Landscape lighting using parabolic optics

By Ian R. Ibbitson B.Sc., Ph.D., MIET, CEng.

Monday, January 14, 2008

In any landscape lighting application, a majority of the objects in the environment are illuminated using a parabolic optic.  Whether it’s a tree, bush, column, sign, statue, or even flagpole, a parabolic optical system is used most of the time.  Understanding the basic principles of how a parabolic optic works is the key to creating a professional design for most applications.

A parabolic optic consists of a lamp and reflector.  Lamps such as the very popular MR-16, and commonly used PAR 20, PAR 30 and PAR 38 integrate the lamp and PARabolic shaped reflector into a single package. These lamp types are mostly found in low-voltage and 120V line-voltage applications, and are available in both incandescent and metal halide versions.  A typical application is shown in Figure 1, where various spotlights are used to light trees and shrubs bordering a pathway.

Figure 1 - Spotlights used to light trees and shrubs bordering a pathway.

More sophisticated commercial applications typically use parabolic optical packages created by the fixture manufacturer.  In Figure 2, in-ground fixtures with metal halide spot optics are used to light some of the statues, trees and the flagpole in a 9/11 memorial.  Custom reflector designs, and a lamp such as an E17, T6, or T4, are used.  The reflector is parabolic in shape with the most common finish specular.  When the arc tube of a MH lamp is positioned at the focal point of the parabola, the light rays are output in a parallel beam from the reflector, creating a spot light.  Lamps with smaller arc tubes concentrate more light at the focal point, producing a narrower spot light pattern. 



Figure 2 - Spotlights used to light trees, statues and flagpole at 9/11 memorial.
Photo courtesy of Brad Bonnet Allred Architects.

Different lamp types placed in the same parabolic reflector will produce varying results because of their arc tube size. For example, a 100W MH E17 lamp has a much smaller arc tube than a 175W E17, even though their bulbs are the same size. The 100W E17 lamp will produce a much narrower spot with a greater concentration of light or center-beam candlepower.  Figure 3 illustrates the difference between these two lamps. Caution should be taken when changing lamp initial lumen values in a photometric package to simulate a different wattage lamp from that originally tested and recommended by the fixture manufacturer. The results in the field may not be as expected. The light may be to bright, or to dim and not as sharply focused in either a spot or flood pattern as required.

Figure 3 - E17 100W (top) and 175W (bottom) arc tubes.

A lamps wattage, and the light (lumens) it outputs, factored by the effectiveness of the reflector produce the real light output or effective lumens of the parabolic optical system.  Today, new low-wattage metal halide lamps, such as the 20W T4 or BT5, have very small arc tubes that can produce powerful narrow beam spot lights.  These perform significantly better on a maximum candlepower per watt basis than their higher wattage relatives.  A typical spot optic used by ALLSCAPE^® in their in-ground product with a 20W BT5 lamp from Philips, produces 46,999 center-beam candlepower compared with the lower 40,516 center-beam candlepower generated by a 39W T6 lamp.  The BT5 beam angle is narrower and more focused at 6°, compared to 8° for the T6.

When a narrow spot optic (less than 10° beam) is required, the best results will be derived from the lamp with the smallest arc tube for a given parabolic reflector.  A narrower beam throws light a greater distance, allowing taller landscape objects (trees, columns, and flag poles) to be illuminated more effectively.

A spot optic requires the arc tube of the lamp to be placed at the focal point of the parabola. What happens if it is not?  Moving the arc tube a small distance out of the parabolas focal point widens the spot, producing a flood optic.  The arc tube movement that is required is often small, less than 1/4 inch. 

Figure 4 illustrates this using a 39W T6 lamp with its arc tube positioned in (spot) and out (flood) of the focal point.  As a result, the light beam widens but is less powerful at the center.  In the flood position, the beam diameter increased to 7ft. from 3.4ft. in the spot position at a distance of 10ft. from the fixture. The center-beam footcandle level reduced from 405fc in the spot position to 95fc in the flood position.


Figure 4 - T6 lamp in the spot (left) and flood (right) position in a parabolic reflector.

Understanding the basic workings of a parabolic reflector allows the landscape specifier to select the best lamp and reflector combination for a particular application.  One common application is illuminating a column on a building. How do you decide what the best spotlight is to light the column?

Small columns less than 2ft. wide should be illuminated with a narrow-beam spot reflector.  A good lamp choice would be a 20W BT5 or T4. Wider columns up to 3 or 4 feet in diameter should use the 39W T6.  Even wider columns could use an E17 lamp or the flood versions of a BT5, T4, or T6.  If column height is also an issue, narrow-beam optics must be used, or a fixture must be placed at the bottom and the top of the column, or in the middle using an up/down light.  If height and width is required, several narrow beam light fixtures spaced appropriately apart may be required.  To achieve a smooth effect, make sure the beams of each fixture overlap.

When using a narrow-beam spotlight to uplight a column or tree place it as close to the object as possible.  If the beam width is approximately 2ft., as in the BT5 example above, placing the fixture 1ft. from the object will only graze the surface of the object. 

Placing the fixture further from the object may require the optical assembly to be aimed (tilted) towards the object, to get sufficient light on it.  Also, if an in-ground fixture is being used to light a column or palm tree, placing the fixture too far away can result in the base not being illuminated creating a floating effect which is not recommended.

In many landscape applications the fixtures are installed when the trees and planters are immature.  Typically, the lighting looks good at the start after the initial positioning and focusing but not so good a year or two later when the landscape has matured.  One efficient way to compensate for the maturing process is to use spot-to-flood adjustable optics provided with product such as the SL-51 Ciello landscape floodlight from ALLSCAPE. 

A flat-blade screwdriver is used to move the lamp in and out of the parabolic reflector.  Fixtures such as this allow adjustment of the beam width to suit the landscape. Spot optics would be used at the start, then adjusted to flood as vegetation grows and expands.

Figure 5 - SL-51 spot-to-flood adjustment mechanism.

Additional options may be occasionally required to provide extra light control.  Refractors are often used to widen or smooth light emanating from the reflector.  A linear spread lens is used to spread the beam in one axis, while a radial lens is used to widen and smooth the light in all directions. 

External fixture controls, such as barndoors, are used to provide a sharp cutoff perhaps to stop light from getting into a nearby window.  Other devices, such as visors and louvers, both internal and external, are also available to control glare.  It is important to control glare because it can be a distraction and even annoyance to the pedestrians in close proximity and can greatly reduce the impact of the lighting design.  Filters can be used to color the light or smooth the light output. Note: the light output is reduced every time additional filters or lens are added.

Figure 6 - Spotlights with visors lighting statue from two angles.

There is more to parabolic optics than literally meets the eye. Creating a great lighting design requires some artistic flavor but also requires a basic understanding of how lamp technology and parabolic reflectors interacts with each other.  Computer tools such as 3dOP Pro, available at http://www.3dOP.com, help make the correct spot optic selection at the design stage of any project.  Compare the optics available and experiment with the placement of the fixtures relative to the objects to be illuminated.  A few minutes of research and computer design will greatly improve the look of your project and ultimately make the customer happy.