View Full Version : Need some advice on footcandles and spacing

05-24-2008, 11:56 PM
OK so Im doing a considerable install, but am not doing the lighting cause that would be illegal. However I am involved in the lighting selection as to work with the new landscape. Basically the situation is a walkway with 15 stairs in groups of 2-4 steps every 6-8 feet. They have selected 3 possible lights from Hadco. The CUL10, CUL9, and MSL12. The recommended spacing ranges from 8-20'. My question is what amount of footcandles is really necessary for a well lit semi awkward path without being too much or too little. It seems that the spacing leaves a small area of .5-.2 footcandles in between. My understanding was that you want at least 1 footcandle on a stairway. Also any other suggestions about the lights or placement would be good. Thanks

INTEGRA Bespoke Lighting
05-25-2008, 06:54 PM
Some photos would help a lot. Path/Step lighting is hard to design with no visuals, no understanding of obstacles, plant materials, path texture, path shape, adjacent planting beds, adjacent lawns, etc etc etc.

05-26-2008, 09:49 AM
The issue of optimum fc levels for stairways in residential lighting is an interesting one. There are many considerations.

As a baseline comparison, IESNA guidelines for commercial lighting (Table 2, DG-5-94) recommend a minimum fc level of 0.5 fc for stairwells. This is a good rule of thumb for residential lighting also. There are, however, many considerations that could either increase or decrease the optimum level.

One major consideration is the presence or absence of other lighting in adjacent areas. If adjacent areas are very dark, then stairway lighting can be less bright (although certainly no less than 0.2 fc - twice as bright as a full moon). If adjacent areas are very bright then the stairway needs to be comparably brighter.

The above guideline should also consider the walking path of the viewer. The eye requires a certain length of time to adapt from high levels of illuminance to low levels and vice versa. For example, a stairway leading from a bright deck into a darker landscape, justifies higher stairway illumination than a stairway connecting too relatively dark areas.

Another consideration is the surface material of the stairs. Apparent brightness is dependant upon the reflectance of the surface. Dark stones require greater levels of luminance than lighter stones.

To further complicate the decision, one must consider the age of the viewer. In the IESNA guidelines for lighting for seniors (ANSI/IESNA RP-28-07), recomended exit stairway illumination is set at 30 fc! Visual acuity, sensitivity and adaptation is greatly reduced in the elderly. Landscape lighting designers should always provide higher levels of illumination for seniors - and be especially carefull to provide pathways of more continuous levels of illumination. We shouldn't forget that even young homeowners will quite likely have elderly guests walking their property.

With all this in mind, I suggest shooting for stairway illumination in the region of 1 to 3 fc. Care should be taken to avoid direct glare from any fixtures. Also important (in many stair types) is that the stair treads have greater illumination than the stair risers (this maximizes the contrast between steps so treads are easily distinguished from risers). While staircases don't need to be evenly illuminated (side-to-side and top-to-bottom) the illumination should clearly define the bounderies of every stair.

05-26-2008, 10:01 AM
Quick correction to my last post - 0.2 fc is 20 times the illuminance of a full moon.

Chris J
05-26-2008, 11:42 PM
Quick correction to my last post - 0.2 fc is 20 times the illuminance of a full moon.

That is subject to debate Steve. More later.

Chris J
05-26-2008, 11:49 PM
That is subject to debate Steve. More later.
Pulled from another site:


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The full moon typically provides only a faint illumination of about 0.2 lx, so the full moon is about 500,000 times fainter than the sun. When the moon is viewed at high altitude at tropical latitudes, the illuminance can reach 1 lx.[1]

The above comes from Tom's reference...as you can read the illumination from the moon will vary depending on the altitude and latitude, ( and I would suspect the season of the year as well) that it is being viewed at.

The SI derived unit of illuminance is the lux. One footcandle is equal to 10.76 lux, although in the lighting industry, typically this is approximated as 1 footcandle being equal to 10 lux.

The above is also a reference from tom's research...If full moon light is .2 lx (from above). And If 1FC = 10.00 lx ( I will use the appox. value to make the math simpler.) Then 1/10 FC = 1 lx. As we can see the 1/10 fc number for full moonlight in our study guide and exam is correct only if you are at the equater standing on top of a mountain during middle of summer. At .2 lx (ie. full moon light, no reference as to what altitude and latitude) the FC equivalent would be .02 FC or 2/100ths FC not 1/100th FC.

To make this information even more suspect read (below)the disclaimer to the article that this was taken from.

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This article does not cite any references or sources. (March 2008)
Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed.

So as you can see even this source is not definitive...

Chris J
05-27-2008, 12:03 AM
Another analogy:

I saw this discussion about the light of a full moon being 1/100th or 1/10th...and I remember Naomi saying it was 1/100th Below is an excerpt from an article on the web from the Lighting Research Center which explains this in laymans terms.

When considering the needs of an outdoor lighting installation, it is still helpful to think in terms of moonlight levels. Under very dim visual conditions (i.e., those found in rural areas), moonlight, with an illuminance of approximately 0.1 to 0.5 lux, or 0.01 to 0.05 footcandles (fc) on the ground, often provides enough lighting for people's basic needs such as walking, or finding a house or car. NLPIP proposes that moonlight illuminance can be used as a reference value for setting expectations of suitable light levels in rural or remote lighting applications. For the purposes of this Lighting Answers, one moonlight is defined as 0.1 lux (0.01 footcandle). This is equivalent to the average illuminance on the earth's surface provided by a full moon.


05-27-2008, 10:59 AM

Thanks for keeping this offshoot discussion on the moon alive. I guess you and I are both moon-lovers and have similar fascination of its light. Have you read my article on the subject in the CAST website? It doesn't delve into the more technical aspects, but since you bring it up I'll share more info on the subject that I un-earthed during my research.

The variables that affect moon brightness include phase (full, of course is the brightest); distance between the earth and moon (eliptical orbit accounts for a 30% change in brightness); distance between moon and sun (eliptical orbit accounts for a 7% change in brightness); and atmospheric attenuation (dependant upon presence of dust and moisture; and the incident angle of the moon to the viewer - this attenuation can result in huge differences (multiple magnitudes) in brightness.

In short, full moons under normal conditions range in brightness from 0.01 to 0.05 fc. The higher value only attainable on clear dry nights with the moon almost directly overhead, the lower value more typical of full moons in our climate and latitude.

I find the LRC's suggestion interesting and hadn't heard that before.

Chris J
05-27-2008, 01:57 PM
I have not read your article, but I will seek it out in a moment. This discussion was brought up some time back, and I have never really gotten to the bottom of it. Thanks for the info, I'll check out your article.

05-27-2008, 09:34 PM
Wow, that actually helps and quite interesting as well. Any thoughts on the Hadco brand of lighting they are looking at? Seemed like a good product when I saw it, but I wouldnt know the difference other than it looked much better than the junk at the box stores. And back to the issue, the path is textured concrete, straight with several small stair sets, and wooded rural with little ambient light from adjacent properties or the house itself. It does bring up a good point though that the amount necessary will be affected by whether they have house lights on. I think they are leaning toward brighter, but not looking for a lit runway either. This is real interesting stuff. I would really like to see the laws around here change so I could get into this service. Thanks for everything so far.

06-02-2008, 10:23 AM
Shovel Racer,

Since I represent a manufacturer, I won't comment on Hadco but I do suggest that you visit an Aquarius Irrigation location where you can evaluate several brands and make a more informed choice on manufacturer. Just keep in mind that a successful landscape lighting business targets the higher end homeowner - they demand the best products and service and so should you.

Regarding changing the NJ law, you may have read in other posts about our landscape lighting task force (directed by the New Jersey Green Industry Council). We're making excellent progress but really need support from guys like yourself. We're looking for donations, letter-writing and phone calls to your legislators. I'll have more info later today or tomorrow.