Table 6.1 Color temperature of various sources
Source Kelvin MIREDs Match or candle flame 1900 526 Dawn or dusk 2000-2500 500 Household bulb 2800-2900 357-345 Tungsten halogen bulbs 3200 312 Photo flood bulbs 3400 294 1 hour after sunrise 3500 286 Late afternoon sunlight 4500 233 Blue glass photoflood bulb 4800 208 3200 K lamps with dichroic filter 4800-5000 208-200 FAY lamps 5000 200 Summer sunlight 5500-5700 182 White flame carbon arc light with Y-1 filter 5700 175 HMI light 5600 or 6000 179 or 167 Sunlight with blue/white sky 6500 154 Summer shade 7000 141 Overcast sky 7000 141 Color television 9300 108 Skylight 10,000-20,000 100 Energy 6000 K 4000 K Visible Spectrum Wavelength in Nanometers 3000 K 100 400 700 1000 1300 1600 FIGURE 6.2
Distribution of energy for various color temperatures across the visible spectrum. Note the distribution of energy in a tungsten source (3200 K) favors the red end of the spectrum, while in a daylight source (5600 K), the curve slopes downward at the red end and the distribution of energy is more even over the visible spectrum.
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diagram: moving from the bottom-left corner clockwise, it starts with violet and deep indigo blue (380-455nm), blue (430-455nm), then cyan, fades to green around the top of the diagram (520nm). Along the right side of the triangle green fades into yellow (500-588nm), orange (588-647nm) and finally to red at the bottom-right corner (700nm). Along the bottom of the diagram are the hues created by mixing red and blue light: lavender, purple, and violet. Theline of purples, as they are called, can be made only by combining colors; they do not exist as single wavelengths. The outside edges of the dia- gram represent the most saturated pure colors possible. As you move into the center of the diagram, the colors become less and less saturated, and the center portion represents white light—this is the portion that we are interested in for this discussion. The curved line leading from orange to blue is called the Plankian locus; the color makeup of black-body sources—light sources that have a continuous spec- trum—will be represented by a point that is somewhere on this line.
Along this locus, we find a point at 3200 K that will be represented as pure white on tungsten- balanced film or 3200 K balanced video camera. We find another point at 5600 K, which represents the color of light that will be represented as pure white when using a daylight-balanced film stock or video camera. Any incandescent light will fall somewhere on the Plankian locus. If you were to dim an incandescent light, the color temperature would decrease, and the color would move toward
0.9 520 540 560 580 600 620 700 0.8 0.7 GREEN CYAN BLUE PURPLES RED WHITES ORANGE YELLOW TC(K) 10000 ∞ 6000 40003000 2500 2000 1500 0.6 0.5 500 y 0.4 0.3 490 480 470 460 380 0.2 0.1 0.0 0.0 0.1 0.2 0.3 0.4 x 0.5 0.6 0.7 0.8 FIGURE 6.3
The CIE 1931 Chromaticity Diagram shows the gamut of human visual perception. The Plankian locus describes the colors of light possible from a black-body radiator, such as the sun, or an incandescent lightbulb.
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orange, but it would remain on the Plankian locus. When you apply CTO or CTB gel (color temper- ature orange or color temperature blue) to a light source, the resulting light is shifted up or down the Plankian locus. The gels redistribute the spectrum by reducing the intensity of selected wavelengths in a given source. Full CTO must be applied to daylight sources to match with tungsten film. Full CTB must be applied to incandescent sources to match to daylight film. In film and television lighting, we often like to use tints that shift the color one direction or another along this locus using increments of half and quarter CTO and CTB. This type of shift appears natural, perhaps because we experience it in everyday life.
Correlated Color Temperature
The color makeup of sources that do not have a continuous spectrum (HMIs, fluorescents and LEDs) can also be given a Kelvin rating, termed thecorrelated color temperature (CCT). The correlated color temperature is the Kelvin rating that—to human color perception—most closely matches the color makeup of the source. You’ll notice that above the Plankian locus the color of the light moves toward green, and below the locus it moves toward pink or magenta. These color points are not on the color temperature scale, but they can be given a correlated color temperature. The hash marks that cross the Plankian line at an angle connect points that have the same correlated color tem- perature. “Plus green” and “minus green” gels are used to shift the color along this axis. A magenta gel can be applied to a green fluorescent to neutralize the green appearance. Conversely, a green gel might be applied to a tungsten light to match it to a greenish fluorescent. (If the correction gel were not used, then when the colorist attempts to reduce the green appearance in the telecine or at the lab, the tungsten light will turn pink.)
The CCT gives the effective color temperature of the source to the human eye when the spikes in the color curve are combined together. Fluorescent lights come in various color temperatures. Day- light tubes are designed to light spaces that have supplementary daylight, such as offices with large windows. The light is color balanced toward the blue end of the spectrum (5000–6500 K) to blend with the window light. Warm lights, which have a color temperature closer to that of household bulbs (3000 K), are for use in enclosed spaces where supplementary light comes from table lamps and wall sconces.
Color Rendering
In addition to the correlated color temperature, manufacturers of fluorescent, metal halide and LED lamps and light fixtures typically provide an indication of the lamps ability to render the full spectrum of colors accurately. This data can give the gaffer valuable information about the general performance characteristics of a given source, however comparing the CRI values of two sources with relatively good CRIs can be misleading for reasons we shall discuss shortly.
The color rendering index (CRI) is a rating, from 1 to 100, meant to express the accuracy of a light’s rendering of color when compared with a perfect reference source (daylight is a perfect 100). A rating above 90 is considered accurate color rendition for photography. With a CRI above 80, the eye can still make accurate color judgments and the color rendering is termed acceptable. Between 60 and 80 color rendering ismoderate. Below 60 color rendering is poor or distorted.
The CRI index developed in the mid-twentieth century when fluorescent and metal halide light sources started to proliferate. Color scientists became interested in quantifying the ability of artificial lights to reproduce the full spectrum of colors. The CRI of a lamp is determined by illuminating
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eight standard test colors with the artificial light and comparing it mathematically to the same test colors illuminated with an ideal source. HMI and LED manufacturers sometimes express frustration that a lamp with a pretty good spectrum of color, but having peaks and valleys in its spectrum, may not hit the eight test colors dead on, and can end up with a lower CRI than another lamp that has no greater color rendering on average, but happens to represent the eight test colors well. Scientists have developed more accurate modern methods for quantifying color rendering, but lamp manufacturers still use CRI data, and none of the improved methods have yet been universally implemented to replace it.
On location, you will run intostandard fluorescents and full-spectrum (high-CRI) fluorescents. Table B.2 lists specifications for various types of fluorescent tubes. A high CRI tells you that the tube has a nearly complete spectrum of light frequencies and is therefore capable of rendering colors well; however, they often have a strong green spike and require color correction, which typically involves a combination of magenta and CTO gels. The amount of magenta filtering varies and does not necessarily correspond to CRI rating. For example, the Optima 32 (CRI 82) has excellent color rendering on film and requires little or no color correction. A Chroma 50 (CRI 92) shows a strong green spike on film that must be removed with half minus-green gel. (Both Chroma 50s and Optima 32s were manufactured by Durotest, which is now out of business.)
A low CRI rating indicates the lamp emits a very limited spectrum of light and is incapable of rendering all colors. The color rendering of fluorescent tubes in office buildings, warehouses, fac- tories, and commercial buildings will depend on the lamps being used. If the tubes have low CRI ratings they should be replaced with better tubes for filming.
Color chips and gray scale
When the film laboratory makes a print or video transfer from a developed negative film, the timer or colorist adjusts the exposure and colors of the print to make them look natural (or however the DP instructs). A DP who introduces colors into the lighting must take steps to prevent the colorist from removing the color in the transfer or print. To give the lab a reference at the beginning of each film roll, the camera assistant films about 10 seconds of color chips or gray scale under a selected color balance of light. Thechip chart, or color chart, has a set of standard colors or a scale of gray tones from white to black from which the colorist can work. The chip chart must be filmed under light that is exactly the proper color temperature. When a DP wants to render colors normally, he uses a per- fect “white” light for the chip chart. A light that has been checked beforehand with a color tempera- ture meter should be standing by for chip chart shots. When filming the chip chart, no other light should fall on the chart. You may need to turn off or block lights momentarily to prevent extraneous light from discoloring the chip chart. It is also helpful to the colorist to see skin tone with the chip chart, and even to see the scene in the background with all its “nonstandard” color.
A DP who wants the lab (or telecine house) to give the film print (or video transfer) an overall color cast might modify the color of the light hitting the gray scale. For example, for the lab to warm up the dailies (make them more golden and yellow), the DP might film the gray scale with ¼ or ½ CTB on the chart light. When the lab puts in the color settings to compensate for the blue chart, it will add yellow, which will show in the footage.
When working with multiple video cameras, the engineer puts up a chip chart in front of each camera before beginning shooting and electronically adjusts the signal from each camera so that they match each other and give proper color rendition.
Color-temperature meter
A color-temperature meter gives a Kelvin reading, and a reading of the green/magenta shift of the light hitting the cell of the meter. It is important to realize that, although a cinematographer’s color meter, is calibrated at the factory, the readings of two identical meters will often differ. This type of color meter is accurate for comparing the color temperatures of several continuous light sources, but no Kelvin reading should be taken as an absolute value. When more than one meter is being used to take readings on the set, the meters should be checked side by side under the same light to determine the variation between them. Common color meters, like the Minolta II or Minolta III F, make their calculation of the color temperature based on an assumption that the light source has a continuous spectrum. The meter extrapolates the color temperature from the ratio between blue and red in the source. This is an accurate method when reading incandescent sources; however, color readings of LEDs and HMIs have been shown to be misleading for both CCT and green/magenta shift. A tri- stimulus meter (such as the Sekonic C-500 (Figure 6.4), the Gossen Colormaster 3F, or the Minolta CL-200) is a much more sophisticated (and expensive) instrument, capable of greater accuracy.
FIGURE 6.4
The Sekonic Prodigi Color C-500 Color Meter incorporates four color sensors: one green, one blue, and two red sensors—one that emulates film response, and one that emulates digital and human visual response. The meter reads color temperature (Kelvin degrees) and provides LB (orange/blue correction in MIREDs) and CC (green/magenta color compensation CC values, and the filter number needed to correct the source). The meter also reads luminance in lux and foot-candles.
(Courtesy Sekonic.)
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Light-balancing scale: Orange/blue shifts
Most color meters automatically calculate the MIRED shift from the Kelvin reading. This shift is generally referred to as thelight-balancing (LB) index number. This number indicates the amount of amber or blue correction gel needed to match the source to the color temperature of the film stock. To use the LB scale, the color balance of the film stock must be preset on the meter—film type B, 3200 K; film type A, 3400 K (which is used for some still film stocks); or film type D (daylight), 5500 K. Many meters also have a manual mode that allows you to enter any target Kelvin tempera- ture. The manual mode comes in handy when the cinematographer needs to have all the lights balanced to a nonstandard Kelvin rating. Many color meters also give the Kodak filter number and the amount of exposure change corresponding to the LB reading. Kodak filters are used in the camera in situations where an overall color correction is needed. For example, when filming a scene in which the practical lamps are unavoidably yellow, the DP can match the movie lights to the practicals by adding ¼ CTO gel, then, to the degree desired, cancel out the yellow cast with a blue filter on the lens (or filter slot), (however this is more commonly accomplished in the lab or telecine) or with the white balance function on a video camera.
Color-compensation scale: Green/magenta shifts
The color temperature meter also gives acolor compensation (CC) reading that indicates the amount of green or magenta gel needed to correct a source to appear white on film. Such a correction is typically required by commercial fluorescent lights and sometimes HMIs and LEDs. A CC reading of 30 M indicates that full minus green (M is for magenta) is necessary. A reading of 15 M requires half minus green. Table I.9 lists green/magenta correction gels and the corresponding color meter readings.
Color-correction gels
Correction gels come in densities of full, half, quarter, and eighth correction. Correction gel typically comes in rolls 48 or 54 in. wide and 22 or 25 ft. long. When gel is cut from the roll, the cut pieces should be labeled (F [full], H [half], Q [quarter], or E [eighth]) and organized by size and color.
As we have said, CTO gels are orange; they “warm up” a light source. The 85 gel corrects sunlight and HMI sources to 3200 K. Full CTO or extra CTO correct cooler skylight to tungsten, or 5500 K sources to a warm yellow 2800 K.
Conversely full CTB gel corrects a tungsten source to daylight balance. However, it is impracti- cal to use fully corrected tungsten lights when shooting with daylight-balanced film. Tungsten lights are very weak on the blue end of the spectrum. Full CTB reduces light transmission by 85%, or about two stops. (The gel has to absorb 85% of the energy from the light; consequently, the hot-burning lights also burn out blue gel very quickly.) When working with daylight-balance, tungsten sources are typically avoided. HMI and fluorescent fixtures provide daylight-balanced light that is cleaner, brighter, cooler, and a more efficient use of power.
Eighth, quarter, and half CTO and CTB gels are often used to warm up or cool down a source. Subtle tints can enhance the colors of the actors’ faces, their clothes, and their surroundings, and can give the scene. A fire light might use half, full, or even double CTO. A sunset or dawn scene might be filmed with a full CTO on the lights to simulate the golden light of the low sun. Quarter CTO and quarter CTS (color temperature straw) are used for simulating a warm interior source such
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as practical lamp, candle or oil lantern. Commonly a ¼ or ½ CTB is used when the DP desires a cool blue look to a particular light source. An exterior winter scene shot on a sound stage might use soft lights gelled with a ½ or3
/
4CTB to cool the scene. Skylight entering a window, twilight, or moon-light also call for a cooler light.
When using HMI lights in a tungsten-balanced scene, the amount of blue tint in the light is con- trolled by the amount of CTO correction applied to the light. When a slightly cool light is desired, a half CTO is applied, bringing the 5600-K source down to about 3800 K. By the same token, when a daylight-balanced scene requires a warm source it is reasonable to use a tungsten light gelled ½ blue.
Gelling windows
When the outside of the window is accessible, gel can be stapled or taped over the outside window frame, which hides the gel more easily. When stapling gel, apply a square of gaffer’s tape to the gel and staple through the tape to prevent the gel from ripping. The key to gelling windows is to keep the gel tight and free of wrinkles. CTO correction is also available in 4-ft 8-ft acrylic sheets. Using acrylic sheets avoids the problems of wrinkles, movement, and noise that gel makes.
With a very light dusting of spray adhesive (Spray 77), you can stick gel directly onto a window or acrylic sheet. Press bubbles and wrinkles out to the edges with a duvetyn-covered block of wood. If gelling windows promises to become an everyday process on a particular location, an elegant sys- tem is to cut acrylic inserts to fit the windows, then add neutral-density (ND) and CTO gel (using the spray adhesive technique) as needed to suit the light conditions and time of day. Move the inserts around from shot to shot depending on the camera angle.
You can also use snot tape (3M transfer tape) or secure the gel by carefully taping around the edge of it with tape that matches the color of the window frame. Another fast method is to spray water on the windows and apply the gel with a squeegee. This method will not last all day, but it saves so much time that it doesn’t matter if it must be redone.
Neutral-density and combination neutral-density/CTO correction gels
ND gel is gray; it decreases the intensity of a light source without altering color. Neutral density gel is also available in combination with CTO, for gelling windows.
Type Light reduction ND 0.3 1 stop
ND 0.6 2 stops ND 0.9 3 stops ND 1.2 4 stops
A combination of CTO/ND gels and acrylic sheets is commonly used to reduce the brightness of windows.
Using MIRED units to calculate color shifts
One can make calculations of color temperature shift using values calledMIREDs.
Although the Kelvin scale is useful for defining the color temperature of a source, it is an awk- ward scale to use when quantifying the effect of a color correction gel or filter. You cannot simply
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state that a particular gel creates a 200-K shift. The Kelvin shift of a given correction filter depends on the color temperature of thelight source. A blue gel that alters a tungsten source 200 K (from 3200 to 3400 K) alters a daylight source 650 K (from 5600 to 6250 K).
These calculations have been made for you in Table 6.2 and in Appendix I. Table 6.1 shows the MIRED value of a variety of sources. Table 6.2 shows the color temperature resulting from applying common color correction gels to different light sources. Tables I.1–I.3 tell you what gel to use to get