The second type of skylight is a tubular device marketed under generic and brand of names including Solatube®, Sun Tunnel, sun or solar tubes.
Tubular skylights are just what the name implies - a narrow reflective metal tube which conveys natural light from a small dome on the roof to a lens or diffuser on the ceiling of an interior space.
The dome shape allows light to be collected from all directions and thus maximizes the amount of light at all hours of the day.
Full disclosure; I have a tubular skylight (brand unknown) in my family room and, even though it would be dramatic to have a couple of venting skylights included in an upcoming kitchen remodel, I love the solar tube and will probably opt for them solely on the basis of cost.
Tube lights have interior lens sizes ranges from 8" to 21" with 10" and 14" predominating. Larger units are available for commercial applications. Manufacturers claim that a single tube will furnish as much natural light as a skylight many times its size. For example, the Solatube manufacturer offers the following size to coverage guide:
10" Solatube - 150 - 200 square feet
14" Solatube - 250 - 300 square feet
21" Solatube - 500 square feet
These skylights can be installed almost anywhere including rooms without direct roof access and in small spaces such as closets where daylighting is not usually an option.
Tube lights are much easier and cheaper to install than regular skylights. Even if there is a drop through attic space from the roof to the interior ceiling no structural reframing or dry walling is needed. The tube serves the function of a casing. Several manufacturers state that a professional should be able to install a tube light in two hours.
In addition to ease of installation, the tubular light has less surface area than flat skylights and are better insulated (to around R-22) to prevent drafts and heat buildup and the lens serves as a diffuser to scatter the light and eliminate hot spots in the room. The round shape of the tube virtually eliminates leaking.
Diffusers come in many styles depending on the type of light desired and some even permit a sky view. Mine has a prism type that occasionally turns the room into a rainbow. The interior appearance can be porthole-like or dome-shaped like contemporary ceiling fixtures.
The exterior dome is billed as being self-cleaning and the diffuser is easily removed for cleaning.
The small bubble on the roof is not particularly noticeable although multiples wouldn't be pretty. Some manufacturers provide split tubes that allow light from one dome to be funneled to diffusers in two different rooms but other manufacturers do not recommend this.
Many of the tubular lights come with a lighting option that projects fluorescent or incandescent light through the tube at night. Other options include dimmer switches to control the level of natural light and venting fans.
There is a wide price range for the tubes. We found 10" models starting at $162 (most come in a kit with 4' of tubing) to $230. The 21" models range from $378 to $750. Light kits are $20 at Home Depot.