BY JAMES BENYA
» In modern architectural lighting, like in theater, we use a combination of general or ambient lighting ('fill') and accent or focal lighting ('key') to create scenes. The greater the contrast between focus and background, the greater the dramatic effect. But unlike in the theater, architectural accent lights must be small, efficient, relatively inexpensive, and innocuous (if not attractive). We tend to use directional lamps, like AR, MR, PAR and R, because the lamp is the reflector, and the resulting luminaire is fairly small. There are also luminaires that use small lamps and have an integral reflector; these are especially important for the new low-wattage ceramic metal halide lamps.
The problem with most architectural lighting equipment is that the quality of the projected beam is poor. The best lamps, like the incandescent PAR36, have beautiful beam patterns that can be aimed at artwork and architectural details and create predictable results. But the filaments and reflectors in many modern lamps like the MR16 and the PAR halogen lamps create beams of light that have striation, halation, and ragged edges. Aimed at artwork, the average MR16 lamp will create a scallop, a hot spot, a halo ring or two, and uneven streaks of light, not to mention a slight green tinge and color temperature about 300K greater than the ambient. This can be-and should be-fixed as part of a process many lighting designers call 'focusing.' One might ask why we use lamps and luminaires that have so many foibles. The answer is simple-cost and availability. MR16s are available at Home Depot, but good luck finding an AR lamp in stock anywhere in Des Moines.
Focusing is a term borrowed from theatrical lighting, and in architectural lighting it has a similar result, even though the 'focus' of a condensing lens system is almost never involved. Rather, focusing consists of four main activities: beam selection, beam modification, color, and shielding.
Exclusive online content. (View this PDF file for a how-to on choosing the appropriate luminaire for the required beam spread.)
Most projector lamps, such as PAR and MR types, are sold according to candlepower type.
To determine the intended beam spread, find the angle needed to illuminate the object using Diagram 1 (this page). The diagram is a version of the distance squared law that lighting designers have long since memorized. Then, find a lamp that produces the candlepower and beam spread matching the requirements.
Most conventional lamps are 'axial beam' lamps, meaning that the filament is oriented along the axis of the reflector to create a round beam. But certain lamps, especially high-wattage PAR56 and PAR64 lamps, employ a longitudinal filament, so the projected beam is oval in shape. For instance, a 1000W PAR64 narrow spot has a beam that is 8 by 20 degrees; the 1000W PAR64 medium flood has a beamspread 10 by 30 degrees. This means that during focusing, the beam must be rotated correctly to achieve the desired effect.
In order to tame the bad beam behavior of lamps like the MR16, one typically uses an accessory lens in place of the clear glass lens usually supplied with MR16 fixtures. For other lamps, an accessory holder may also be needed. Professional lighting gear, especially for museums and high-end residential lighting, is often designed to permit stacking two or more lenses or other media. The three most common beam shaping lenses are:
• Soft-focus lens, which consists of a very fine pattern of ridges and depressions. This lens is about 92 percent efficient, removing almost all halation and striation without significantly affecting the beam spread or candlepower. For instance, when added to a narrow spot lamp, the beam will be softened to a spot distribution, adding a few degrees to the beam. A soft-focus lens preserves the 'punch' of the beam while removing striation and halation.
• Spread lens, which can either be a sandblasted lens, an acid-etched lens, a prismatic lens, or a lenticular (pressed) lens. A spread lens changes the beam of the lamp significantly in all directions, creating a flood out of a spot. A spread lens tends to remove the punch from the beam, with the resulting light becoming a wash with very soft edges. Efficiency varies with lens type, but in general spread lenses are 50 to 70 percent efficient. Personally, I tend to use this lens less often than the soft focus lens; a typical application for a spread lens would be an extremely broad and even wash.
• Linear lenses, which consist of linear prisms running in one direction. This lens, also called a beamsplitter, elongates the beam in one direction and shortens it in the other, turning a round beam into an oval. When used with a spot lamp, the effect is dramatic, often creating a long, skinny beam of light. When used with a flood lamp, the effect is more subtle, turning a round beam into a moderate oval. Linear lenses are not very efficient, but the effects are unique.
Two or more beam control lenses can be used together, but the resulting loss of efficiency makes this concept undesirable. The 'art' of focusing is combining the appropriate wattage and beamspread, with the proper lens, to achieve the right result.
Radial and Cross Hair Louvers maintain most of the original candlepower and beam spread, and are relatively innocuous; however they provide limited shielding angles and may not fit many luminaires.
Tube Baffles or Snoots attach to the front of the luminaire to permit tight shielding. They preserve full candlepower and beam spread, but are unattractive.
Barn Doors allow field adjustments and tuning, and preserve full candlepower and beam spread, but are large and unattractive.
Large Square-cell Louvers, like radial or cross hair, maintain most of the original candlepower and beam spread, and are relatively innocuous; however they provide limited shielding angles and may not fit many luminaires.
Most architectural light sources like PAR and MR lamps emit light throughout the hemisphere beyond the face of the lamp. Generally, only the beam and a bit of the field is useful, the rest of it becoming glare. While it seems that glaring lamps are part of many European and Asian projects, in North America and especially in museums and galleries, glare control is an essential part of good lighting design practice.
Glare reduction amounts to preventing the outside field from leaving the luminaire at all. Most recessed luminaires and some monopoint/track luminaires shield the glare with a combination of recess depth and cone, which 'baffle' the light from exiting the luminaire at unacceptable angles. A good example is the matte-black ribbed baffle commonly used on low-cost downlight trims. There are a number of track luminaires that employ a recessed lamp and ribbed baffle to effect the same result.
But when accessorizing most architectural lighting, including recessed, track and monopoint accent lights, luminaire size often restricts shielding options to very few choices.
One of the MR16 luminaire's better qualities is that it is generally required to have a safety glass in front of the lamp itself. In almost every luminaire, you can replace the clear glass with an accessory lens, and often a snoot as well. To light moderately-sized artwork in residential and hospitality settings, for example, I use a 25 degree, 37W IR or 50W standard MR16 with a soft-focus lens and snoot in an inexpensive gimbal ring track or recessed luminaire.
To accessorize a PAR luminaire, it is often necessary to select a product with clips that hold lenses and other media. These clips are usually standard equipment on 'museum grade' track lighting, but for ordinary track and recessed lighting, the clips may need to be specified separately. For MR16 luminaires, you can add a screw-on accessory holder that clamps only onto the lamp-a very useful accessory, especially when working with the delicate lampholders used on low-voltage monorail systems.
The increase in vibrant colored light using theatrical color-changing instruments has been a great development in lighting design over the past decade. But for accent and display lighting, fixed color lenses, including filters for UV and IR light, can also be used, perhaps in a more subtle manner. Commonly used color lenses include:
• Saturated color lenses, including red, green, blue, magenta, cyan (aqua), and yellow. I also like to use amber and orange. Mostly these are applied for fun, creating an effect in which the light becomes artwork.
• Color-correction lenses, including color temperature warming (cosmetic peach), color temperature cooling (light blue), and minus green (cosmetic pink). For instance, every glass-backed MR16 lamp and most quartz metal halide have a tendency to turn slightly green, so adding a cosmetic pink filter creates a more attractive scene. These lenses can be stacked; for instance, a warming lens with pink will make an MR16 look like an R20, but with candlepower.
• Special filters, which are usually employed for artwork protection. UV filters are commonly used in museums, and cold mirror IR filters can also be used in critical applications. (Note: Be careful with the cold mirror lens. Ninety percent of the energy of an incandescent lamp becomes heat, and reflecting the IR back into the luminaire can overheat the lamp, socket, and wiring, causing disastrous results.)
Lenses can be stained or dyed glass, dichroic-coated glass, or theatrical gel. Each type has advantages: The stained glass lens is durable and the color of the beam is consistent. The dichroic can produce vibrant color, but often creates color shift and color halation. The gel has a short life and is seldom used with smaller luminaires and lamps like MR16s owing to heat; glass lenses, however, can expand without cracking.
It has become popular to throw a light pattern onto a wall or floor. However, while there are 'add-on' accessories that turn track luminaires into pattern projectors, great results require using a fixture specifically designed to project a focused, collimated beam of light. Most projectors use ellipsoidal optical systems and condensing lenses that permit the use of gobo patterns or slides, iris or shutters, and various focal-length lenses. While interesting patterns are possible by placing perforated metal or other materials in front of a clear MR16 lamp, don't expect to project a bright, clear image-for example of a company logo-without moving from ordinary architectural lighting into the realm of heavy-duty display and theatrical equipment.
James Benya is a professional lighting designer and principal of Benya Lighting Design in Tigard, Oregon. He serves on the editorial advisory board of A|L.