Light, Object, Viewer and the Rubber Ruler

Overview

Light is electromagnetic energy. White light is composed of all colors humans see, and these colors are measured by their wavelengths in nanometers. Spectrophotometry is the science of measuring light. The CIE chromaticity diagram uses tri-stimulous values to designate colors the human eye can see. These tri-stimulous colors are specified by their X and Y coordinates. Color temperature is measured in degrees Kelvin, derived from the glow or light output of a black body radiator. More recent, although somewhat vague, terminologies to describe this phenomenon are Correlated Color Temperature (CCT) and Color Rendering Index (CRI). The light's energy in intensity is measured in foot-candles or lux. This measurement is derived from the CIE weighted scale, called the human eye sensitivity curve.

What is the "Rubber Ruler?"

The Rubber Ruler refers to the many flawed systems used to measure color. To understand colorimetry, the science of measuring color, one needs to understand the language of light measurement. Words like nanometer, spectrophometry, CIE chromaticity chart, color temperature, degrees Kelvin, foot-candles, lux and human eye sensitivity curve are terms often used but not always understood. I'll attempt to define and make these terms understandable, and, hopefully, show how they relate to our craft-painting with light and color.

Unlocking the secrets of color, or How I met "Roy G. Biv."

In 1702, Sir Isaac Newton published Optiques. It was the treatise about the physics of color. At that time, scientists knew that putting white light through a prism would divide it into bands of color. But Sir Isaac had the idea to take a second prism and regather the colored light back into "white light." This is how we learned that white light is comprised of all the colors of the rainbow. Red, Orange, Yellow, Green, Blue, Indigo, and Violet (hence the acronym, ROY G BIV), Sir Isaac's own description of the color spectrum.

Newton speculated that white light was energy waves of different lengths, and those differences resulted in the color spectrum. Years later, Maxwell and Helmholz were able to accurately measure the wavelengths of light, now known as electromagnetic energy. These wavelengths are measured in nanometers. A nanometer is one billionth of a meter. The red end of the spectrum has long wavelengths from 650-700 nanometers. At the dark blue or violet end of the spectrum are shorter wavelengths from 400 to 450 nanometers.

Additive Color Mixing…The Road to White Light

Sir Thomas Young theorized that three primary colors, red, blue and green, could stimulate the full range of color perception. There were some things missing like brown, gold and silver, but nonetheless, his theory of tri-color mixing is the basis for color television and color film today. Simply put, if the primaries red, blue and green are added in the proper proportions, white light will result. Mixing green and red light will make yellow, blue and green creates cyan, blue and red produces lavender or purple. This process is called "Additive Color Mixing."

Prior to Newton's work, there was a great deal of speculation as to what gave an object its color. Thanks to Newton we know that "white light" strikes an object that we perceive as red, because it absorbs the green and the blue energy of the spectrum and reflects the red energy to the observer. A yellow leaf absorbs some of the red, more of the blue and little or none of the green and the leaf is perceived by the observer to be amber in color. A green leaf absorbs much of the red and much of the blue but reflects almost all of the green, giving it a green appearance. The shade of green depends upon the mixture of reflection and absorption of the different wavelengths of light. Without light there is no color and light must strike an object for us to see it. Light passing through space appears to be black, we have no sensation of color. Once the light strikes an object, it releases its energy.

Additive color mixing is illustrated in the color star ("How to Effectively Use the Gray Card," International Photographer Magazine, September 1997). The primary colors are red, blue and green. Secondary colors yellow, blue-green (cyan) and magenta are a mixture of the primaries as are the tertiary colors like orange, amber and violet. The color star is a visual expression of additive color mixing. It's helpful in understanding the colors you can achieve by using color filters on various light sources and blending them. Interestingly enough, it is the basis for the GamColor wheel and numbering system which organizes the colors in primary, secondary and tertiary hues. The GamColor wheel is identical to the star in that the complimentary colors are opposite each other. The GamColor numbering system indicates where on the spectrum the filters' color lies.

Subtractive Color Blending, or Finger Painting

We are familiar with subtractive color mixing since it is what you do when you mix colors to finger paint. I don't intend to explore subtractive color mixing where the primaries are different and the results are very different. Subtractive color mixing results in the color becoming darker (remember those finger paintings you, or your, child brought home that started life with color, but ended in shades of brown?). Additive color mixing results in the color becoming lighter. It's important not to confuse these two systems, particularly when dealing with light.

The Human Eye's Response versus Film and Video

The theory is that the cones in the eye respond to the three primary colors and various wavelengths, to mix them and create the full range of colors. If you compare the average human eye's response to a typical film's response to color, you will see the film designer's attempt to mimic the human eye's perception of color. The main exception is the film's red register, which is deliberately shifted to enhance skin tones. The full color spectrum of (visible light) blue through green, yellow, orange and red is represented in the 400 to 700 nanometer bands. Energy just below 400 nanometers is ultraviolet, while energy just above 700 nanometers is infrared. Most humans do not "see" energy in the UV and infrared regions but some film emulsions and video systems do. This is one reason why cinematographers filming in high altitudes use UV filters to lower the sky's density, and other unwanted density from Ultra Violet exposure.

CIE Chromaticity Chart-A Valuable Tool

In 1931 the CIE meeting in Paris, the City of Light, adopted the CIE chromaticity diagram. This important development was an attempt to map and quantify every color. Using this system, it was hoped, would allow them to accurately designate a color's appearance so that it could be duplicated. The need to develop a system grew from the Industrial Revolution and the expanding use of synthetic dyes. Prior to mass manufacturing, natural dyes were used and those dyes were used in small lots. But with large quantities of products being manufactured, it was necessary to quantify and specify colors. For example, if J.C. Penney orders a train car full of red shirts and burgundy shirts showed up instead....You can see the magnitude of the problem.

The way the CIE chromiticity diagram works is quite ingenious. Visualize this graph as a triangle with blue 400nm (nanometers), green 520nm, and red 700nm at the corners. The dominant wavelengths follow the outside curve of the chart where they are numbered. Now, if an imaginary straight line ran between 400 and 700, you would see purple, the result of mixing red and violet. This line is referred to as a minus, or a complimentary number. Colors can be designated by their XY coordinate. The Y axis represents green, the X axis represents red, and the three numbers must add up to one. The Z axis is blue. For example, if we have the coordinates for X and Y, we can determine the Z axis. This system is based on Thomas Young's tri-stimulous theory.

The curved line, or black body locus (or black body radiator), drawn on this CIE diagram indicates what happens to a black body radiator as its temperature is raised. There are points designated along the black body locus for incandescent, daylight and other frequently used light sources. I will discuss this separately later with Kelvin Temperature.

The black and white CIE diagram shows the area designated for each color. Along the outer curve are shown the wavelengths of color. The dominant wavelength of a color is determined by drawing a line from the light source coordinates, usually located along the black body curve, through the coordinates of the color to the outer edge. This CIE standard was the first color system that enabled us to numerically designate a color and visualize its appearance.

Using the CIE chromaticity diagram, it's easy to overlook the deeper problems the committee had to solve when they adopted this standard. The results were not something measured with a "ruler" but were the documented observations of 1000 selected male viewers between the ages of 20 and 40. No senior citizens, children, or females were included. The men's responses were then averaged. As flawed as this system was, it was the first successful attempt to quantify color, and describe it numerically. The system still has many useful functions.

Note the colorimetry of film and video systems overlaid on the CIE chart. Remember the additive theory, all the colors of the spectrum can be achieved by mixing the three primary colors. The choice of the primary colors will limit the size of the triangle and the range of colors that can be created. The whole chart represents all visual color perception possible for human beings. The triangles representing film or video indicate all the colors they can reproduce. It can be quickly seen from this chart the numerous colors outside the film and television triangle. These colors cannot be duplicated in these mediums. By the way, you might notice that film can produce a slightly wider range of color than video. Many production designers, cinematographers, and others concerned with sets and wardrobe are very aware how problematic some colors are. This chart indicates which ones and why.

Kelvin Temperature and That Pesky Black Body Radiator

We measure the color temperature of a theoretical black body radiator in degrees Kelvin. A theoretical black body radiator is a device that radiates no color or light at minus 270°C. As the temperature of the black body radiator rises, it begins to radiate long wave energy. When the temperature reaches about 600° Kelvin (about 870°C) it reaches the threshold of visibility. At 770° Kelvin the black body is dark red. As the temperature rises, the eye can perceive the visible spectrum emitted from the black body as orange, yellow, "white," and finally blue. On the Kelvin temperature comparison chart, sunrise is about 1800° Kelvin. This is in the very red range, much like a candle flame. As the sun moves away from the horizon, the ambers and yellows increase and the light takes on the characteristics of an incandescent lamp. Incandescent lamps vary in color temperature from 2600° to 3200° Kelvin. The noonday sun (in Washington, D.C.) has a nominal color temperature of approximately 5400° Kelvin, or "daylight" for film. The amount of blue energy in the spectrum of the noonday sun varies and is dependent upon the region and the weather conditions. If the sky is overcast and cloudy, we will see less blue energy in the spectrum. If the day is clear and bright, the blue sky could be as high as 28,000° Kelvin and the resulting blue energy is greater.

Degrees Kelvin is an important tool for measuring the light sources used in theatre, television, and film production. Degrees Kelvin is a term very often misused, unfortunately, mostly by those who should know better. It's important to understand that degrees Kelvin can only be attributed to a black body radiator simply because it's very predictable what happens to the balance of all colors of the spectrum as the black body radiator's temperature rises. Think of the sun. At early morning, the sun has very little blue energy and a great deal of red energy, therefore a low Kelvin temperature. As the sun rises, the color temperature increases: 1000, 1200, 1400 degrees and so on, the ratio of blue to red shifts in a predictable fashion. Although the black body radiator is a theoretical device, sunlight, carbon arcs and incandescent (Tungsten filament) lamps are very good black body simulators. In fact, early Kelvin temperature meters measured only the blue and red energy and gave an accurate Kelvin temperature reading.

New color temperature meters also indicate the green/magenta balance. This is needed since discharge lamps such as fluorescent, Xenon, Mercury Vapors and HMIs aren't black body radiators. These lamps have such an interrupted spectrum, and spikes in their spectral distribution, that it's inaccurate to describe their color in degrees Kelvin. Unfortunately, it has become common practice to describe these light sources as having a color temperature in degrees Kelvin. In the past, manufacturers referred to the "apparent color temperature" of a fluorescent lamp. The use of "apparent" warned us that something was different. Today's lamp manufacturers misuse the term Kelvin temperature in reference to their product, or adopt one of two new designations, CCT or CRI.

Correlated Color Temperatures (CCT) or, What is the Apparent Color Temperature?

Correlated Color Temperature (CCT) is the new term for "apparent color temperature." The lamp manufacturer uses degrees Kelvin as a reference point and states that it indicates the Correlated Color Temperature in degrees Kelvin. CCT can be very misleading since the implication is an accurate reading of the light source's color temperature. This is not true. Light sources such as fluorescent lamps with a 3200° or 5400° CCT rating can be very unpredictable in rendering colors visually, and more extremely in video and film because they do not have a continuous color spectrum like the black body radiator.

Color Rendering Index (CRI)-It's Only an Opinion

CRI or Color Rendering Index is a very subjective method, with little standardization, of determining how well a light source renders color to the average observer. For example, sixteen colors are selected and the light source we wish to rate is projected on them. Next, we average the response of a group of human subjects as to how accurately the colors appear when compared to the same colors under either Tungsten or daylight sources. The ratings are 0 to 100. By definition, daylight and Tungsten are 100 and everything else is measured from that point down. For television production, it has been recommended that sources with a CRI below 80 not be used. I believe that you can't depend on the Color Rendering Index rating in film and video production. It shouldn't be used because it's neither consistent nor predictable. For example, the Color Rendering Index number for both Tungsten and daylight is the same: 100. Try shooting the same film stock the same way under those two very different sources. It simply won't work! It would be more useful to have a full spectral distribution chart of the light source to determine just what problems might exist with the light source, what solutions you may need to correct them.

The Weight of a Foot-Candle

The foot-candle is a unit of measurement of light. The chart below shows how the value of a foot-candle (metric system lux) is determined. One foot-candle is 12.57 lumens of output on one square foot of surface and in the metric system we use one square meter and call it lux. Think of foot-candles as a unit of measurement of the energy of a light source. The higher the foot-candles indicates more energy, or light. Too bad, it's not that simple. Let's see how we got to this seemingly very accurate measurement. Once again, an "average observer" is used. This person looks into a black box in which two colors are presented. The observer indicates which color appears brighter. By the way, these observers were all males, between the ages of 20 and 40, no senior citizens, no children and certainly no women. Most observers felt that colors in the range of 550 nanometers appeared brighter than colors with either higher or lower wavelengths. Based on these subjective tests each part of the spectrum was given a weighted value. The results of those tests are shown in the Human Eye Sensitivity Curve. When a foot-candle meter is used to measure light output of a source, it gives more weight to that part of the spectrum that generates yellow/green light and less to those areas in the red and blue. It's no accident that "high efficiency" and "high output" lights tend to have a lot of yellow green in their spectral distribution. (See "From Candlelight to Daylight," International Photographer Magazine, January 1998, for a more detailed review of the film/light meter response to this.)

Full Spectral Analysis Chart

A spectrophotometer is capable of making a full analysis of all of the wavelengths of a light source. This is accomplished by taking the beam of light, dispersing it through a prism, thus dividing it into its various colors. Each color is passed through a tiny slit and the spectrophotometer measures the energy of the specific wavelength. The resulting full spectrum analysis chart doesn't state the color's appearance in the same manner the coordinates on the CIE chart do. From just examining the numerical data, you may not be sure what the color is. However, this information is far more accurate and complete. Louis Erhardt states in his book, Radiation, Light and Illumination, that "measurements with the spectrophotometer are the most detailed and accurate possible. The resulting data (i.e., measurements) are used when accurate information must be exchanged about light sources and color filters. Superimposing a color graph over the GamColor spectral distribution graph makes it easier to see which wavelengths stimulate a color response."

Tungsten light and daylight at sunset have similar charts. The high blue energy of clear noon daylight is significantly different from sunset where there is more red than blue. Fluorescent lamps contain an "artistically rendered" range of colors and some strange bars on the graphs. More about these bar graphs later.

ANAYLSIS OF SOME DIFFERENT LIGHT SOURCES

Sodium Vapor Lamps

On the HMI lamps spectral energy chart we see more interesting bars again, they go up in a straight line, cut off, and drop straight down. There appears to be color in all parts of the spectrum, unlike the low pressure sodium vapor lamps with color in only one part of the spectrum, yellow. What can a director of photography do when he needs to film a full color spectrum, but encounters sodium vapor lamps? The best answer I have is to "shoot in black and white!" You can filter out colors you don't want, but you can't add colors that don't exist in the source's spectral distribution. It's impossible to get good color rendering with low pressure sodium lamps without augmenting the light with other color correct sources.

HMI and Xenon Lamps

Now about those bars, it is acceptable engineering practice to designate higher peak energy by fattening the indicator bar and chopping it off, so the spike doesn't go off the paper. The engineers create a ratio that crushes the height while expanding the width. Think of the anamorphic format. If these charts were more accurately diagramed they would appear in the chart indicating the typical spectral power distribution of a HMI discharge lamp. The HMI lamp does produce color across the full spectrum. However, it has spikes in the blue region at about 440 nanometers, a huge spike in the green range around 560 nanometers, and several spikes in the red region. The green spike in an HMI lamp is often overlooked, and creates a problem for film and video cinematographers. This green spike also increases with the age of the lamp and varies from lamp to lamp. On the set, minus green gels (magenta) are often used to eliminate this problem. However, HMI lights are considered reasonably good daylight sources, but they aren't as good as Xenon. The Xenon chart may be a little disturbing at first but, if you examine the visible spectrum for humans, 400 to 700nm, the light output is consistent with only a few small spikes. The larger spikes, for the most part, are outside the photo/visible spectrum.

Mercury Vapor Lamps

The high intensity mercury vapor lamps also have unusual bars indicating high energy (spectral color intensity) peaks in those regions. There also is a large area of interrupted spectrum with little or no color. Mercury vapor lamps are not good for shooting color film or video and they are certainly not great for viewing colors. Although they may be ideal for an effect, or reproducing street light in a scene.

Industrial Fluorescent Lamps

One of the most often used light sources today in schools, offices and public buildings is the fluorescent lamp. Fluorescent lamps are often described as having a color temperature in degrees Kelvin. As I mentioned earlier, this is a misnomer. Terms like Color Rendering Index (CRI) and Correlated Color Temperature (CCT) have been substituted for a true Kelvin color temperature. This new terminology implies an exact science of measurement that isn't supported by science. If you really need to know how a light source or color filter will accurately perform, use the full spectral analysis chart and learn how to read it. When you look at the spectral analysis charts published on fluorescent lamps, they are often misleading because of the accepted practice of using bars to represent peak energy. Also, it is important to note that the smooth line indicating the colors drawn across the full spectrum is not quite true either, it's "artistic license." In truth, the spikes look a lot more like the spikes in the HMI chart except much of the spectrum is missing. Fluorescents typically have an interrupted spectrum, therefore much of the full color spectrum is not there. Hundreds of different fluorescent lamps are manufactured by a variety of companies and the range of "apparent color temperature" (CCT) of fluorescent lamps can be anywhere from 2600° Kelvin to 5600° Kelvin. Fluorescent lamps are not very good for rendering color and change significantly with age. In the theatre, where fluorescents are rarely used, you should never select a color filter under this light even if told it is a "Tungsten equivalent."

Back to the Rubber Ruler, and a Reality Application to Filters

Let's return to the CIE chromaticity map. It's possible to designate two colors on the map and to mix colors anywhere along the line between those two colors. An example might be appropriate to see if the system works. Look at two colors, GAM 690, Blue Grass, and GAM 950, Purple. Place them on their CIE coordinates. Any color along that line can result with a blend of these two colors intensity. On this line is GAM 840 (Steel Blue). Look at two other colors, GAM 835 is Aztec Blue, and GAM 720 is a Light Steel Blue. If a line is drawn between these two colors, where they intersect is the GAM 840 (GelFile was used to make this selection). In theory, the 840 Steel Blue color may be achieved with two lights using either of these combinations. Are they the same?

Metameric Colors, i.e., Color Filters and The Source "Color" of Lighting Fixtures

The resulting G840 Steel Blues created with these two different color combinations are called metameric colors. Metameric colors are colors that look the same under one light source but different under another. They look different because the spectral energy of the two colors are different. A full spectral analysis (spectrophometry) will show us these colors are different and we can predict a different performance under different light conditions. Spectral distribution graphs would show the different wavelengths of visible light from 400 to 700 nanometers of the two colors. The charts may show ultraviolet (below 400) and infrared (above 700). The G840 created with two different sets of color filters in this illustration would have the same CIE tri-stimulus coordinates. The dominant wavelength would be the same. However, with different light sources, they behave very differently. This really is common sense. If a CTO is placed over a Tungsten 10K the resulting light is not the same as if the same CTO filter is placed over a 12K HMI. This is an extreme example, but on set, the source light must be monitored, or the total resulting effect must be monitored. There is no in-between for the careful cinematographer.

Conclusion

This article has made an effort to cover the measurement of color and light, and the terms used in theatre, television and film. It did not discuss every system for measuring color, but only those most used in the lighting profession. If you explore other books and technical information on this subject you will understand the language of color more clearly as a result of this presentation. You will understand that these measuring systems were created with great care and as much precision as possible; however, they are flawed and subjective. If you think about the most basic element the scientists are investigating, it's quantifying the human eye's response and brain's interpretation of light and color. More simply put, it's quantifying the human experience, or in computer jargon, digitizing color so it may be evaluated in a binary universe. If you think further about the problem, color bars exist on the heads of tape masters with the thought that the machines can be adjusted to accurately record and playback the "original colors." There are no easy answers. It's important to remember that the eye "sees" in ways meters can't measure, and film and tape can't record.

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