Light On Color: Illuminate

Light On Color: Illuminate

In the last several Artissima blog posts we have taken a  journey through some of the aspects of color that many of us find challenging to truly understand, and wrap our minds around.

It has been fascinating to tackle the topics of metamerism, spectral reflectance, spectral power, the color rendering index, and color temperature.  I wanted to take a few moments, and summarize what I have learned, and hopefully what you have learned too!

As discussed,  color is a function of light. The color that we  see  is  reflected light waves.   “Visible light is made of seven wavelength groups.  When light hits objects, some of the wavelengths are absorbed and some are reflected, depending on the materials in the object. The reflected wavelengths are what we perceive as the object’s color.”

We recognize four types of Metamerism, the phenomenon of colors seeming  to match under one light source, yet appearing different under another/in a different environment.

Sample metamerism: when two color samples appear to match under a particular light source but do not match under a different light source.   Sample metamerism has to do with differences in each sample object’s spectral reflectance distribution, or its response to light, characterized by the wavelengths that it primarily reflects. It happens because of differences in the objects (or samples) themselves.

Illuminant metamerism:  occurs when the spectral reflectance distributions of the two color samples observed are identical, and are seen under different lights sources with differing spectral power distributions  (SPD, or output of a light source, characterized by its relative strength at each wavelength.  It happens because of the light sources (or illuminants) themselves.

Sample Metamerism occurs as a result of  differences in the reflectivity of the color samples themselves, and Illuminant Metamerism occurs as a result of differences in the output of the light source itself, under which we are viewing the color samples.

Observer metamerism: can occur because of differences in color vision  from one person to the next. The physical act of seeing, that which happens in our eyes and brains when we see an object in whatever light, can alter  our color perception. A common source of observer metamerism is color blindness,  but it occurs with the “normal-seeing”  as well.  What  may be a color match for one observer may not be for another.

Geometric metamerism: The angle, distance or light position from which identical colors are viewed may change the color that we see. The distance between a woman’s eyes is, on average, slightly less than a man’s.  This slightly different angle of stereoscopic viewpoint may be why men and women have been known to perceive colors differently!  Most of us have had the experience of  two samples appearing to match when viewed from one angle, but then not matching when viewed from another angle.

CRI, or The Color Rendering Index is an international measurement scale that measures or  describes how colors are rendered, IE “show up” to our eyes,  under an artificial source of light as compared to daylight. Daylight renders the widest variety of colors, as compared to artificial lighting, which depending on the nature of its light source, can render many or few colors.  The International Commission on Illumination (Abbreviated C.I.E. because of its French name, Commission internationale de l’éclairage)is recognized as the  international authority on light and color.  It defines  CRI/color rendering as the effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.

in my own words, the color rendering index describes the  effect of a light source on how the color of an object appears to us.  It is the measurement of how much an object’s color appearance shifts when illuminated by an artificial (other than daylight)  light source compared to the color appearance of the same object when  illuminated by a “reference” light source (daylight), of comparable color temperature.

Color Temperature… refers to the actual color and type of  light emitted by a particular light source. High color temperatures, those over 5,000K (K = Kelvin) are termed cool colors  and are bluish white, while lower color temperatures (2,700–3,000 K) are called warm colors  and are yellowish white through red.  

Thus, color temperature refers to the actual color of light and  CRI refers to the ability of a light source to render color, in comparison to daylight.

The relationship of color temperature to CRI?

The color rendering index can be used as a basis of comparison between light sources only if they have the same color temperature; if they are the same degree Kelvin, and thus the light they emit is the same color.

CCT, or Correlated Color Temperature, also expressed in Kelvin, is a numerical description of a  light’s color appearance,  and describes whether a white light source appears more yellow/gold, or more blue.  CCT can be used as a means of correlating the color of an artificial light source with the color of daylight.

As artists, colorists, visual makers of any kind, and humans upon this earth…we will always come back to our response to color, how it is perceived or rendered through light, its effect on our interior and exterior environments, and upon our hearts, minds and souls, to say nothing of our work. Color, and thus light, are a frame of reference for our physical and emotional  experience as we move through our lives.

I hope these posts have shed some light on the color in your life….and that you walk in beauty…and illumination.

 

Color Temperature

Color Temperature

What is “Color Temperature“, and how does it relate to CRI, or the Color Rendering Index?

We know CRI to be an international measurement scale or rating of how accurately an artificial  light source renders, or shows the color of an object (often called its “color appearance” ) as compared to daylight, which is capable of depicting, rendering or showing the widest range of colors. Thus, CRI rates, describes or measures how the  colors of objects  appear (compared to daylight) under a specific light source…IE, in a given light.

Color temperature however refers to the actual color and type of  light emitted by a particular light source.  High color temperatures, those over 5,000K (K = Kelvin) are termed cool colors  and are bluish white, while lower color temperatures (2,700–3,000 K) are called warm colors  and are yellowish white through red.

Yes, counter-intuitively, the higher color temperature describes a cooler color, while a lower color temperature describes a warmer hue. When considering interior lighting,  color temperature can play a significant role in how we feel and function in a space.  A warmer light, which has a  lower color temperature, can  promote our relaxation, while a cooler  light with a higher color temperature may enhance or help to increase our concentration.

Thus, color temperature refers to the color of light, and CRI refers to the ability of a light source to render the color of objects in a manner comparable to the way daylight does/would.

Whew…what a mouthful!

The rub?  The color rendering index can be used as a basis of comparison between light sources only if they have the same color temperature. To compare light sources, or to compare an artificial light source to daylight, said artificial light source must have the same color temperature as the daylight to which it is being compared.  Remember, daylight renders, or makes visible, the widest range of colors…and there are yet many more colors in the light spectrum that we cannot see with our naked eyes.  We are not able to see the entire light (thus color) spectrum. Light values beneath the visible part of the spectrum are referred to as infrared, and above the spectrum as ultraviolet.

CCT, or Correlated Color Temperature  is expressed in Kelvin, and describes whether a white light source appears more yellow/gold, or more blue.  Thus, it  is  a numerical description of a  light’s color appearance.  It can be used as a means of correlating the color of an artificial light source with the color of daylight. “The correlated color temperature (CCT) is a specification of the color appearance of the light emitted by a lamp, relating its color to the color of light from a reference source when heated to a particular temperature, measured in degrees Kelvin (K).  —http://www.lrc.rpi.edu/education/learning/terminology/cct.asp

Here, we bring our discussion of Color Temperature, and its relationship to CRI to a close, at least for the moment.  It is a complex subject, but one that can come into play when dealing with any matters of color and light in the realms of photography, film, video, interior design, theater, the visual arts, and many other areas.  Good for us to have some idea of what the terms mean!

 

I hope that this series of posts on the relationship between Color and Light has helped to demystify it for you…at least to some extent. As much as we can learn, observe and discover, we will never know it all…and maybe that is as it should be.  Some things, such as the majesty and magnificence of the natural world should retain some mystery…no matter how much physics we attempt to wrap our minds around.

 

 

 

Rendering Color II

Rendering Color II

“What is color? No object of itself alone has color.
We know that even the most brightly colored object, if taken into total darkness, loses its color. Therefore, if an object is dependent upon light for color, color must be a property of light.
And so it is.”
Paul Outerbridge, Photographer 1896 – 1958

In the post, “Color Rendering I”  I delved into the nature of color and light…as Paul Outerbridge  says above,  color is a property of light…the color that we see an object as “being”, is in essence, light…the wavelengths of light it reflects, as opposed to absorbs.

In this post, I am seeking to clarify CRI…what does that mean?

CRI, or the Color Rendering Index, is a scale that measures not color, not light, but ” the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source”.–http://en.wikipedia.org/wiki/Color_rendering_index

The ideal or natural light source being daylight, because “it (daylight) displays (1) a great variety of colours, (2) makes it easy to distinguish slight shades of colour, and (3) the colours of objects around us obviously look natural.” —P.J. Bouma

The International Commission on Illumination  (which is usually abbreviated C.I.E.  for its French name Commission Internationale de L’Eclairage), the international authority on color, color spaces, light, and illumination, has defined CRI as the  “Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant”.

Is the concept and definition of CRI becoming any more clear?

Trying to explain CRI reminds me of trying to translate from one language to another, in a manner that makes the meaning of a phrase in one language, comprehensible in another.  It isn’t enough just to translate the words…the whole meaning, context, and  sense of the phrase must be understood.

How’s this:

  The closer the red of your child’s red beach ball inside, under the light of say, your dining room chandelier, looks to the red the same ball appears to be outside, on the beach, under the sunlight, the higher the CRI is of that dining room chandelier illuminant.  CRI measures the ability of a light source to reveal, render, depict or show color the way daylight would.

Put another way, the color rendering index describes the  effect of a light source on how the color of an object appears to us.  It is the measurement of how much an object’s color appearance shifts when illuminated by an artificial (other than daylight)  light source compared to the color appearance of the same object when  illuminated by a “reference” light source (daylight), of comparable color temperature.

Whoops!  Color Temperature!?!  Suffice it to say, right here, right now, the CRI of a light source can only be determined when it is being compared to a reference illuminant, (natural light/daylight), with the same, or comparable color temperature.  The role of “Correlated Color Temperature”  in CRI will be discussed in a future post.

So…until then chew on the above…and I hope the light bulb goes on for you about what Color Rendering Index is.

May both your days, and nights be illuminated with light sources of the highest CRI!

Rendering Color I

Rendering Color I

“What is color? No object of itself alone has color.
We know that even the most brightly colored object, if taken into total darkness, loses its color. Therefore, if an object is dependent upon light for color, color must be a property of light.
And so it is.”
Paul Outerbridge, Photographer 1896 – 1958

As Mr. Outerbridge so succinctly states, color is a property of light…or, otherwise put, a function of light.  Color and light are intrinsically entwined…part and parcel of each other… mystic twins, or co-dependents, depending on your point of view.

Sunlight, or white light,  is the combination of the entire electromagnetic spectrum ( a light source’s spectrum is a distribution giving its intensity at each wavelength, and most light sources emit light at many different wavelengths)..in essence, all the colors of the rainbow.  Their combination creates white light, while  lack of light, no color, is perceived by us as black.  Though artists may see black as a color, it is actually the lack of color: a state of no color.  We are discussing color, and color mixing from the point of view of light wavelengths, not color mixing as regards to paints.  This is physics…the physics of light and color.

The terms “color” and “wavelength” here may confusing.  Our eyes will perceive the color based on not only the wavelengths of light that the object we are viewing reflects or absorbs, but  also on the actual measurement of those wavelengths…measured in nanometers. We can see the reflected colors of light which lay in a very small region of the electromagnetic spectrum called, aptly enough, “visible light”.  We cannot see color wavelengths that are absorbed by an object or surface.

Wavelengths ranging from about 400-750 nanometers make up the visible spectrum of light that can be perceived by the human eye.  When light strikes an object certain  of its wavelengths are absorbed by that object, and others are not. Those that are not, those wavelengths of light which bounce  or are reflected off an object, are perceived by the human eye as color.   In essence, “An object appears a certain color because it reflects certain light wavelengths, which are then  perceived by the eye.”  —http://www.wisegeek.com/what-is-the-color-rendering-index.htm

Surfaces we see as black absorb all of the wavelengths of light in the visible spectrum which reach them.  All the “color”  thus is absorbed, and we see no color, and thus black. Surfaces we see as white are reflecting, or bouncing back to our eyes, (specifically, the rods and cones -photoreceptor cells- within our eyes), all of the wavelengths of visible light which reach them.  Thus, as all of these visible wavelengths are being reflected “to us”, and their combination, as discussed above, creates white light, as white light (discussed above)  is the combination of entire electromagnetic spectrum, or all the colors of the rainbow!  (Mind you, if you mix a wide spectrum of varying paint colors together, you will  not get white! We are discussing color in reference to light only in this post!).

Each light source emits different wavelengths of light, thus the way we perceive colors varies depending on how the object we are seeing is illuminated, I.E., the wavelengths of light it absorbs and reflects.

Ah..now we begin to get to the heart of the story…the Color Rendering Index, or CRI.  It will take a second post on this complex subject to further demystify it.

“The Color Rendering Index is an international measurement scale that describes how colors are rendered under an artificial source of light. The standard against which artificial lighting is compared is daylight, because daylight renders the widest variety of colors. Artificial lighting, by contrast, can render very many or very few colors, depending on the nature of the light source. The color rendering index has many applications, especially in art and photography.” —http://www.wisegeek.com/what-is-the-color-rendering-index.htm

“The Color Rendering Index (sometimes called color rendition index), is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as photography and cinematography.^[1] It is defined by the International Commission on Illumination as follows:^[2]

Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.

The CRI of a light source does not indicate the apparent color of the light source; that information is under the rubric of the correlated color temperature (CCT).” —http://en.wikipedia.org/wiki/Color_rendering_index

Because of the complexity and relative “thickness” of this subject, I will delve further into the Color Rendering Index, Color Rendering itself, The International Commission on Illumination, and Correlated Color Temperature, in a subsequent post.

Until then…I am wishing you much light and color in your life.  May your spirit be illuminated, and your soul rendered…in all the colors of the rainbow!

Seeing the Light: Metamerism II

Seeing the Light: Metamerism II

To support my  attempt to demystify the baffling phenomenon of metamerism, and enhance our understanding of it, let us review the four types of metamerism described in the previous post, “Seeing the Light…Metamerism”.  We can then  move on from there to deconstruct some of the formal terms used in their definitions, specifically, spectral reflectance distribution, and spectral power distribution, two terms/phenomena that are inter-related, but not the same, and which play an integral part in metamerism.

Sample metamerism: What we think of as “metamerism” is actually one type, sample. When two color samples appear to match under a particular light source but do not match under a different light source, this is called “sample metamerism.”   Sample metamerism has to do with differences in each object’s spectral reflectance distribution, or its response to light, characterized by the wavelengths that it primarily reflects. When the samples respond to different light sources in different ways, reflecting wavelengths of light (which is the color that our eye sees: we see the wavelengths of light that are NOT  absorbed by an object…IE, the wavelengths of light that ARE reflected by an object) differently, they will be seen by our eyes as different colors.

Illuminant metamerism: Easily confused with sample metamerism, illuminant metamerism occurs when the spectral reflectance distributions of the two color samples (which, as mentioned above, is their response to light, characterized by the wavelengths that they primarily reflect)  is identical, and the spectral power distributions of the various light sources under which they are seen, differ. When two  identical samples are seen under different lights sources with differing spectral power distributions  (the output of a light source, characterized by its relative strength at each wavelength), their color will appear to differ, even though as regards to their spectral reflectance distribution, they are exactly the same.

Thus, Sample metamerism occurs as a result of  differences in the reflectivity (ability to reflect) of the color samples (objects/surfaces) themselves , and Illuminant metamerism occurs as a result of differences in the output, or strength of the wavelengths of the light source itself, under which we are viewing the color samples.

Complicated?!

Indeed!  But when we read the definitions carefully, and break them down so that we understand the terms and what they are referring to, we can begin to understand how light, color, and sight work together.

Illuminant metamerism is not often seen, unless  the observer use a  light box to see identical samples illuminated by both light sources separately, yet simultaneously.  Again, this type of metamerism is created by differences in the strength of the wavelengths of the light source only, not in the samples themselves.

Observer metamerism: Do we all see color differently?  It is commonly agreed upon that we do, assuming that each individual  possesses adequate color matching aptitude. Observer metamerism can occur because of differences in color vision  from one person to the next. Again the process of seeing is complex, but for the purposes of this post, suffice it to say  that the physical act of seeing, what happens in each person’s eyes and brain when they see an object in whatever light, can alter  their color perception. A common source of observer metamerism is color blindness,  but it occurs with the “normal-seeing”  as well.  In the case of observer metamerism, two lights or surfaces  may be a color match for one observer but not for another.

Geometric metamerism: The angle, distance or light position from which identical colors are viewed may change the color that we see. The distance between a woman’s eyes is, on average, slightly less than a man’s.  This slightly different angle of stereoscopic viewpoint may be why men and women have been known to perceive colors differently!  Most of us have probably had the experience of  two samples appearing to match when viewed from one angle, but then not matching when viewed from another angle.  Examples would be the color variations that appear in pearlescent auto finishes or “metallic” paper. This may be something to think about when using specialty finishes in interiors, fashion and works of art.

Now, let’s explore,and attempt to define both spectral reflectance distribution, and spectral power distribution, and their curves, and see if this process illuminates our understanding of ” magical” metamerism!

The spectral reflectance distribution of a sample (object or surface) has to do with its response to light…to the light  wavelengths it reflects, as opposed to those it  absorbs.  The color that we see is composed of the wavelengths (which compose white light) that are reflected…not absorbed by a surface.  In essence, what we see is really not a “tangible”  part of the object or surface we are looking at.  We do not see absorbed wavelengths…they are no longer visible. We see the wavelengths that are reflected. The spectral reflectance distribution is a quality, or ability of an OBJECT or SURFACE…not of a light source.  Light sources, whether natural or artificial,  provide light, they do not reflect it.

The reflectance spectrum or spectral reflectance curve  is the plot of reflectance as a function of wavelength, which can be charted graphically.

Spectral power distribution  is how much power a light source puts out, “characterized” by its relative strength wavelength to wavelength.
It is a quality, or ability of a LIGHT SOURCE…not an object or surface.  As stated above, objects or surfaces absorb and reflect light.  They do not provide light.  Light sources provide light that is then reflected or absorbed by an object/surface.

A spectral power distribution curve  ” provides the user with a visual profile of the color characteristics of a light source. They show the radiant power emitted by the source at each wavelength or band of wavelengths over the visible region….” –http://www.gelighting.com/na/business_lighting/spectral_power_distribution_curves/

Whew!  Is this…physics?

I know that  I have  repeated some of this information several times.

I have found that only by repetition, reading and rereading, sometimes even  out loud, can I begin to wrap my mind around these concepts. I hope these efforts help YOU understand metamerism and its inherent phenomena, and that your color practice, consultation, choices, play and enjoyment is enriched.

Happy coloring!  Try it outside the lines…maybe you will make a colorFULL discovery!

Seeing the Light…Metamerism

Seeing the Light…Metamerism

Metamerism, commonly defined as the optical phenomenon of colors seeming  to match under one light source, yet appearing different under another, is fascinating, and perplexing.  Why and how does it occur?

Well, the answer, from my research, can get pretty complex…involving such things as CRI, or color rendering index, SPD, or spectral power distribution,  spectral reflectance distributions  and  reflectance curves.

I became interested in metamerism afresh after viewing a webinar on the subject created by the paint company Sherwin-Williams.  I must admit, I watched and listened to this recorded webinar several times, took notes, and then watched/listened to it  again. Light in tone, with fun images, and plenty of humor, the webinar contained information that has taken me time, focus, and further research to even begin to understand.

Because I find the subject so complex, I have decided to devote a series of Artissima blogs posts to metamerism, and attempt to break it down into comprehensible chunks…siting examples and resources along the way which I hope will be helpful.

Color involves light, the object illuminated, and the observer of the illuminated object.   As color is a function of light, very simply put, the color that we (the observer) see  is  reflected light waves.   In essence: “Visible light is made of seven wavelength groups.  When light hits objects, some of the wavelengths are absorbed and some are reflected, depending on the materials in the object. The reflected wavelengths are what we perceive as the object’s color.” —http://www.devx.com/projectcool/Article/19954/  Put another way; “Objects affect light by selectively reflecting or absorbing light of different wavelengths. So an object that absorbs most blue wavelengths and reflects most red wavelengths will usually appear red to our eyes. The actual color it appears to us is dependent on the spectral composition of the light reflecting off the object.” ––http://photoshopnews.com/2005/04/20/metamerism-%E2%80%93-friend-or-foe/

Let’s  look at four kinds of metamerism…

Sample metamerism: What we think of as “metamerism” is actually one type, sample. When two color samples appear to match under a particular light source but do not match under a different light source, this is called “sample metamerism.”   Sample metamerism has to do with differences in each object’s spectral reflectance distribution, or its response to light, characterized by the wavelengths that it primarily reflects. When the spectral reflectance distributions of the two samples (the objects themselves) differ, the color of each will look different in different lights.

Illuminant metamerism: Easily confused with sample metamerism, illuminant metamerism occurs when the spectral reflectance distributions of the two color samples observed are identical.  These identical samples are seen under different lights sources with differing spectral power distributions  (SPD,  or,  the output of a light source, characterized by its relative strength at each wavelength)

Thus, Sample Metamerism occurs as a result of  differences in the reflectivity of the color samples themselves, and Illuminant Metamerism occurs as a result of differences in the output of the light source itself, under which we are viewing the color samples. (Confusing…I think so…but with careful study of the differences, they can become clear)

Illuminant metamerism is not often seen, unless  the observer use a  light box to see identical samples illuminated by both light sources separately, yet simultaneously.  Again, this type of metamerism is created by differences in the light source only, not in the samples themselves.

The complex phenomena of SPD/Spectral Power Distribution,  Spectral Reflectance Distribution, and Spectral Distribution Curves will be discussed in greater depth in a subsequent  post on Metamerism.  It can take fortitude to keep them all straight!

Observer metamerism: Do we all see color differently?  It is commonly agreed upon that we do, assuming that each individual  possesses adequate color matching aptitude. Observer metamerism can occur because of differences in color vision  from one person to the next. Again the process of seeing is complex, but for the purposes of this post, suffice it to say  that the physical act of seeing, what happens in each person’s eyes and brain when they see an object in whatever light, can alter  their color perception. A common source of observer metamerism is color blindness,  but it occurs with the “normal-seeing”  as well.  In the case of observer metamerism, two lights or surfaces  may be a color match for one observer but not for another.

Geometric metamerism: The angle, distance or light position from which identical colors are viewed may change the color that we see. The distance between a woman’s eyes is, on average, slightly less than a man’s.  This slightly different angle of stereoscopic viewpoint may be why men and women have been known to perceive colors differently!  Most of us have probably had the experience of  two samples appearing to match when viewed from one angle, but then not matching when viewed from another angle.  Examples would be the color variations that appear in pearlescent auto finishes or “metallic” paper. This may be something to think about when using specialty finishes in interiors, fashion and works of art.

I hope this post on Metamerism has lit a fire in your belly to know more about it.  I know will be continuing my research, and delving further into the subject to help demystify it for both YOU and me.

The subject of color, light, and the relationship between the two is so vast, multifaceted and complex, that it will never be demystified completely.  Thus we have a lifetime of color mystery and magic to look forward to.   We can join in the efforts of fearless color explorers through the ages, and add our own special hue…ah, I mean, view (!), to their findings, while enhancing our own knowledge base and experience.