Mirror lighting needs to be bright but not glaring, accurate yet flattering, and for public/commercial spaces low maintenance and low power. There are three considerations for mirror lighting; fixture placement, lamp (or bulb) type, and fixture style.
The best mirror lighting surrounds the face with light. This provides light from all sides that minimizes shadows under the chin, eyes, cheeks, and forehead. This does not mean that you have to use 'Hollywood' style lights (strips that use the round light bulbs). A wall sconce on either side of the mirror can work nicely. Normally you want the center of the fixture at about 66" above the finished floor and at least 30" apart (these specs will vary according to the particulars of the room and fixture). If the mirror is a whole wall mirror you can mount the fixtures right on the mirror. Avoid using ceiling mounted fixtures as the sole source unless the room is very small with light colored walls. Large bathrooms often need additional lighting besides the mirror lighting. Definitely don't rely on recessed cans for mirror lighting, the shadows can be horrendous! If a recessed type must be used, consider constructing a luminous soffit with two 4' fluorescent tubes. The most common placement for mirror lighting is on the wall above the mirror. Though this is not ideal it avoids most conflicts with mirror size, medicine cabinet doors, and room dimensions. However the fixture needs to be long enough to light the sides of the face.
Color qualities, light distribution, and maintenance costs are all heavily dependent on the type of lamp the fixture uses. It is very important to choose the lamp you want to use first and then find a fixture that will hold it properly.
The two main color properties the specifier should know about are color temperature and color rendering. Color temperature refers to whether the light source appears cool (bluish) or warm (yellowish) or neutral and is expressed in degrees Kelvin or K. Most light sources used for mirror lighting range from 2700K (incandescent) to 4200K (cool white). Daylight is considered to be 6500K and sunlight 5000K (these values change depending on season, latitude and time of day). Skin tones look most flattering in medium to warm color temperatures (<3500K) though some people prefer the cooler colors. Color rendering (expressed as CRI) is another key color quality. CRI is a scale from 0 to 100, the higher the CRI the better. Incandescent and Daylight are 100, cool white and warm white are around 60, T-8 and compact fluorescents are about 80. Light distribution is another key element in effective mirror lighting. Even, shadowless light is easiest on the eyes, and fluorescent sources (particularly the large tubes) are well suited to this task. Spot lights, clear bulbs or small halogens require lots of shielding or diffusion from the fixture for glare control. This will tend to reduce the light level necessitating the use of higher wattages. Maintenance costs include power consumption, lamp cost, and frequency of lamp replacement. Many public/commercial lavatories are illuminated 12 - 24 hours a day and over time these costs far exceed the cost of the fixture itself. In a private residence the maintenance costs may not build as rapidly but they can be significant if the bathroom gets more than 3 hours of use per day. For example, at $.05kwh @12hr/day, 2- 100W incandescent fixtures cost $53 per year to operate but 2 - 32watt fluorescents cost $15 per year.
When selecting a fixture style choose one that has a translucent lens. If the lens is clear the fixture may be glaring. If it is opaque (metal, ceramic, etc.) it will not pass enough light directly to your face. Be sure that the lens will not change the color of the light (some glass is rather green). Check that the fixture meets your maintenance requirements for relamping, cleaning, and spare parts. Most 'vanity' fixtures are linear and many of them can be mounted horizontally above or vertically on the sides of the mirror. If you choose to use wall sconces in small rooms select ones that are narrow as these will fit best.
by Eric Strandberg, The Lighting Design Lab