LUMENS versus LUX for OUTDOOR lights
Lumens are a measure of the amount of light coming out of a bulb or light fixture. Lux is a measure of the brightness in the beam.
In a directional light — a light that does not shine in all directions like a tradition bulb, while the amount of light can be quantified in terms of lumens, the brightness in the beam is controlled by the light concentrating device, which is either a lens or reflector. The narrower the beam, the brighter the light which can be quantified in terms of lux in the center of the beam.
For INDOOR Lighting Use Lumens: Indoor lighting in rooms where reflective surfaces allow the light to bounce around multiple times generally are brighter with more reflective surfaces. In those situations, the amount of light released into the room is an indication of how bright the room will be. Lumens, which represents the amount of light, is the preferred specification to examine in selecting the type and quantity of light fixtures for indoor lighting.
Generally fixtures and bulbs that illuminate in a hemisphere pattern (ceiling light) or omnidirectional pattern (table lamp) do well for interior lighting in terms of efficiently illuminating the desired area.
For OUTDOOR Lighting Use Lux: Outdoor lighting typically has direct illumination on a surface such as wall, deck, table or garden, and then scatters. The light is not contained into a multi-sided reflective container such as a room. Thus the perception of how bright the illumination will be is NOT from lumens, but from lux in the beam. Generally the center of the beam will have the brightest lux value.
At a certain beam angle, the brightness will drop to half of its peak value in the center. And the useful beam width for outdoor lighting will be some arbitrary value that people can agree on. At 1 lux, which is about 3 times the brightest of a full moon, most people agree this is sufficient to clearly see for safety and security.
Generally, fixtures and bulbs that illuminate in a hemisphere pattern (recessed light with white diffuser) or omnidirectional pattern (wall-mounted coach light) do poorly for outdoor lighting. There is too much light close to the fixtures and not enough light for the desired reach, hence these fixtures are inefficient and also create undesirable effects with the wasted light such as shining in bedroom windows at night.
Comparison Using Example Products: Below are 4 charts which better explain directional lighting and how bright it is compared to full moon and a traditional 60 Watt frosted bulb as two reference points that people can easily related to. In this comparison, we’ve selected the most popular 4 of our many product choices — our 2 Watt wide and medium lens fixtures in both warm and cool white.
Another Comparison: We’ve also compared our medium lens 2 Watt light to an 10 Watt LED outdoor fixture that has a white diffuser instead of a lens.