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.

 

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!