Other Free Encyclopedias » Online Encyclopedia » Encyclopedia - Featured Articles » Contributed Topics from P-T » Twentieth Century Photographic Lighting - Introduction, Chemical Flash Lighting, Open Systems, Enclosed Flashbulbs, Duration and Synchronization, Modern Flashbulbs, Electronic Flash Lighting

Continuous Electric Lighting Systems - Introduction, Arc Lighting, Early Filament Lamps, Improved Filament Lamps, Tungsten-Halogen Lamps, Early Discharge Sources

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Introduction

The advent of continuous electrical lighting brought with it the need for a clarification in terminology. This has not been entirely successful throughout history, as the careless modern use of the term “lightbulb” illustrates, and has also been confused by the adoption of different nomenclature in England and in America. Therefore it is worth noting that a device that produces light is often referred to as a “fixture” in the United States and as a “lantern” in Great Britain (where the term can be traced back to such machines as magic lantern film projectors). A largely successful attempt has been made to unify the terminology by adoption of the word “luminaire,” which comes from the French for light source. Since this term is used only in professional circles, this essay will generally refer to light fittings simply as lights and to the sources themselves as “lamps.”

The story of continuous electric lighting starts in 1879 when Joseph Swan, in England, and Thomas Edison, in America, both demonstrated carbonized filament lamps. It appears that Swan had the idea first, having demonstrated an open-air filament that lasted only a few seconds in 1840, but Edison was the more astute businessman who obtained not only American but also British patents for his design. After much legal wrangling the two men subsequently joined forces to manufacture carbon-filament lamps from 1883 until 1906. It was true that carbon-filament lamps possessed only a low color temperature and therefore, like the chemical lighting systems such as kerosene and gas lamps that preceded them, had a relatively low actinic effect. Swan’s and Edison’s inventions opened the floodgates for the development of new ways in which electricity could be turned into light for the benefit of photographers.

Unfortunately, the invention of the carbonized filament lamp came forty years after the invention of photography itself, and as a result the most important early photographic electric lighting system used a completely different principle.

Arc Lighting

It is important to realize that Swan and Edison were primarily concerned with distributing electric lighting to the masses: other workers had already considered ways in which the discovery of electrical energy could be used to provide light in more specialized applications. Once again the genius of Sir Humphrey Davy played an important part. It was Davy who first observed the electric arc phenomenon, which can be thought of as a “continuous spark.” Davy did not construct his device as a means of providing lighting but rather to investigate the phenomenon in its own right.

The essential ingredients of an electric arc lamp are two carbon rods and a DC electricity supply similar to what might be provided by a chemical battery. Such lights are rated not by the amount of power they draw (as is more normal) but by the current they require when running: typical figures range from 40 to more than 200 Å. When the two pointed carbon rods are brought together these very high currents flow through them, and when the rods are pulled slightly apart the current continues to flow causing a bright light to be emitted. The tips of the rods are white hot and erode, leading to a pit developing in the positive rod that eventually extends the gap to such a degree that a current can no longer flow and the light ceases. It was therefore necessary to design a mechanism that would maintain the appropriate distance between the two rods over a prolonged period of time.

The negative rod is also eroded but this process is slower and does not degrade the rod’s pointed profile. Around 95 percent of an arc lamp’s light is generated from the incandescent crater in the positive rod with only a minority of the light coming from the arc that joins to the negative rod.

In 1836 Englishman William Staite developed a way to regulate the movement of the carbon rods using a clockwork mechanism, and in 1876 Russian engineer Paul Jablochkoff (Yablockhkov) devised an even more ingenious design featuring parallel vertical rods that required no adjustment. The Jablochkoff Candle, as it became known, was brighter than a gas lamp and also simpler and cheaper than any arc lamp. It was used to light streets and dock areas in Paris and London but was eventually superseded by lamps that had reliable rod-moving mechanisms, which overcame the need for the Jablochkoff Candle to have its entire rod assembly replaced each time it was reused. The final two improvements to the arc lamp were the addition of color-producing combustible salts to the carbon rods and the introduction of enclosures around the rods, which reduced erosion considerably.

The use of “cored carbons” stabilized the arc and transferred the light generation from the positive rod to the arc itself (described as “flaming” in such lights). The colored effect of the salts was exploited to correct the cyan tinge of an ordinary carbon arc lamp principally through the deployment of rare earth fluorides that gave spectral lines so closely packed they seemed to be virtually continuous. The effective color temperature of a high-intensity arc lamp was around 5000 K compared with the very discontinuous spectrum of an ordinary carbon arc lamp, which approximated to 3700 K.

From a photographic perspective arc lamps were important not only because of their brightness but also because of their actinic effect and point-source geometry. In certain applications, including motion picture and theater use, the attendant disadvantages of noise, smell, and high UV radiation were acceptable drawbacks. The point-source geometry of arc lamps was particularly useful because it allowed lights to be fitted with reflectors and lenses that could project near-parallel beams over a considerable distance. This advantage was also put to use in military searchlights by Colonel R. E. B. Crompton, who was one of the foremost advocates of electric lighting in Britain during the late 19th century.

Creatively, however, the point-source geometry was at a disadvantage since the light produced by arc lamps is hard-edged and unflattering. This fact led the leading mid-20th century American portrait photographer William Mortensen to comment: “the point source is of no use by itself.” Nevertheless, it is a general principle that hard lighting can be softened more easily than soft light can be made harder, and that a harder light source is better for revealing surface texture—which is useful in some situations but is clearly not the aim when trying to record portraits with a smooth skin tone.

Early Filament Lamps

The advent of the filament lamp did not depose arc lamps, instead it extended the use of electric lighting in general thanks to lower costs and less hazardous operation. That said, cost has to be put in the context of salaries at that time. Brian Fitt and Joe Thornley, in their 1995 book Lighting By Design: A Technical Guide , estimate that in the last two decades of the 19th century the price of one domestic lightbulb was equal to 16 percent of an engineer’s weekly wage. In addition, carbonized filaments could not be made hot enough to generate actinic light so their use in photography, especially considering the low emulsion sensitivities at that time, remained impractical.

In 1902 the first lightbulbs using osmium filaments were produced commercially and in 1906 the tungsten-filament lamp was launched. These filaments were more versatile than their carbon ancestors in that they could be looped and coiled and operated at much higher temperatures to produce a whiter (though still distinctly yellow) color of light. Tungsten, which has the higher melting point of the two metals, melts at about 3410°C and can be heated to almost this temperature inside a lightbulb to generate light over a considerable period of time. Ultimately, the evaporating tungsten that turns from solid directly into vapor darkens the glass bulb and reduces the diameter of the filament, which causes the filament to break wherever it becomes thinnest. This problem was eventually solved by the advent of tungsten-halogen lamps but was previously aided by changing from evacuated glass bulbs to ones that were back-filled with a mixture of nitrogen and inert gases to reduce tungsten evaporation—an idea that was introduced in 1913 by New Yorker Irving Langmuir (who also went on to design a mercury-vapor condensation pump that made possible nearly perfect vacuums).

The fact that tungsten filament lamps contain a considerable length of wire—there is more than a meter of wire in a modern 100W domestic lightbulb—made possible different geometries to produce different qualities of light. The larger light-emitting area of a filament lamp contrasted strongly with the point-source nature of arc lamps and brought with it the ability to create a naturally softer lighting quality. As a result many of the photographic lights found in studios at this time used very large lamps contained within proportionately large housings that were intended either to scatter the light generally or to produce a more directional effect with soft-edged shadows.

Although the size of a lamp was directly linked to its wattage, more light (and a greater actinic effect) could be obtained from any given lamp by over-running it, albeit with the disadvantage of a considerably shortened lifetime. Once again this was a drawback that photographers could tolerate even though it would have been unacceptable in lamps that were intended for general use. At a time when ordinary domestic lightbulbs were designed to last at least 1000 hours with a typical color temperature of 2850K, Type B photographic lamps used a slight over-voltage to give a 3200K color temperature but with only a 50- to 100-hour lifetime. Type S photographic lamps employed an even higher over-voltage to give a color temperature of 3400K, but a lifetime that was just 1/10 that of a Type B lamp.

As a general rule the power rating of a photographic lamp was indicated by its fitting. Low-power lamps up to 250W had bayonet caps, high-power lamps from 1000W upwards had bi-post fittings, and intermediate lamps had screw caps.

Improved Filament Lamps

To provide a greater projected brightness without paying a penalty in terms of reduced lifetime some photographic lamps were manufactured with integral reflectors. This was achieved by applying an aluminum coating to the inside of the back of the glass bulb (a process sometimes referred to as “silvering”) and was often accompanied by external frosting of the front of the bulb. The overall effect was an increased forward illumination without a hard, specular effect. Banks of lamps could be assembled to create a bright yet soft lighting effect—something that would have been impossible if each lamp had to be mounted in its own external dish reflector. This same idea was developed in the later 1960s by Colortran in the guise of its Cine-Queen, which became the ubiquitous parabolic aluminum reflector lamp (PAR-64) that was used in applications as diverse as stage lighting and car headlights. Banks of six or nine PAR-64 lamps, which had the highest lumens-per-watt rating of any 3200K lamp then developed, were used to replace the Fresnel-lensed 225A Brute arc lamps that had hitherto been regarded so warmly in the film industry. The later alternatives were more compact and offered a range of different coverage angles thanks to a variety of different sealed-beam designs. PAR lights are not used extensively in stills photographic studios but may be employed when it is necessary to light a large area.

A more recent development, Derek Lightbody’s Optex AuraSoft, is now widely employed in the film industry and also finds similar use for area-lighting in stills photography. The AuraSoft, and its AuraFlash cousin, relies on a large (60, 70, or 80cm) specially designed dish reflector that is covered in spheroidal convex mirrors and is fitted with a semi-opaque front baffle. Behind the baffle can be placed a variety of light sources; tungsten-halogen (with switchable output levels), MSR (conventional or high-frequency), and electronic flash. The light is favored for use outdoors thanks to its ability to simulate natural sunlight, but it also provides very flattering flash illumination for studio-based fashion and portrait stills photography. AuraSoft is unique among light fittings because it received accolades from both the Academy of Motion Picture Arts and Sciences and the Academy of Television Arts and Sciences.

Returning briefly to the pre-PAR era another problem, caused by the expanding use of color film, was the change in color temperature that tungsten lamps suffered as they aged. According to the 1952 book Artificial Light and Photography by G. D. Rieck and L. H. Verbeek, quoting the authors’ own work and that undertaken by others, the color temperature of a 500W Photolita lamp dropped by approximately 150W over its (maximum) six-hour lifetime. This is a significant shift in a very short space of time that could result in unpleasant color differences if new lamps were mixed with old in the same lighting arrangement. It might also produce inconsistent results if the same sitter or object were photographed at different times or even during one extended session.

Tungsten-Halogen Lamps

The solution to this problem came in the form of tungsten-halogen lamps. These employed a halogen gas in the bulb’s atmosphere rather than opting for an inert environment. Improved lamp life and color stability results because the halogen is able to combine with any evaporated tungsten vapor. This prevents both thinning of the filament and darkening of the glass bulb by retaining the tungsten in a gaseous state provided that the bulb’s surface temperature is kept above 250°C. When tungsten-halide gas molecules approach the hot filament, by random movement inside the bulb, they decompose as soon as they encounter a temperature above 1400°C. This deposits metallic tungsten on the filament, thereby maintaining a more uniform wire thickness than would otherwise be possible. The liberated halogen is then free to combine with other vaporized tungsten atoms and continue the tungsten-halogen cycle. Sadly for users, but fortunately for lamp manufacturers, the process is not perfect because the hottest parts of the filament, where there is greatest metal vaporization, are at about 3000°C and therefore not prime sites for tungsten deposition. Even so, failures are most often due to mechanical damage caused by movement or rapid cooling of the lamp when it is switched off not to evaporation of the filament.

The first tungsten-halogen lamps were developed in the early 1960s using a quartz envelope (no longer globular in shape and therefore not correctly referred to as a bulb) that contained iodine vapor in its atmosphere. Quartz was needed because the tungsten-halogen cycle requires the glass envelope to be kept very hot to prevent the tungsten halide compound from solidifying on its inside surface, and the temperatures required are in excess of those that can be tolerated by ordinary glass. Because of their distinctive components the new lamps were referred to as “quartz-iodide.” Although these lamps were a significant improvement over the plain tungsten bulbs that they replaced, they suffered from a slight pink tinge that is characteristic of iodine vapor. In addition, quartz is easily attacked by mild alkalis, leading to premature failure of the envelope itself. Consequently, bromine was later used in place of iodine and synthetic “hard glass” (borosilicate glass, i.e., heat-resistant Pyrex) replaced pure quartz.

With carbon-arc lights it was felt that the whitest light was obtained by the addition of fluoride salts, therefore fluorine would be the best halogen to use in a tungsten-halogen lamp. This would indeed be the case were it not for the fact that fluorine is a very aggressive gas that attacks glass (hydrofluoric acid is used for etching glass). Another less reactive halogen, bromine, is preferred because it has a pale yellow color consistent with the color temperature of incandescent lighting and therefore can be regarded as colorless in this context.

A further complication introduced by the use of tungsten-halogen lamps was UV radiation, which is absorbed by conventional glass but transmitted by pure quartz. Focusing lights, which employ glass lenses, do not suffer from this problem. But in close-up portraiture it is necessary to avoid sunburn by fitting a toughened glass safety screen that protects against both lamp failures and unwanted UV emissions.

Such lights are still used in photographic studios today, often in the guise of 800W “redheads” and 2000W “blondes.” These nicknames were coined by the Italian manufacturer Ianiro because of the casing colors used for its lights. In general these terms are used even when referring to the same wattage open-face lights produced by manufacturers such as Arri, all of whose lights are cased in blue.

Recent developments have focused on compact low-voltage lamps and high-quality optical designs to improve efficiency. One of the most advanced ranges is that designed by Dedo Weigert and sold under the Dedolight name. These lights are renowned for their high-light, low-heat output together with ultra-compact size and precision engineering quality. They are low-voltage systems that run at either 12 or 24V and employ a patented focusing mechanism with dual-lens aspherical optics to ensure perfectly uniform illumination with no stray light, coupled with a 20:1 flood-to-spot ratio that is beyond the capability of any Fresnel lens system. Weigert was given a Technical Achievement Award by the Academy of Motion Picture Arts and Sciences in 1990 for his work.

Early Discharge Sources

The first attempts to pass electricity through a gas, with no solid conductor to carry the current, hark back to the experiments of Englishman Michael Faraday, who developed a device comprising two metal rods inside a glass container, called the “Electric Egg,” which could be evacuated and backfilled with different gases. The results of these early investigations into discharge phenomena were reported in Faraday’s 1839 publication Experimental Researches in Electricity . Discharge phenomena also captured the interest of Dutchman Heinrich Geissler, who worked with his brother Friedrich to seal platinum wires through glass-walled containers so that very low pressures could be maintained inside. Other experimenters found that mercury vapor could be made to emit a dull glow, but it fell to Geissler to produce a sustained light from a discharge tube. Heinrich Geissler used his own low-pressure mercury pump to create the rarefied atmosphere needed for electrical conduction through gases, and he is now considered to be the father of gas discharge lighting. As reviewed in the section on Silver Halide Materials, the low sensitivity of then-available emulsions meant that Geissler’s lighting technology was not immediately considered for photography, but rather was seized upon as both a scientific achievement and an attractive curiosity (because of the various colors that could be created).

French physicist Georges Claude experimented with Geissler tubes while developing a commercial method for liquefying air. The two aspects came together when Claude was able to extract xenon and neon from the air, in spite of their exceptionally low concentrations, and thereby obtained the gases that would be most important for electronic flash lighting (see below) and the everyday discharge lighting that was unveiled in Paris in 1910 and is known today as neon tube lighting. Neon lighting was both too dim and too ineffective (because of its red color) to be of any use in photography. Carbon dioxide tubes, which were used by Daniel McFarlan Moore in 1898 to light up a chapel in Madison Square Garden in New York, emitted a whiter light but remained too dim to be of much use for photographic lighting. The same cold-cathode technology was, however, subsequently developed as a light source for black and white darkroom enlargers until the demise of optical photographic printing at the end of the 20th century.

In all cases discharge lighting works by exciting the electrons in an atom from one state to another using the electricity supply provided. It then allows those same electrons to return to their former stable state accompanied by the emission of photons of light that may be inside or beyond the visible range of the spectrum. The changes in electron energy state are very precisely defined (quantized) and as such result in light of a very definite color (frequency) being produced. The spectral characteristics of a discharge lamp are therefore very different from those of daylight and incandescent lamps, both of which comprise a wide variety of colors emitted at the same time.

Controlling Color

The overall color of an incandescent lamp is directly related to its temperature, which in turn depends on its resistance and the applied voltage, and is described as a color temperature that is measured in degrees Kelvin. Although zero degrees Celsius is approximately 273K, the two scales have the same incremental value so that, for example, 100°C is equal to 373K and so on. If an incandescent lamp is too red in color (has too low a color temperature) then its light can be made more yellow, and ultimately more white, by increasing the applied voltage and accepting a shorter lifetime for the lamp (at a higher color temperature). The same is not true of discharge lighting, where the color of the light produced depends on the energy-level changes that the electrons undergo. The result is a fixed, discontinuous spectrum of light that is dominated by certain colors and bears little similarity to the appearance of natural daylight.

The next leap forward addressed this problem by using the phenomena of fluorescence and phosphorescence, whereby some substances are seen to glow (emit visible light) when irradiated with UV light. Geissler had previously placed a droplet of mercury in his discharge tube and then evacuated the atmosphere so that only low-pressure mercury vapor remained. Under these conditions the emitted light is mostly UV, which is highly actinic on photographic film but also largely invisible. In 1893 the Serbian-American physicist Nikola Tesla, whose multiphase AC electrical systems deposed the DC systems preferred by Edison to become the dominant technology for electricity distribution, applied phosphor coatings to Geissler tubes to increase the amount of visible illumination. The result was a phosphorescent effect that differs from fluorescence because it can persist after the exciting energy has ceased. Even today this can be seen in some so-called fluorescent tubes that continue to exhibit a dull glow after they have been switched off.

By choosing a coating that fluoresces or phosphoresces toward the red end of the visible spectrum it is possible to balance the visible violet glow with a UV-induced red light, thereby giving a more natural overall color. The visual impression is one of white light; this is in fact an optical illusion. Sadly, photographic emulsions are not so easily fooled and the highly discontinuous spectrum emitted by a discharge tube will often create a strong color cast in pictures that are captured on film even when the lighting looks natural to the human eye.

At the end of the 1980s, after many years in the wilderness, fluorescent lighting became fashionable with stills and motion picture photographers with the advent of Kino Flo. These lights employ either daylight or tungsten-balanced fluorescent tubes and were born from a need to provide unobtrusive lighting in one scene for the 1987 film Barfly . Frieder Hochheim, who worked as gaffer to the film’s Director of Photography, Robby Mueller, was so frustrated by the variable color of the fluorescent tubes he was forced to use that he set about obtaining lamps that could be deployed alongside other sources without introducing color casts. The result was Kino Flo, a brand name formed by truncating the German word for cinema with an abbreviation for fluorescent lamps.

To emphasize the difference between general-purpose fluorescent tubes and those intended for critical lighting applications a Color Rendering Index figure is used. Figures above 95 (on a scale that runs to 100) indicate good photographic quality and are a characteristic of Kino Flo lamps. Hochheim’s work was recognized by a Technical Achievement Award from the Academy of Motion Picture Arts and Sciences in 1995.

The 1990s saw a growth in digital capture for which low-temperature, modest-cost continuous lighting was required. Among the manufacturers that responded was the now-defunct British lightbox company Hancocks, which mounted a bank of photographic-grade fluorescent tubes in a single housing to provide the same light output produced by a 1000 W incandescent lamp while drawing just 216 W of power and providing around 6000 hours of daylight-quality illumination. A different design, which bundled six 24 W tubes in a protruding cylindrical assembly, was devised by American softbox designer Gary Regester and sold under the Scandles name. In each case the softness of the source was complemented by the tubes’ low operating temperatures and, therefore, the ability to place such lights very close to a set without risking undue heating of the sitter or any object photographed.

Banks of photographic-quality fluorescent tubes can also be employed as “active reflectors” to lighten shadows in areas that the light from other sources cannot reach. This possibility becomes especially important when each and every light source is baffled to affect a specific area, leaving no stray light to be directed into the remaining shadows. Large Kino Flo assemblies, which may be more than two meters long and typically comprise four tubes in a single panel, are ideal for this purpose.

High-Power Discharge Sources

Lighting technologies come full circle when considering high-power discharge sources, which are the most advanced of all the available photographic lighting systems. Just as tungsten lamps evolved into tungsten-halogens, so discharge and arc lamps combine to produce ultra-bright sources epitomized by Osram’s Hydrargyrum Medium-Arc Iodine (HMI) lamps, which first appeared in 1969. Other manufacturers followed suit with other acronyms, such as Philips’ Medium Source Rare-Earth (MSR) lamps, but the technologies are all similar and Osram’s HMI designation is often used generically.

These lamps were daylight-balanced and had four times the watts-to-lumens conversion efficiency of a tungsten-halogen lamp. Like fluorescent tubes they rely on the excitation of electrons in a gas or vapor environment, but in this case the distance between the electrodes is short enough for an arc to be formed. Unlike carbon-arc lights, which usually draw a direct current, HMI/MSR type lamps use an alternating current that can be either matched to the mains frequency or up-rated via a high-frequency ballast. The latter was important to avoid flicker when such lights were used with the scanning digital backs that were common in the mid-1990s, but is a less important consideration today now that digital capture is as instantaneous as film capture.

Residual drawbacks of HMI/MSR type lights include high capital and lamp-replacement costs, coupled with relatively modest lamp lifetimes (around 250 hours) and high UV emissions. This is not entirely surprising given that the spectrum produced is very similar to that of daylight, and were it not for the UV-absorbing property of the earth’s ozone layer we would all be horribly irradiated with high-energy UV light from the sun. Fortunately, a simple glass screen is all that is needed to absorb the UV radiation produced by photographic HMI/MSR lamps.

At this point the largest single lamp of this type is used in the SoftSun light manufactured by Lightning Strikes. This lamp is rated at 100 kW yet can be dimmed down to just 3 percent of its maximum output without any color shift.

Light Modifiers

Modern continuous lights have evolved to become perfect solutions for particular lighting requirements. The uses for which specific lights are best suited can be judged by the external appearance of the unit and also by the geometry of the lamp that is inside. Small lamps tend to be used in lights that are fitted with lenses and generate hard-edged shadows. Larger lamps, which may be tubular or employ baffles to mask the filament itself, are better for area lighting. In many cases the geometry of the reflector mirrors the geometry of the lamp itself: tubular lamps are generally fitted inside reflectors that have cylindrical symmetry whereas near-point-source lamps are often used in circularly symmetrical parabolic reflectors.

Lightweight lens assemblies are created using the Fresnel design, which recognizes the fact that light is refracted when it crosses the boundary between two different optical media and that the volume of glass between the front and rear surface of a lens does nothing to focus the light. Fresnel lenses, which have also been employed in lighthouses, may feature a dimpled rear surface that helps to disperse the image of the filament that might otherwise be projected onto the set being lit. Fresnels differ from conventional lenses because their front surfaces feature a series of steps that retain the curvature of the lens without its bulk. Conventional lenses are employed whenever an image is deliberately projected, using a gobo shadow mask, for example, or when ultra-bright, hard-edged pools of light are required.

Although hard lighting can be softened by placing a diffusing screen between the source and the set (the same way that clouds in the sky diffuse the hard-edged light of the sun), this can be difficult in a smaller studio since it requires additional space. It also involves a loss of both brightness and contrast owing to the fill-in effect arising from light that is reflected from the rear of the diffuser and, thereafter, the studio walls. As well as avoiding such difficulties specialist softlights also permit greater control of forward illumination. One way in which this can be done is by fitting an “egg crate” (a series of protruding baffles) on the front of a softlight to restrict its angle of coverage without affecting the hardness of the illumination.

Overall, given the fact that continuous light sources are normally designed for specific purposes at the outset, there are fewer accessories available for these units than there are for modern electronic flash heads (see below), which are frequently designed to be as versatile as possible.

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