine. Our answer on whether sun tunnels work on cloudy days covers that in detail. Flat Fenland skies help here: with no hills to block the low sky, a dome on a Cambridge roof sees more of it than one in a valley town.
Roof position and what you see in the room
Where the dome sits on the roof and which diffuser finishes the ceiling decide how the light that arrives is actually experienced in the room.
A dome shaded by a chimney stack, a taller neighbour or a tree loses direct sun for part of the day. A dome on a north slope still works, since it gathers sky light, but it misses the direct beam through much of the year; our answer on fitting a sun tunnel to a north facing roof explains how much that matters. Choosing the spot is covered in where to put a sun tunnel on the roof.
At the ceiling, the diffuser shapes the light rather than adding to it. A clear or prismatic lens throws more light downward with a brighter disc on the ceiling; a frosted diffuser spreads it more evenly with less glare. The options are compared in sun tunnel diffusers. At the survey we look at all of this together as part of how we plan the light, and the written light plan with your quote says which diameter, run and position we recommend, and why. To have that done for your room, book a light survey.
Questions people also ask
Is a sun tunnel as bright as a window?
Not usually. A window lets in a wide view of the sky; a tube passes on the light from one dome. A well-placed, wide, short tube can light a hall or bathroom as well as a small window would, but it will not match a large rooflight over a kitchen.
Will two small tubes give more light than one large one?
Two 250mm tubes collect about the same area as one 350mm tube, and spread the light over two points. One larger tube loses less on the way down, so it is often the brighter choice, while two tubes suit a long, narrow room.
Does a sun tunnel give any light at night?
No. Once the sky is dark the tube is dark. An integrated LED kit can light the same diffuser in the evening, as explained in sun tunnels with LED lights.
Can a sun tunnel make a room too bright?
In a small room on a bright summer day, a wide tube can give a strong glow. A frosted diffuser softens it, and a dimmer kit can reduce the light when you want the room darker.
