Published In April 2000
We always see luminaires outside that are tilted, or attached to poles at an angle other than 90 degrees. I speak not of the floodlight, whose shape does not lend itself to alignment with the pole. Rather, these are boxes or 'rectilinear' shape luminaires, which tend to blend into the lines of the buildings . . . if only they were at right angles to those buildings. There are intended tilts (usually an 'up-tilt') and also unintended ones (otherwise known as sagging luminaires). This aspect of a highly successful lighting job heavily depends on the installer. Luminaires are intentionally up-tilted by someone wanting a "free lunch," which we all know does not exist. There are gains to be had via a correctly done up-tilt and losses to offset those. In practical terms, the urge to up-tilt a 'shoebox' comes from the misconception that the most intense light, or "beam," comes out perpendicular to the glass. The lighting-layman sees more light directly under the pole, and assumes the "beam" is concentrated there. Having no understanding of the "distance squared" law, or cosine corrections, these installers erroneously "aim the beam" away from the pole, where more light is desired. You see this often where a "Dusk-to-Dawn" luminaire is "aimed" at a barn or farmhouse. The result is two-fold: 1) There is more glare and less light on the barn, and 2) Great quantities of light are wasted into the night sky. There is little sympathy for the first result, but light emitted above horizontal is detrimental to astronomic observation and those of us who enjoy seeing a blanket of stars in the night sky. With the advent of reflectors that create a beam directed forward, away from the pole, we have begun to see some advantage in tilting-up. Generically, this type of shoebox/optic is called cutoff floodlight, forward throw, or Type IV Shoebox. They are sometimes called a combination of all three. These optical systems produce a "half bat-wing" which is not greatly different from a floodlight, except that the uplight is controlled, or "cut off." There can be value in "aiming" these fixtures above 90 degrees, and there can be lighting disasters, as with our farmer friend. To benefit from uptilting, you need to know three things: § The luminaire you intend to uptilt is indeed a cutoff floodlight (or the same optic called by a different name). § You know the vertical angle of maximum candela for the particular luminaire with the specified lamp or "where is the center of the beam with respect to the housing?" § You are reasonably certain that the high-angle light will not have a negative effect on your lighting design. The first two items can be learned by obtaining a photometric report from an independent laboratory. Once the vertical angle of maximum candela is known, you can start calculating the up-tilt angle. The main fact that must be understood is that the highest illumination possible at a single point by a luminaire is when the maximum candela is 'aimed' at that single point. This might be a "no-brainer" for you and confusing to others. The luminaire on top is up-tilted such that the maximum candela is "aimed" at the critical area. The luminaire on the bottom is up-tilted by someone who imagined the beam coming out perpendicular to the lens. Notice the angle of the real beam angle when the luminaire is overly up-tilted. The angular difference between the "aimed" angle and the angle of maximum candela is the up-tilt angle. Tilting the luminaire higher than this angle will cause light pollution and spill unwanted light into the heavens. It will also reduce light on the area where you need it. Once the luminaire is up-tilted to achieve maximum illumination at a particular spot, one trade-off remains: There is additional glare/brightness on the street side because the shielding (usually the lens frame) is 5 to 15 degrees higher than it was before it was up-tilted. It is possible that visibility and/or comfort are compromised on the street side of the luminaire. Certainly this is worth considering in the design. Type II, Type III, or (ugh!) Type V optic system patterns were developed with the intent of never being up-tilted. And even if that were true for every luminaire, these products have a sharp cutoff, and their beams are shaped to intersect with the ground at a certain angle. If a Type III reflector is up-tilted 15 degrees, then the house-side beam will now intersect the ground at an angle 15 degrees steeper than intended. This can cause a hotspot or striation of light behind the pole, and a cutoff 15 degrees closer to the pole than intended. The "street-smarts" you should now be armed with are: § Never consider an up-tilted luminaire unless it is a cutoff-floodlight-type shoebox. § Never uptilt so that the beam is above horizontal. The worst tilts, "down-tilts" or sagging, are often seen on luminaires mounted to wood poles. Luminaires usually sag because the material on one side of the arm flexes or moves in response to gravity. Some luminaire manufacturers even pre-plan for a one- or two-degree sag due to flexible housing materials. This one- or two-degree up-tilt is usually accomplished by cutting the arm at 88 or 89 degrees, instead of at a right angle. Having worked for one of these manufacturers, I was constantly chagrined when I found the arms installed upside-down, yielding a luminaire down-tilted four degrees. The label on the arm was clearly visible from the ground and stated "TOP, this end to pole" . . . an obvious installer error. Die-cast aluminum luminaire housings have a built-in up-tilt of one or two degrees due to the draft designed to allow the housing to pop out of the tool. Unfortunately, all sides of the housing have this draft, resulting in the street-side of the housing being out-of-plumb by double the draft (assuming the house-side is exactly vertical). The moral to this story is to have as little draft as possible. I have been going out of my way to examine down-tilted fixtures for many years (much to my wife's dismay) to see every form of construction and design that gives way to a down-tilt. Material failure causes most of these. Let's face it, every material has its failure mode: sheet aluminum bends, extruded aluminum twists, cast aluminum fractures, and wood (poles) crushes. Not knowing what extremes of wind or quaking earth caused these isolated saggings, I usually write them off to happen-stance. However, when every luminaire on a site has the same down-tilt, I can predict that it is either a design flaw, or a poor installation. One more type of tilt, the side-tilt, or fixture "twist," is attributable to two things: loose attachment bolts and gravity (from the ballast). Outdoor fixture manufacturers rarely attach the ballast on the same centerline as the mounting arm. In a rectilinear fixture, there is always more room in the corner, resulting in more weight on one side of the fixture than on the other. If the arm-bolts are not torqued tight enough, the fixture can twist or rotate on the arm. This can also result from too much pole vibration, or too-rough handling during pole erection. I don't know of a product out there that does not have enough "play" between the arm, bolts and fixture to allow this to happen, if the installer is not careful. We owe it to everyone's eyes to do anything in our power to eliminate unwanted tilts. Without our help, they remain eyesores and light polluters for a very long time. PAULIN is the product manager at Ruud Lighting, vice president of design and application at IESNA, and past chair of the IESNA Security Lighting Committee. He can be reached at (262) 884-3320.