Controlling Color Temperature: Light Sources and Filters |
The development of fast film emulsions, highly sensitive video cameras, and a wide array of improved lamps have freed the cinematographer from shooting exclusively by daylight. Today there are a wide range of options when it comes to selecting light sources for film and video production. The diversity of lamps and stock, however, bring up the problems of balancing and filtering sources of different color temperature.
The eye is remarkably adaptive to color and perceives a variety of light sources as white light sources. The light from any given source, however, is dominated by certain wavelengths that determine its overall hue (or color) temperature. Film is designed to react correctly only to light sources with specific combinations of wavelengths. Panchromatic (black-and-white) film is overly sensitive to the blue portion of the spectrum, as illustrated in Figure 5.1. Hence, when outdoor scenes are photographed with panchromatic film, skies appear white and flesh-tone values appear dark, unless a filter is used in front of the camera lens. A filter is a colored sheet of glass or clear plastic that transmits its observable hue and absorbs all others. Yellow and red filters, which block blue hues, are the most popular with black-and-white photography.
The problem of color balance and temperature is more critical with color films. The same film that gives faithful results under incandescent light renders a scene blue under daylight. Conversely, a film that reproduces accurate color in sunlight records scenes in reddish orange tones under incandescent light. Thus, the cinematographer must always take into account two factors—the color temperature of the light source and the color balance of the film emulsion.
In contrast, the electronic circuitry of the video camera provides the videographer with the option of shooting color-corrected scenes under lighting conditions of differing color temperature, provided the camera is white-balanced (set up for the balance of ambient light) beforehand. The camera will correctly balance for any continuous spectrum source, thus eliminating filtering the camera lens.
FIGURE 5.1 Normal photographic section of the electromagnetic spectrum showing the blue bias of panchromatic film.
Color temperature is measured according to a scale devised by Lord William Thompson Kelvin in the late nineteenth century. This scale measures the color temperature of a light source by comparing its light to that of a perfect black body (a theoretical carbon block that, at room temperature, reflects zero light). The black body is heated until it glows, much like a piece of iron in a fire. At low temperatures, the black body gives off a red glow; at higher temperatures, it gives off orange, yellow, and finally a blue hue. The temperature of the black body is taken when its color matches the light source. This temperature reading is taken in Celsius and then converted to Kelvin by adding 273° to the original measurement (0°C equals the freezing point of water, while 0°K equals –273°C—the point at which all molecular activity stops). The Kelvin system is accurate for measuring the color temperature of a source that generates its light through incandescence (glowing with heat). Sources that create light through fluorescence and discharge arc cannot be accurately measured in degrees Kelvin, because they do not emit all hues of the visible spectrum. The term correlated color temperature is used to describe the spectral energy distribution of these nonincandescent sources.
Since film does not adapt to light variations as does the human eye, film manufacturers balance their emulsions for one of two general color temperatures—3200°K for interiors (tungsten) and 5500°K for exteriors (photographic daylight). For the sake of uniformity, most filmmakers use 3200°K-balanced film and filter the camera outdoors with a #85B orange filter, thereby matching the 5500°K daylight to the 3200°K emulsion. Because still photographers use tungsten sources less frequently, film for still photography is usually balanced for daylight.
Though film emulsions are balanced for either 3200°K or 5500°K only, the actual color temperature of different sources may vary widely. The deep blue sky may reflect light measuring as high as 30,000°K, compared to candlelight, which may measure a very warm 1900°K (see Figure 5.2).
Daylight may vary in color temperature, depending on the time of day, the position of the sun, the presence or absence of clouds or smog, and other factors. Pure sunlight reaches the earth’s atmosphere at about 6000°K–7000°K. As the light penetrates the air, the short wavelengths (blue and ultraviolet) are scattered. The filtered sunlight arrives at about 5400°K, while the atmosphere diffuses the blue wavelengths and radiates them back to earth as skylight, measuring between 10,000°K–25,000°K. The resulting overall color temperature depends on the ratio of sunlight to skylight. On a clear day at noon, skylight may comprise from 10%–20% of the total illumination, for a combined color temperature of 6100°K–6500°K.
As the sun approaches the horizon, the light slowly changes color from white to yellow to red, as the rays pass through thickening layers of atmosphere (see Figure 5.3). Dust and other particles in the air scatter much of the light, leaving only the long wavelengths (red) to penetrate to the earth’s surface. At sunset, the color temperature of daylight may be somewhere around 2000°K, while twilight may be bluer than noontime daylight. Anytime a cloud passes over the sun, light will emanate from the blue sky, resulting in a color temperature that may exceed 15,000°K.
FIGURE 5.2 Variation of color temperature in common light sources, in degrees Kelvin.
FIGURE 5.3 As the sun approaches the horizon, its light must penetrate increasing layers of atmosphere, which scatter the shortwave violet and blue rays, and give the sunset its familiar reddish cast.
FIGURE 5.4 Standard incandescent lamp.
The primary artificial light source used in film and video production is the incandescent lamp, which comes in two forms—standard tungsten (incandescent) and tungsten-halogen.
Thomas Edison did not invent the incandescent light bulb, but he improved the design significantly so as to make it a practical lighting instrument for the home. Edison’s lamp used a carbon filament that glowed too dimly to be of use to the infant motion picture industry in the late nineteenth century. The tungsten filament was perfected in 1909 and is far superior to the carbon filament, as it creates a much brighter light.
The standard incandescent lamp has evolved very little since 1909 and consists of a coiled tungsten filament within a thin glass bulb or envelope (see Figure 5.4). The filament resists the flow of electricity that passes through it, which causes the filament to heat up and glow. Nitrogen gas within the envelope prevents the filament from oxidizing. Electrical current is conducted through the base, which fits within the socket of a lighting fixture.
Household light bulbs are the most common example of the standard incandescent lamp. They are not very efficient sources in terms of the power they consume (measured in watts), compared to the actual amount of light they generate (measured in lumens). The standard measure of electric light efficiency is measured in lumens per watt. The average standard incandescent lamp’s low wattage (10–250W), low efficiency (14–18 lumens per watt), and low color temperature (2900°K, when new) make it unsatisfactory for film and video work. The only standard incandescent lamp used professionally is the photoflood lamp, which is used primarily by portrait photographers. The photoflood, with a color temperature of 3400°K, is designed to be used with panchromatic and type A (balanced for 3400°K) color emulsions. The useful life span of the photoflood is limited, however, because the color temperature begins to drop after only a few hours of use.
Although standard incandescent bulbs may last from 750–1000 hours, color temperature drops as the lamp ages, due to a phenomenon known as boil-off. The lamp’s filament glows at very high temperatures, causing tungsten molecules to boil off. The particles settle and cling to the inside of the envelope, creating a black deposit (see Figure 5.5). While this occurs, the filament thins, passes less current, and glows with diminishing intensity. The combination of these factors results in diminishing color temperature—the light gradually becomes weaker and more reddish with age. These shortcomings have been overcome with the advent of the tungsten-halogen lamp.
FIGURE 5.5 Boil-off in a standard incandescent lamp.
In 1880, Edison suggested that if a halogen gas, such as iodine, was pumped into an incandescent lamp bulb, it would allow the tungsten element to glow at a temperature of at least 3000°C, thereby redepositing the boiled off particles back onto the filament. In order to facilitate the regenerative process, the envelope had to be extremely compact. Unfortunately, no silica glass could withstand such intense heat. In the 1950s, the General Electric Company developed a quartz glass that could withstand extreme temperatures; this advent made possible the tungsten-halogen (or quartz) lamp (see Figure 5.6).
Tungsten-halogen lamps have almost completely superseded the standard tungsten lamp in the film and television industries. Because the regenerative halogen gas is contained within the quartz envelope and redeposits tungsten particles back on the filament, the lamps do not blacken and change color with age. Tungsten-halogen lamps are designed to emit light of a great intensity—about 20 lumens per watt. Average bulb life is 4000 hours. Tungsten-halogen lamps used in the motion picture and television industries are generally designed to burn at a constant 3200°K.
Tungsten-halogen lamps have one distinct disadvantage—the intense light they give off is accompanied by extreme heat. Therefore, tungsten-halogen lamps must be kept a safe distance from drapes, painted walls, wood, and sprinkler systems.
Do not touch the quartz bulb of a tungsten-halogen lamp with your bare hands, even when it is cold. The oils deposited on the glass will create a hot spot on the bulb, causing it to crack and possibly explode. Handle bulbs with gloves or a towel. Vibration is the main enemy of tungsten filaments. Do not jiggle or shake them, particularly when they are hot.
The fluorescent lamp, a problematic source for the cinematographer, is discussed in this text because its very ubiquity has made it an unavoidable light source when shooting on location. Introduced by the General Electric Company in 1938, the fluorescent lamp has become the predominant lighting fixture in nearly all office buildings, schools, factories, and stores. The highly efficient fluorescent lamp can emit 40–80 lumens per watt over an average life span of 10,000 hours. As you will see, it is not a favorite light source of photographers or filmmakers; its peculiarities rate a discussion.
FIGURE 5.6 A tungsten-halogen lamp.
FIGURE 5.7 A standard fluorescent lamp. Electrodes at either end of the tube heat up as current passes through, emitting free electrons. The electrons strike atoms of mercury vapor in the tube, causing the atoms to give off ultraviolet (UV) radiation. The UV rays, in turn, strike a phosphorous coating on the inside of the tube, thereby stimulating the phosphors to emit visible light. This conversion of one kind of light into another is called fluorescence.
How the Fluorescent Lamp Works
A typical fluorescent lamp consists of a long glass tube that is sealed and has an electrical connection and filament at both ends (see Figure 5.7). The tube is coated on the inside with a mixture of fluorescent powders (called phosphors) and contains argon gas and a small amount of mercury. Electrical current passes through the filament and causes it to incandesce. The filament, called an electrode, emits electrons that shoot down the tube and collide with mercury atoms, which vaporize and fill the tube with mercury gas. The electrons bombard the mercury atoms, which give off ultraviolet radiation. This radiation stimulates the phosphors to give off visible light in certain wavelengths.
The fluorescent lamp requires alternating current to operate; thus the incandescent process reverses twice every cycle of AC current (or 120 times per second). This fluctuation is responsible for the phenomenon known as strobing or flicker and it may cause a pronounced effect on motion picture film. A camera with a shutter angle of less than 180° is more susceptible to strobing than one with a wider angle. Thus, flicker is likely to occur in film that is shot with a spring-wound or variable-speed camera, such as a Bolex H16, rather than with a synchronous sound camera, such as an Arriflex SR. Flicker does not pose a problem in video as it does in film, because video equipment and fluorescent lamps are pegged to the same 60-Hz AC standard.
Incandescent lamps emit all the colors of visible light; they have a continuous spectrum, which may be represented on a graph as an unbroken line. These spectral energy distribution (SED) graphs are used by the lighting industry to describe the different color temperatures of various light sources.
Standard fluorescent lamps emit high radiation in one or more wavelengths, but they do not contain all the hues found in the spectrum. The dominant wavelengths will register as spikes on the graph (see Figure 5.8). These dominant wavelengths translate on color film as an unmistakable, overall greenish blue light, which is typical of fluorescent lighting.
In general, fluorescent light must be filtered in some way if one intends to mix it with tungsten lamps or daylight. The eye compensates for this lack of spectral continuity—film does not. To compound the problem, there are at least six types of fluorescent lamps, each of which emit different portions of the visible spectrum.
The brightest and hardest artificial light source available is the carbon arc, which was invented by Sir Humphrey Davy in 1801. Originally used for lighting city squares, fairs, and outdoor exhibitions, the carbon arc lamp enabled early film crews to move their cameras into the studio. These lamps are frequently used as the searchlights that sweep the skies at grand openings and public events.
The carbon arc lamp produces its light when high-amperage direct current is applied to one or two carbon rods or electrodes within the lamp housing. A stream of electrons in the current forms a brilliant arc as it jumps a narrow gap between the two carbon rods. As the lamp burns, the feed rod oxidizes and must be continually adjusted (or trimmed).
The carbon arc lamp most frequently used for film and television work is the 225-amp brute. There is no lamp that can better simulate the intense light of the sun or the hard light of the moon. The correlated color temperature of a white flame, carbon arc lamp is 5800°K, which closely matches photographic daylight.
For all its advantages, the carbon arc lamp has some distinct limitations. A bulky and heavy fixture, the carbon arc lamp requires a DC generator for power. The typical carbon arc lamp requires a voltage of 72 volts, which is supplied by a resistive grid (or ballast) that converts the normal 120 volts DC provided by most generators, and requires a technician to continually trim and replace carbons every 40 minutes, thus making it an expensive lamp to operate.
The most revolutionary artificial light source to be developed in recent years uses an enclosed arc lamp, which incorporates a medium-length mercury arc that is augmented with metal halides to alter the color of the emitted light. Enclosed arc lamps operate on alternating current only and require a high-voltage starter and a ballast to limit the current. Like fluorescent lamps, they exhibit a flickering that is noticeable on film if the illumination fades or decays too rapidly with each change in AC polarity. Enclosed arc lamps include the HMI, the compact iodide daylight (CID), the compact source iodide (CSI), and the industrial mercury and sodium discharge lamps.
FIGURE 5.8 Spectral energy distribution of tungsten lamps, daylight, and fluorescent lamps.
The HMI lamp is the most widely used, enclosed arc lamp for motion picture and television lighting. Instead of a filament, the HMI lamp incorporates a sealed arc within a bulb filled with mercury vapor and metal iodides (see Figure 5.9). The great advantage of the HMI lamp is its tremendous efficiency, which is usually expressed in lumens per watt of light. An Osram HMI 2500-watt bulb can produce 240,000 lumens of light, compared to a standard 2000-watt quartz bulb output of 50,000 lumens. This means that the HMI lamp can provide roughly four times as much light for the same amount of power required. Furthermore, the HMI lamp produces roughly half the heat generated by a similar tungsten lamp. Unlike the 3200°K tungsten lamp, the HMI lamp produces highly consistent, 5500°K illumination.
Because of their high efficiency and daylight balance, HMI lamps are used frequently for exterior applications, largely supplanting the power-devouring carbon arcs. HMIs are particularly useful for filling in shadows outdoors and for supplementing daylight in interiors, and are a much better alternative than filtering tungsten lamps.
FIGURE 5.9 An HMI lamp. Light is created when current arcs between the two electrodes.
CID lamps also provide 5500°K illumination. CSI lamps provide light at 4200°K, which is readily filtered to either tungsten or daylight. CID and CSI lamps, though popular in Europe, are not used widely in the United States.
Metal iodide sources have some notable disadvantages. The HMI lamp is a heavy piece of hardware—the head weighs 60 pounds and the ballast adds an extra 145 pounds, for a total of 205 pounds. The fixtures are also very expensive; a 2500-watt lamp may well cost more than four 2000-watt tungsten lamps. Like the fluorescent lamp, the HMI lamp pulses at a rate equal to double the AC frequency (60Hz in the United States), resulting in flicker at a rate of 120 times per second. While this is of no concern to the videographer, the cinematographer will find that if the motion picture camera shutter is not synchronized with the 60-Hertz frequency, a noticeable strobing will result in the footage. Synchronization between shutter and lamp must be an even multiple of the 60-Hertz pulse rate. Thus, the standard 180° shutter found in most cameras will not perform satisfactorily. The camera speed must also be constant, through crystal sync or AC camera motors. These sync problems have been mitigated by means of a reduced decay flux in the newer lamps. Thus, flicker can almost be eliminated.
Recent years have seen the increased use of discharge lamps in streetlights, stadiums, shopping malls, parking lots, and industrial sites. Thus, the cinematographer is certain to encounter this type of lighting at some time when shooting on location. Discharge lamps, which exhibit high efficiency (more than 100 lumens per watt) as well as odd-colored hard illumination, include mercury vapor and sodium lamps.
The clear mercury vapor lamp produces light that lacks red, blue, and blue-green wavelengths. It is impossible to add color where none exists, therefore no amount of filtration will transmit the absent hues. The only way to compensate for this deficiency is to overpower the mercury vapor light with continuous spectrum illumination, such as tungsten lighting.
Some mercury lamps have a phosphorous coating on the inside of the envelope, much like fluorescent lights. These lamps, known as color-improved mercury vapor lamps, have a more complete spectral energy distribution and are becoming more common in interior lighting situations.
High-pressure sodium lamps are used to illuminate boulevards and parking lots. Because they emit light primarily in the yellow wavelengths, they are easily recognized by their familiar orange-yellow color. Low-pressure sodium lamps, used widely in Europe, exhibit a similar, amber-colored light. No amount of filtering can even out the spectrum of low-pressure sodium lamps.
Filters modify light by transmitting only a portion of the light they receive. They can be used to create special effects, vary the tonal renditions of a scene, or adjust the color temperature and dominant color of light. As most filters absorb some of the light passing through them, they vary in their transmittance, expressed as a percentage (in lighting gels) or as a filter factor (in camera filters). The filter factor is expressed as the number of times exposure must be increased to compensate for light absorption. A doubling of exposure equals a one-stop increase. Thus, a filter factor of three would indicate an exposure increase of 1.5 stops.
Camera lens filters are usually made of glass or acetate. Expendable light source filters, made of acetate or polyester, are called gels, as they were formerly made of fragile gelatin. It should be noted that gels used for lighting are not optically clear enough to be used on camera lenses. As a rule, the thinner the filter, the less it will affect the performance of the lens.
Filters are made by a number of manufacturers. Camera filters are available from Eastman Kodak, Tiffen, and Harrison and Harrison. Rosco Laboratories offers a wide variety of gels for lighting. Manufacturers often have different names for the same filters. For example, the MT2, the CTO, and the #85 all refer to the same color-balancing amber filter used to convert daylight to tungsten.
Ultraviolet (UV) filters are often used on camera lenses to cut haze in distant landscape shots. Because they absorb little visible light, they are often left in place at all times as a lens protector. UV filters are also used with arc, HMI, and sometimes fluorescent lamps to cut out the UV radiation emitted by those light sources.
The polarizing filter is used to reduce or eliminate reflections from glossy surfaces, such as glass and water, to increase color saturation and to darken the sky in landscape photography. Light, which normally vibrates in all directions at right angles to the direction of its propagation, is polarized when it is reflected by a glassy surface. The polarizing filter acts like a grill, blocking all but the light vibrating in a single plane.
Neutral density (ND) filters allow for the use of a wider lens aperture by cutting down the amount of light passing through the lens. This feature is particularly useful when using fast films on bright, exterior locations. ND filters do not conform to standard filter factors. They are, instead, calibrated in one-third-stop increments. An ND3, for example, cuts light by 1 stop. ND filters are often combined with other filters such as #85s.
Graduated filters are composed of a neutral density (or other color, such as rose) filter that gradually modulates to clear glass or acrylic. These filters are used for dramatic sky darkening and find wide use in music videos, commercial spots, and feature films.
Low-contrast filters, diffusion filters, fog filters, and sheer fabric filters such as gauze, all soften contrast and image definition in varying degrees. They are usually used to impart an impressionistic or dreamlike quality to a scene.
In general, still photographers who work primarily with daylight and electronic flash use color film that is balanced for 5500°K. Because it is simpler to filter daylight than tungsten light, the cinematographer usually opts for a tungsten-balanced stock for maximum flexibility. The filmmaker usually wants grain, contrast, and color uniformity; this is ensured by purchasing film stock of the same batch.
Because the eye tends to adapt to the color temperature of a given source, it prevents an objective evaluation of a scene’s color reproduction on film. Therefore, a color temperature meter is often used to judge the true color of the light in question. When the meter is pointed directly at a light source, it tells the color temperature in degrees Kelvin along with the color and amount of filtration needed to bring the light up to the desired color temperature.
When daylight and tungsten light are used simultaneously, one source is usually filtered to match the other. Daylight is always more intense than tungsten light, so it is more efficient to filter daylight using tungsten film in outdoor situations than it is to do the opposite. Daylight is converted to 3200°K with a Wratten #85 series filter; tungsten is converted to 5500°K with a Wratten #80 series filter. A #80 filter typically transmits half the light that a #85 transmits.
Contemporary cinematographers enjoy a great deal of freedom and regularly depart from strict adherence to 3200°K color temperature norms in order to create a more evocative image. The trend in recent years has been toward a warmer, more orange light for everyday situations. To this end, lights are often covered with orange or amber gels, like the traditional #54, the CTO, and the Rosco MT series.
As noted earlier, fluorescent lamps present color-balancing problems when used in production. Unfortunately, it is often impossible to replace existing fluorescent lights on location. Therefore, the fixtures are sometimes left unfiltered for greater realism. Video cameras can be color balanced for fluorescent lamps, providing there is no tungsten light source in the scene. When filtering is called for, conversion gels and color-compensating filters may be used.
A scene lit entirely by fluorescent light can be filtered at the camera lens. In general, fluorescent light may be converted to 5500°K by using a CC30 magenta filter or converted to 3200°K with a combination of yellow and magenta filters. Another way to filter fluorescent light at the camera lens is to use a FLuorescent to Daylight emulsion (FLD) or FLuorescent to “B” type emulsions (3200°K) (FLB) filter. The FLB filter corrects warm fluorescent light, while the FLD filter corrects the daylight variety.
Usually, existing fluorescent lights must be augmented with more light. The idea is to create a balanced illumination of uniform color temperature. If windows (i.e., natural lighting) allow a large amount of daylight and the lamps are fluorescent daylight or cool white, the windows can be filtered with a green gel, such as Tough Plusgreen/Window-green (Rosco Laboratories) to balance illumination. If tungsten sources are also used, they need to be filtered with Tough Plusgreen 50 (Rosco Laboratories). If 5000°K lamps, like FAY lights, are used as well, they should be treated as daylight and filtered with Tough Plusgreen/Windowgreen (Rosco Laboratories).
Sometimes it is more convenient to filter the fluorescent lamps themselves. In this case, magenta filters such as Tough Minusgreen (Rosco Laboratories), available in sheets or tube sleeves, are used. Don’t be fooled by the resultant sickly purple illumination—it closely approximates 5500°K light. There are certain fluorescent tubes that eliminate filtering, such as the daylight-balanced Chroma 50 (General Electric) and the tungsten-matching Deluxe Warm White (General Electric).
Even though it is possible to have the lab timer correct for fluorescent light, it is best to correct as much as possible in production. The timer cannot correct different sources selectively; only the overall color can be adjusted. Too much correction in timing may boost contrast, grain, or both.
The correlated color temperatures of white and yellow flame carbon arc lamps are rated at 5800°K and 3350°K, respectively. Therefore, some filtering is necessary to match arc light with daylight and tungsten standards.
The Y-1, a pale yellow filter, is commonly used with white flame carbons to match daylight; its transmission is rated at 90%. When it is desirable to match tungsten sources, a Y-1 plus an MT-2 (or #85B) is needed to provide 3200°K illumination. An MT-Y conveniently combines the Y-1 and MT-2 into one filter. Yellow flame carbon light may be converted to 3200°K with YF-101 filters. Other filters in the MT and CTO family may be used as necessary to warm the carbon arc light in varying degrees.
HMI lamps emit very high amounts of UV radiation, which may cause severe sunburn and possible eye damage in subjects. Therefore, HMI fixtures are fitted with special UV filter lenses. Under no circumstances should an HMI lamp be altered to operate without this lens. Although HMI lamps are designed to match daylight, they may vary in their correlated color temperature according to manufacturer and age, so it is best to use a color temperature meter to determine the amount of filtration needed. The Rosco Jungle Book, a 3-inch square booklet of color-compensating and color-balancing filters available free from Rosco Laboratories, is ideal for figuring proper filtration of light sources when used with a color temperature meter. Photographers also find the Jungle Book handy for trying various filters over a camera lens when testing a given film emulsion under illumination of unknown color temperature. For a complete summary of color-balancing and color-correcting filters, see Appendix A at the back of this book.
QUESTIONS
1. Boil off occurs as ——— lamps age.
a. carbon arc
b. HMI
c. standard incandescent
2. Incandescence results when a filament ——— electricity.
a. conducts
b. resists
c. polarizes
3. Photographic daylight measures ———.
a. 3200°K
b. 5500°K
c. 2900°K
4. Tungsten sources can be cooled to 5500°K by filtering with ——— gels.
a. Minusgreen
b. Tough Blue
c. CTO
5. HMI lamps are often preferred to tungsten-halogen lamps because ———.
a. they are less expensive fixtures
b. they are lightweight and compact
c. they produce more light per watt use
6. Match the items on the left with the choices on the right.
|
——— Minusgreen (magenta) ——— #80A (blue) ——— K2 (yellow) ——— #85B (amber) ——— Plusgreen (blue-green) |
a. converts daylight/tungsten film b. converts tungsten light/daylight film c. fluorescent light/daylight film d. corrects black-and-white film blue bias e. converts tungsten light to fluorescent light |
ANSWERS
1. c. |
Although oxidation progressively shortens the life of arc lamps with use, tungsten filament boil off is a phenomenon peculiar to standard incandescent lamps. |
2. b. |
Incandescence takes place when a tungsten filament resists electron flow. |
3. b. |
Photographic daylight, a standard for balancing daylight emulsions, measures 5000°K. Tungsten-halogen lamps for film and television lighting are rated at 3200°K. Standard tungsten lamps average 2900°K. |
4. b. |
Tough Blue filters will boost the color temperature of tungsten-halogen lamps to 5500°K. For this reason they are referred to as booster blue. Minusgreen, a magenta filter, is used as a color-compensating filter to match fluorescent light with daylight film. CTO is used to warm 5500°K daylight for balance with tungsten lamps. |
5. c. |
While HMI lamps are much more expensive than tungsten-halogen lamps, and the ballast they require to operate is heavy and cumbersome to transport, they are, however, much more efficient in their lumens-per-watt output. HMI lamps average three to four times as much luminous intensity as a quartz lamp of similar wattage and they are cooler burning, too. |
6. c.,b.,d.,a.,e. |
Minusgreen (magenta) is used for fluorescent light/daylight film combinations. #80A (blue) is used to convert tungsten light (3200°K) to match photographic daylight (5500°K). K2 (yellow) is used to correct the blue bias of panchromatic film. #85B (amber) is commonly used to convert daylight to tungsten film. Plusgreen (blue-green) converts tungsten-halogen light to fluorescent light. |
PROJECT 5.1: IDENTIFYING ARTIFICIAL SOURCES
Purpose:
To gain familiarity with the design, function, and features of the various artificial sources available.
Materials Needed:
one new, standard incandescent light bulb
one old, standard incandescent light bulb
a tungsten-halogen lamp
a fluorescent lamp
a discharge-type light
Procedure:
Without looking directly at the lamps, observe each of them in operation, noting the differences in heat output, color temperature, and noise generated. Note the differences among the various lamp designs.
1. Compare the old, standard incandescent light bulb that has been in use with the new bulb.
2. Examine the tungsten-halogen lamp; be careful not to touch it with your bare hands. Note the compact envelope and the heavy, coiled filament.
3. Observe the fluorescent lamp while turning it on and off. Note how the lamp does not immediately come to full intensity like the incandescent lamps. Can you detect the strobing? Does the lamp emit any noise when it is on? What kind of noise is it? What causes it?
4. Look at the light given off by the discharge-type lamp. What wavelengths of light or hues are predominant? (Look at something of a known hue, such as a flag, to best gauge the cast.) Based on the perceived cast of the light, what type of lamp do you suppose it is? Mercury vapor? High- or low-pressure sodium?
PROJECT 5.2: USING COLOR-BALANCING AND COLOR-COMPENSATING FILTERS
Purpose:
To identify and correct the color temperatures of various light sources.
Materials Needed:
35 mm (SLR) camera with a through-the-lens (TTL) metering system
tripod
one roll Ektachrome 160 (tungsten-balanced)
Rosco Laboratories Jungle Book (available free from Rosco Laboratories, Inc.; 36 Bush Avenue; Port Chester, NY; 10573 or 1135 North Highland Avenue; Hollywood, CA; 90038)
camera log (see Figure 5.10)
Procedure:
Shoot a series of photographs, each framing a person in medium shot (including head and shoulders), as follows:
1. Take the first exposure in each group without a filter; then choose a different filter for each subsequent picture, holding the gels over the lens (either by hand or by holding it in place with tape). Use the most obvious filter choices in order to correct the color balance. For example, daylight must be filtered to match tungsten film. Thus, a warming, amber-colored filter in the CTO series would be the logical choice in this instance. Try for a perfectly corrected and properly exposed scene in each group. Don’t mix light sources of different color temperature in the same shot. For each exposure note the f-stop, shutter speed, lighting conditions, type of filter (if any) used, and a short description of the subject in the camera log. Circle the exposure you think uses the proper filter for the ambient light of the scene.
FIGURE 5.10 Camera log.
2. For Group A, exposures 1–5, photograph outdoors in daylight.
3. For Group B, exposures 6–10, photograph indoors under standard incandescent household bulbs.
4. For Group C, exposures 11–15, photograph indoors under fluorescent light.
5. For Group D, exposures 16–24, photograph a night scene outdoors under sodium or mercury vapor light (frequently found in parking lots of business and industrial locations). You need a tripod and you will need to shoot a variety of exposures to ensure at least one picture is shot with the proper exposure (e.g., f-2, f-2–2.8, f-2.8, etc.). At least one exposure in each set should be of a fully color-corrected scene.
Evaluating the Results:
View the processed slides with a projector if possible; compare the images within each lighting group and with the data you recorded in the camera log.
For Group A, the color temperature should be corrected if you used filters in the CTO or #85 (amber) series. Note the very subtle differences between each exposure; often, the correct filter will be a half or quarter CTO for the optimal desired effect.
For Group B, it is not usually necessary to correct tungsten light for tungsten film under normal circumstances. Standard incandescent lamps often need to be filtered in order to boost their color temperature to 3200°K. In this case, a third or quarter blue will boost color temperature 600° or 300°, respectively, and will provide the most accurate correction.
For Group C, color-balancing filters are used for boosting and decreasing color temperature in degrees Kelvin. In order to correct the light emitted by fluorescent and other discharge lamps, however, it is necessary to use color-compensating (CC) filters. The most useful CC filters for fluorescent correction are the magenta and green filters. Magenta is used to make fluorescent light compatible with 3200°K film, while green filters are used to match daylight sources, such as windows, to fluorescent lights. Therefore, filters of the Minusgreen or magenta series are the most logical choice.
For Group D, industrial discharge sources present the biggest challenge to the photographer. Sodium lamps are often difficult to correct, while mercury vapor lamps are virtually impossible to fully correct. Often, the best thing to do when faced with industrial lighting is to try to determine its overall cast, be it green, amber, or magenta, and then filter it with the closest complementary filter.
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