How bright do you want it to be, as a rough approximation a single 400W spotlight on a 10m pole, positioned at one end of the 50m radius perimeter pointing in on an angle COULD cover the middle area with around 40 lux and the edges with around 8 lux. Note that this depends a lot on the type of light distribution pattern the spotlight has so take these figures with a grain of salt (ie they are rough but likely close).

Trees and leaves will generally reflect and block the light creating shadows and areas of darkness (especially when using a single light source).

The short answer is yes, BUT 40 lux isn't a lot and shadowing and low light levels near the edge of the perimeter would likely be unacceptable), also a 10m tall pole may be too high if this is a residential application, so I would suggest more than one on shorter poles (especially if the area is going to be used for entertainment, reading, etc).

Hope this is clear as illumination engineering is an art form and a little of a "black art" in itself.

Do you intend to run all night every night? That's a kilowatt hour to be paid for every 2.5 hours.

milo

Neighbour light pollution is becoming an issue at some locations. If the intent for the light is decorative, consider using low-level extra-low-voltage lamps or LED fittings instead, installed at a low level, so as to give a kinder impression of a beautiful garden at night.

There will be a negligible effect on trees from garden lighting.

Amen!

milo

a bright enough light can interfere with the way the plants sense day and night. Many plants are tuned to length of day. the light will not make night look like day, but it may add an hour of light at dawn and dusk as the plant senses it by lengthening twilight and dawn times.

Sodium vapor, Mercury vapor, or incandescent? Sodium vapor will give the brightest light, and it will be primarily in the yellow-orange part of the spectrum. Mercury vapor will give the next brightest, balanced in the blue-green part of the spectrum. Incandescent will give the least, balanced in the red-yellow part of the spectrum. Check the species of your plants for photoperiod response and wavelength sensitivity, they vary quite a bit. Your 400 Watt security light can interfere with growing poinsettias outdoors, but can keep them in bloom longer.

Lighting that is intrusive and spills needlessly into the night sky is indeed becoming a loud conversation, especially in rural areas.

I've helped a number of towns and counties here in the east write local ordinances and urge you to check with local code enforcement officials before unilaterally choosing a light source.

As for your question: wattage is not an indicator of light output but of power used. When most every light was tungsten, that was the common method for making choices but not any more. The number of lumen's generated will vary according to type so there is no way for us to properly answer your question.

There are several things I would urge you to do.

1. Position the light source so it illuminates the target but in such a way that the light source itself, cannot be seen. This will eliminate the bulk of light trespass complaints from neighbors , pedestrians and drivers.

2. Try as best you can to eliminate light be scattered into the sky. A horizontal cutoff is ideal and is what you will often find in most new parking lots. It's not always easy to do both so if iut becomes a problem go with #!

Lastly, consider purchasing a set of low wattage colored LED string lights. One property owner here in Western NJ did just that to his tree.

The trunk is wrapped in red. the lower half of the tree is white and the top half blue. It's striking, patriotic and unintrusive.

Thanks for taking the time to ask. Many simply do what they wish.

L.J.

Hello felixsemion,

Try these sites which explain how to work out the 'Lux' from a given light source. Or, how to figure out how much light/power you will need to light a given area.

Some of these site offer calculator to work out the lux.

Others show Formulae.

Hope you find what you are looking for. Please forgive any duplication if other posts have the same sites.

http://www.ndlight.com.au/lighting_calculations.htm .

http://www.voltimum.co.uk/find/page-11-10-2/collec-vltsearch/how-to-calculate-the-lux-level.html

http://www.lightsearch.com/resources/lightguides/formulas.html

http://www.flg.co.uk/data_lighting_design_request/data_lighting_design_req.html

This site has a calculator to work out the Lux

www.flg.co.uk/data_lighting_design_request/data_lighting_design_req.html

www.lightsearch.com/resources/lightguides/formulas.html

bb

1. Certainly light will affect the plants.

2. Consider using more number of lights covering the area (but not exceeding the toatl of 400W) and use only the lights where some body is present. Avoid using lights all at a time. This will save your national electricity, natural resources and reduce the global warming ( how ever small it may be)

3. Also see sites of International Darksky association.

400w metal halide lamps generate approximately 36000 lumens

it it's for security purposes, strategically located, low-wattage lights can serve the purpose. you can even configure them that they light up once any movement is detected.

for general lighting, be innovative. try this instead.

All the suggestions by the other contributors are good.

As a rule of thumb the "dirtier" the face of the bulb, the wider its throw.

A clear flood lamp will produce an intense beam, of more narrow coverage, that many will find less pleasing.

As a practical matter, you may not want to put your light way up in the air, where it will be difficult to change when it burns out.

I'd suggest that you drive or walk around at night where some others may have done something similar, and find out what fixtures they might have used.

Way too bright, check with the locals and see if they mind you lighting up their homes too.

The normal security lights are barely 200W, so unless you need to read outdoors and have a lot of money to spend on electricity, use something smaller.

Sawraye had a very good answer. Lumens and lumens per Watt are more important that Watts. Light patterns are also important. A type 5 light will give you a round patternand is typically used for area lighting. A type 2 or type 3 pattern will give you an elongated pattern that is normally used for lighting long narrow areas, such as streets. Placement is important so that you don't block the light. I have been to GE's outdoor lighting institute in North Carolina and their indoor lighting school at Nela Park. My thought is that 400 Watts is way too much. Most residential dusk to dawn lights are 175 W Mercury Vapor or 100 W HPS, as are most residential street lights. I do not recommend MV because the depreciation factor is steep and they lose output to quickly, and they have disposal issues. High Pressure Sodium gives more light for the energy, but the CRI is poor, so colors don't reproduce, and it may not look as bright. I recently put in a 65 W compact fluorescent that is equivalent to a 250 W incandescent and it gives a nice white light. If you are in a really cold climate, you may have trouble getting CFL's to start in winter. If you really feel like you want more output, a Metal Halide fixture will give you a white light but it is not as efficient as HPS. Also, consider a timer so that the lights go off after a certain time, such at after midnight, unless you plan to stay outside all night.

PS - I am a member even though I am answering as a guest. That is my choice.

Are there trees all over the garden? Or just around the perimeter?

Erecting a pole for a single flood light will be expensive: probably cheaper and more aesthetically pleasing to go for a distributed solution.

Interesting.