Dome magnifiers are half-spherical magnifiers (sometimes also called "brightfield" or "paper weight magnifiers"). They sit directly onto the reading material. The advantages are that they are always in focus and have the ability to "gather light". This type of magnifier is particularly beneficial to those with myopia (short sightedness) who often find normal magnifiers of little or no use. The Optima range includes domes of 50mm, 65mm,80mm and 95mm.
Another similar product called the Lumiloupe incorporates 6 LEDs with 3 switchable levels of brightness into a magnifier. Very useful where light is needed.
Bar magnifiers are semi-cylindrical lenses, which lie on top of a page and magnify just one line of print at a time. They are available with or without LED illumination. Bar magnifiers are helpful to people with relatively good vision who wish to read telephone directories or other reference material with columns or lists of information.
The main disadvantage of bar magnifiers is the limited level of magnification (up to 2x). The other disadvantage is that they tend to suffer badly from reflected light and distortions.