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.