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Landscape Lighting The Magazine for Exterior Site/Area Lighting11-01-89 | News

Landscape Lighting
The Magazine for Exterior Site/Area Lighting

Distance, flare, and cone size all effect the ability to see beyond the light

Good landscape lighting is, in a large part, simply glare control. The key word here is glare. So, what is glare? Well, how about “unpleasant brightness,” or “light that shines so brightly that it hurts the eyes.” In landscape lighting, glare is especially important because you are dealing with extreme contrasts. You see, the background is generally black – complete darkness. So, the ratio of light to dark is almost always extreme.

Now, the minute you introduce an unshielded light source into this very dark picture, that’s all you’re going to see. That light source will totally dominate that scene.

On the other hand, you can shield or somehow hide and control the source so that you see only the product of the light. Essentially, the light is reflecting what you want to light – what you want to see – and what you want to feature.

I think it’s necessary to quantify this glare; to put some numbers to it so that we can at least compare it.

To understand this data, we must first understand the measurement units:

Brightness is measured for our purposes here in terms of candelas per square meter (CD/M2) or footlamberts (FLB). To convert from CD/M2 to FLB, simply multiply the CD/ M2 by .029. Brightness is also known as luminance.

OK, so here are some common luminance values from Page 8-68 of the I.E.S. Handbook, Volume 1981, reference:

Now, there’s another factor that influences this data that’s known as adaptance. Notice when someone approaches you during the daytime with the bright headlights of their car on; it may be annoying but at night time, it would be blinding. The eye has adapted to lower levels so it is more sensitive to higher levels.

Glare can be used as an effect

The size of the light source is also important. When the light approaches, being a point source, it takes on the character or quality of sparkle or festivity. Examples of this are the tiny Christmas tree lights. Even though they are very bright, they are generally not considered to be glare sources. Stars would be another example.;

OK, so with this data in mind, what’s this magic number above which we have glare, and below which we do not? This certainly is some what subjective and dependent on age, etc. But that’s probably unimportant anyway since the number we are looking for is probably around 4,000 footlamberts (FLB).

Looking at the chart, you see that the tungsten and mercury lamps are well above this number. That, of course, means that metal halides and high pressure sodium (HPS) would be well beyond this range; also, low pressure sodium, HWI, Xenon, etc.

While reducing glare can be a key factor in your design, sometimes a lighting fixture is specified due to its daytime synergy in a project.

In short, you don’t have a lot of choice if you wish to observe this 4,000 FLB value, or anything even close to it. You simply cannot use bare HID lamps, or even shield with refractors or diffusers that don’t adequately control the brightness.

To carry this a bit further, note that the streetlights and parking lot lights frequently use glass or plastic refractors or diffusers that shield these extremely bright light sources. They are, of course, well above the 4,000 footlambert value, and they surely are glare sources.

So, what’s to do about this? First, become aware of it; second, don’t do it: third, prevent it, or change it if you can. There are many light fixtures that have excellent photometric design (efficiency and distribution,) and yet, have shielding that eliminates glare.

R. W. Swarens has been in the lighting industry since 1956, as representative, designer and consultant. In 1976, he opened his own lighting consulting and design firm.

Swarens is past president of the Illuminating Engineering Society, Southern California section and past vice president of United States Institute of Technical Theatre. He has won numerous awards for custom lighting design, is an instructor of lighting design at the Otis Art Institute of Parsons School of Design, Los Angeles, and frequently gives seminars on lighting to architects and designers.