What’s in a Spotlight?

By Ian Ibbitson, PhD., ALLSCAPE Quality Lighting

This is a typical application where various spotlights are used to light trees and shrubs bordering a pathway. These types of lamps are mostly found in low-voltage and 120V line-voltage applications, and are available in incandescent and metal halide HID versions.

In any landscape lighting application, a majority of the objects in the environment are illuminated using a spotlight. Whether it is a tree, bush, column, sign, statue or even flagpole, a spot lighting optical system is used more often than not to get the job done. Understanding the basic principles of how a “spot optic” works enables professional landscape contractors to select the best choice of spot optic for any given application.

A spot 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 one convenient package.

More sophisticated commercial applications typically use spot optical packages created by the fixture manufacturer. When the arc tube of a MH lamp is positioned at the focal point of the parabola, the light rays are out-put in a parallel beam from the reflector, creating a spotlight. Lamps with smaller arc tubes concentrate more light at the focal point, producing a narrower spot light effect.

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 thought their bulbs are the same overall size. The 100W E17 lamp will produce a much narrower spot with a greater concentration of light or center-beam candlepower.

In-ground fixtures with metal halide spot optics are used to light some of the statues, trees and the flagpole in this 9/11 memorial. Custom reflector designs, and a lamp such as an E17, T6, or T4, are used. The reflector is usually in shape with a specular finish.

A lamp’s wattage, and the light (lumens) it out-puts, should be tempered by its ability to work with the reflector to produce effective lumens. Today, new low-wattage metal halide lamps, such as the 20W T4 or BT5, have very small arc tubes that can produce powerful narrow spotlights. These perform significantly better on a maximum candlepower per watt basis than their higher wattage relatives. A typical spot optic with a 22W BT5 lamp from Philips, produces 46,000 center-beam candlepower compared with lower 40, 516 center-beam candlepower generated by a higher 39W T6 lamp. The BT5 beam angle is also narrower and more focused at size degrees, compared to eight degrees for the T6.


So what does all this mean? If a narrow spot optic is required, the best results will be derived from the lamps with the smallest arc tubes. A narrower beam throws light a greater distance, allowing the taller landscape objects (trees, statuary, monuments, flag poles) to be illuminated more effectively, generally with less energy, two benefits for the contractor’s client.

If a spot optic requires the arc tube 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 parabola’s focal point widens the spot, producing a flood optic. The arc tube movement that is required is often quite small, less than 1 inch.Understanding the basic workings of a parabolic reflector allows the landscape contractor to select the best lamp/reflector combination for the job. If a column on a building is to be illuminated, how do you decide what the best spotlight is to perform the job?

Note the difference between these two lamps. Caution should be taken when changing lumen or light-output values in a photometric package, to simulate different wattage lamps from what was originally tested and recommended by the fixture manufacturer. Results in the field however, may vary. 

Small diameter columns, less than 2 ft. should use a narrow-beam spot reflector. A good lamp choice would be a 20 W 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 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 object, 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.


If narrow-beam spotlights are used it is recommended to place the fixture as close to the object as possible. For example, if the beam width is approximately 3 ft., place the fixture 1 ft. from 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 planters and trees have not grown to their full size. Typically, the lighting looks good at the start but not so good a year or two later when everything has grown. A good way to compensate for this is to use spot-to-flood adjustable optics.

A simple 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, and then adjusted to flood as vegetation grows and expands.

Spot-to-Flood Adjustment: The circle at left is a magnified view of this lamp’s spot-flood adjustment mechanism. Installers use a flat-head screwdriver to retract or extend the brown part at far left to set the bulb (far right) to spot or flood mode. Other brands have similar but different spot-flood adjustment mechanisms.

Fine-tune, Adjust

Most of the time, a simple spot optic will do the trick but occasionally additional options may be 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. It is important to control glare because it can be a huge distraction to the pedestrians walking by and take their eye off your great lighting design.

As you can see, there is more to spot optics than literally meets the eye. Creating a great lighting design requires some artistic flavor but also requires a basic understanding of lamp technology and how it interacts with parabolic reflectors. Compare the optics available and play with the placement of the fixtures relative to the object to be illuminated. A few minutes of research and computer design will greatly improve the look of your project and ultimately make your customer happy that they chose you to design and install their project.