Much of what we enjoy visually is presented in a two-dimensional format. Paintings, textiles, photographs, movies, and video (TV) are significant examples. Experts who produce memorable work in these media are well aware of the limitations of two-dimensional (flat) displays and attempt to use every possible method of providing a feeling of depth in their work.
Lighting, with the many variations possible today, provides a significant contribution in deriving a feeling of depth and mood.1,2 For this reason, the photography, film, and video markets are especially demanding of light sources and support new ways of using them.3,4
Specially designed fluorescent lamps are becoming more popular for the image capturing market due to efficiency, stability, and low infrared heat output. Lamp design considerations, such as new phosphor coatings and ballasts, help to minimize color and flicker problems traditionally associated with this type of light source.
A variety of design factors should be considered and utilized to develop fluorescent lighting products for the stage and studio segment. Some of these factors, specifically phosphor types, power level, and lamp size will be reviewed in this white paper.
DESIGNING FLUORESCENT LAMPS TO BETTER MATCH TUNGSTEN LAMPS AND DAYLIGHT
Color photography film was developed in the 1930s for movies and still photography.5 Two types of film were developed: one for indoor light (incandescent) and one for outdoor (daylight). We are fortunate in that these are excellent full spectrum lighting sources, which provide a rich spectrum for effectively differentiating colors resulting from partial reflection and absorption of light by objects being viewed.
New lighting products, such as metal halide and fluorescent lamps, need to achieve the same continuous spectrum performance to be successful in film applications. Any application where accurate color differentiation is critical — digital photography, art studios, museums, and retail displays — should consider similar light sources for good lighting performance. Color temperature restrictions can be less defined relative to film, but full visible spectral output should provide significant benefits.
Traditionally, fluorescent lamps have not been well regarded for use in situations where good color rendition, especially for film, is required.6 The main reason for these color issues is the choice of phosphor(s) utilized to generate light at the inner surface of the fluorescent lamp tube. Commercial and industrial lamp users wish to derive the maximum efficiency from light sources and emphasize photopic lumens as the most important feature, with color rendition as a secondary goal. Conventional lamp designs available today, even with CRI levels in the 80s, often exhibit discontinuous spectra and use narrow band phosphor blends, which are unsuitable for film and other full spectrum applications. Exceptions utilize broadband phosphors, and with minimal color correcting gel, have been used as a soft daylight 5500K light source. Standard fluorescent lamp products give poor results.
MAKING A FLUORESCENT LAMP PRODUCE VISIBLE LIGHT WHICH IS SIMILAR TO TUNGSTEN LAMPS
Consider the smooth continuous spectral irradiance plot for the 3200K halogen incandescent lamp shown in Figure 1. When this is contrasted with the spectra for the tri-band phosphor lamp in Figure 2, a common commercial fluorescent lamp optimized for photopic lumens, one can see the significant gaps in output from the narrow band phosphors used in the modern fluorescent lamp. Emphasis on a photopic response positions light in the yellow-green region of the spectrum, thereby creating the infamous green cast resulting in film exposed under these lights.
To achieve a better match with tungsten halogen, broadband phosphors are selected and blended to more closely match the visible portion of the tungsten light output as shown in Figure 3.
An exact match is difficult because the blue and green discharge lines present in the fluorescent lamp (not due to phosphor) are not easily attenuated. Additional improvements in the fluorescent lamp spectrum can be achieved — with some loss in overall lamp efficiency — by attenuating the 405 and 436 nm mercury emission lines using external filters. Since none of the undesirable infrared emissions from the tungsten lamp are present in the fluorescent lamp, subjects exposed to the “visibly matched” fluorescent 3200K source will feel cooler. Photopic lumens per watt, as listed in Table 1, are not as good for this full spectrum design relative to the triband lamp, but they are significantly better than the incandescent lamp with its high IR losses.
DAYLIGHT FLUORESCENT LAMPS
North sky daylight (skylight) has long been the light source of choice for art studios where consistent lighting is desired. The spectral plot of relative irradiance vs wavelength (Figure 4) for daylight under several sky conditions illustrates that this is a good full spectrum lighting source. Pigments and materials with a variety of reflection and absorption spectral responses can be differentiated well using daylight. Natural daylight is more variable and less continuous than tungsten light. The variability in daylight is due to variations in atmospheric conditions and the position of the sun. The average spectrum at noon has been chosen as the reference (5500K) and is used as the reference for lamp development purposes.7 It is important to remember that natural daylight under varying conditions may often not match well with the 5500K spectral standard or with daylight lamps developed to match it.5,7
Making fluorescent lamps with visible light similar to daylight can be achieved by blending broad-band phosphors in proportions that match the daylight reference spectrum (Figure 5). Some mismatch occurs due to the capability of existing phosphor blends as well as the visible mercury lines, which aren't easily attenuated. However, the result does achieve CRI levels in the mid 90s relative to the reference daylight. It's important to note that the photopic lumen levels tabulated for 5500K products in Table 2 are lower, similar to the 3200K example, for the full spectrum lamp made with broadband phosphors. This reflects the lower emphasis placed on the yellow-green photopic region by the full spectrum design, not an indication of less effective light output. End users may actually perceive that the full spectrum light looks better and is the more efficient source for color critical applications.1 By blending broadband phosphors, fluorescent lamp makers can more effectively match tungsten and daylight spectral output. Photopic efficiency is compromised to achieve the wider spectral output, however efficiency levels are still good compared with incandescent lamps where more than 50% of the input energy is wasted producing infrared radiation.
High frequency ballasts eliminate flicker and support smaller lamps for improved fixture designs
Twenty years ago when 50 and 60Hz ballasts were the most practical options, the flicker problems exhibited by fluorescent lighting and the motion picture filming process were unacceptable. Also, T12 lamps were the most efficient lamps for these relatively low operating frequencies. Advances in electronic high frequency ballast designs during the past 20 years have resulted in many new improvements and practical choices for fluorescent lamps: many are flicker-free for film and video formats.
Electronic high frequency ballasts typically operate at frequencies between 15 and 60KHz (much higher for some compact fluorescent lamps). In addition to offering flicker-free operation, these new ballasts are lighter, smaller, more portable, and more efficient than older electromagnetic ballasts. All of these improvements have enabled enhancements to fixture designs. Light dimming is practical over a wide range for some electronic ballast types. Programmed dimming with DMX compatibility is also available for modern set lighting control systems.
Part of the improved efficiency of electronic ballasts is due to the use of smaller diameter lamps (e.g. T8, T5, and T4), which do not work optimally on 60Hz ballasts. This has driven the change to smaller lamps; much of the new compact fluorescent lamp market has arisen since the advent of electronic ballasts.
THE ROLE OF REDUCED LAMP DIAMETER ON PERFORMANCE
Initial, film-friendly, full-spectrum fluorescent lamps with electronic ballasts were primarily of the traditional T12 diameter in varying lengths. A useful feature of the T12 design is the soft, diffuse light, which it produces due to the relatively large size of the lamp surface where the light, from the phosphor particles, originates. Efficient soft light sources play an important role in modern stage and studio lighting and are increasingly popular as new fixture designs become available. The T12 lamp also has the advantage of being thermally stable, compared with compact fluorescent types, in terms of brightness and color changes during warm up and over lamp life.
Smaller diameter fluorescent lamps, exhibiting high light output, improve the ability to focus the lamp light resulting in fixture designs capable of improved “throw” distances and less spill losses. The improvements in focusing are limited primarily to linear lamps such as the T8, T5, or T4. Folded, single-ended types, although the bulb diameter is often as small as T5, are only slightly more “focusable” than T12 linear lamps. In all cases, fluorescent lamps are going to be more diffuse compared to filament or small arc (HMI) lamps, which better approximate a point source for fixture design purposes.
IMPORTANT OPERATING CHARACTERISTICS SPECIFIC TO FLUORESCENT LAMPS
All practical fluorescent and metal halide lamps contain a small amount of mercury. Mercury vapor is essential for generation of ultraviolet light in the fluorescent lamp arc. This UV light is converted into visible light by the phosphor(s) coating the inner surface of the tubular glass bulb.8 The mercury discharge also produces visible light peaks at wavelengths of 405, 436, 546, and 578 nanometers, which contribute to the difficulty in replicating tungsten and daylight spectra more exactly. Color, lumens, and even the appearance of the operating lamp depend on the condition of the mercury discharge.
Lamp warm-up characteristics depend strongly on the mercury vapor pressure, a function of lamp warm-up time. This is illustrated in Figure 6 for several sizes of fluorescent lamps after a full burn in cycle. It should be noted that stabilization times are much shorter for lamps that are used repeatedly in the same position and location. Moving lamps can result in relocation of the free mercury, which can extend the stabilization time by a factor of two or more the first time the lamp is used again.
In contrast, incandescent lamps are immediately useable after starting, assuming stable supply power. Lighting users need to be familiar with the warm-up characteristics of fluorescent lamps when using them for color-critical applications such as film-set lighting. Lamp color changes due to mercury vapor pressure (thermal) effects can be a significant problem when making set changes such as moving light sources, temporarily closing fixture barn doors, or cool air drafts. Some of the new CFL light sources are not nearly as stable as T12 or T8 types, in terms of thermal effects, which can result in lamp orientation and warm-up issues.
Fluorescent lamp ambient conditions also play a key role, especially when lamps are closely spaced in confined fixtures. It is not productive to operate lamps under extreme temperature conditions. Placing fluorescent lamp fixtures near a heat source or outdoors during the winter are typical examples.
Dimming T12 fluorescent lamps results in much less color shift than observed with tungsten lamps, as shown in Figure 7. This is because the phosphors do not behave like Planckian radiators. However, the changes in mercury pressure due to cooler operation when dimmed can result in color shifting over time, after the lamps are dimmed down or up. The effect of dimming can be similar in magnitude to the startup transients and must be kept in mind by lamp and lighting designers when developing fluorescent lighting products for cinematography applications.
Useful lifetimes of fluorescent lamps can be determined by several factors. Electrical failure of the electrode lasting 10,000 hours or more is the conventional failure mechanism used for life ratings. However, loss of available mercury due to reactions with lamp components over time can also limit lamp life. Furthermore, phosphor depreciation resulting in loss of light and/or unacceptable color drift can affect useful life.
When lamps are operated at high current levels to maximize light output, as is the case in most studio applications, they will probably fail due to excessive lumen loss and/or color drift. This is particularly true of highly loaded T5 and CFL types. Figure 8 illustrates typical light output depreciation results for T12 full spectrum lamps. It is important to realistically rate product life in terms of these aging effects, not just based on electrode failure times.
ADDITIONAL DESIGN CRITERIA FOR PORTABLE LOCATION LIGHTS
Stage, studio, and cinematographic applications of fluorescent lamps have more constraints relative to other full spectrum lamp applications. Probably the most important differences result from continuous relocation and rough handling. Use of protective coverings and rugged lamp fixtures can help to minimize handling issues. Warm-up and thermal issues can become more difficult to address when covered lamps are used under hot conditions. Sometimes operation improves under colder conditions. Use of T12 or T8 lamps, which are inherently more thermally stable, is perhaps the best way to assure rapid color and brightness stability for transient use situations.
When fluorescent lamp fixtures are moved or relocated (or fitted with new lamps), it is important to “burn-in” the lamps in their new positions. This results from the relocation of the small quantity of mercury within the lamp when moved. The first time the lamps are operated in the new positions, the mercury may take much longer to stabilize compared with repeat starts, even from the same starting temperatures. This is important to remember when lighting a set where lamp warm-up time must be kept to a minimum.
Full spectrum fluorescent lamps provide excellent soft light sources for film and studio applications. They provide: relatively cool operation; stable color during dimming; daylight matching capabilities; and long life. New lamp and fixture combinations are resulting in a wide variety of new lighting systems for motion picture and video applications. Given the game-changing nature of brighter fluorescent lighting options now available for set lighting applications, we may need to adjust that familiar phrase: Fluorescent lights, camera, action!
Jon B. Jansma, Ph.D., is a senior development engineer with GE Consumer and Industrial's lighting business. His quarter-century of experience designing lamps began with chemical engineering studies at Case Western Reserve University, includes research and development at GE Global Research in Niskayuna, NY, and more recently involves leadership of the team behind GE's unique Starcoat® fluorescent coating.
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