All x15, x10 and lower magnification lenses are triplet. This means that to get to that magnification, 3 lens are used inside the loupe. For x20 magnifications, only the highest quality loupes, such as the Ruper range, offer triplet for x20 magnification. Such field lenses offer outstanding clarity and little distortion, but are of course much more expensive.
Five-elements: For x20 loupes, the standard is five elements, whereby 5 lens are used to get to x20 magnification. These loupes are still high quality, but have a reduced clarity and have some degree of image distortion. Only the expensive brands offer triplet in x20 magnification due to the increased cost in the manufacturing process.
Achromatic: Achromatic loupes are designed to limit the effects of chromatic (failure of a lens to focus all colors to the same convergence point) and spherical aberration ( imperfection of the produced image). Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane. Such loupes are the highest quality you can obtain, and normally much more expensive.
Higher magnifications: You may find places advertising higher magnification loupes, often x30, or even x60. Sadly, it is physically impossible to get to this magnification. The true magnification of these loupes are often between x10-x20.
LED Loupes: These loupes with built in lighting, are ideal for indoor working. The bright LED lights offer much better clarity in all levels of lighting.