Not all LED bulbs are dimmable.  Not all LED bulbs and dimmers are compatible with each other.  Some signs of incompatibility are flickering bulbs, bulbs may not turn on, humming/buzzing noises.  An on line search for the lamp or dimmer can help you determine compatibility.  LED technology changes very rapidly, so be sure you are looking at the most current information.

In 2012 the City of Tucson/Pima County adopted an outdoor lighting code (ordinance) for protection of access to the dark night sky.  This ordinance is in place to accommodate the observatories in the area, such as Kitt Peak.  Less light pollution allows for better viewing.  The ordinance requires exterior light fixtures to have full cutoff so that “no light can occur at or above an angle of 90 degrees”, and “the rated color temperature of light sources shall not exceed 3500K”.  The ordinance allows for class 2 misdemeanor fines for entities that do not comply.  Most fixtures that can be purchased on line do not comply with the ordinance.