Light is visual radiant energy. Princeton's WordNet defines radiant energy as "energy that is transmitted in the form of electromagnetic radiation; energy that exists in the absence of matter." It defines electromagnetic radiation as "radiation consisting of waves of energy associated with electric and magnetic fields resulting from the acceleration of an electric charge." X-rays, ultraviolet, gamma rays, radio waves and infrared energy are all included in the electromagnetic spectrum, and visible light is only a very small part of the electromagnetic spectrum. The Article "The Science of Light", from the website of Sylvania, one of the oldest lighting suppliers in the world, says "As visually perceived radiant energy, light powers the mechanism of sight. It is light, reflected from objects to our eyes, which allows us to see." Sight is our perception of the light that is reflected to our eyes, without light we cannot see.
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There are two measurements used to describe the color properties of a light source: the color temperature of a light, and the color rendering index. The color temperature of light is determined on the Kelvin (K) scale. The system is based around a black body radiator that, when heated, will go from its cold black state to white hot. Its color changes gradually from black to red, to orange, then on to yellow, white, and finally a bluish white color, when the radiator is fully heated. A light source's color temperature will be determined in degrees Kelvin by comparing it to the black body radiator and matching it exactly to the color of the radiator. Oddly enough, warm color's temperatures are actually cool, while the cool color's temperatures are warm. The colors toward the blue end of the spectrum, which is physically warmer, are referred to as cool colors; and colors toward the red, orange, and yellow end of the spectrum, which is physically cooler, are called warm colors. This is because warm, or cool, does not refer to the physical temperature of the light, but rather, the psychological effect of the color. ("Light Color Characteristics")
The color rendering index (CRI) of an artificial light source is its effect on the color of objects and surfaces it illuminates compared to the effect of another common light source. The CRI is a system based on visual experiments. The first step is to determine the color temperature of the light source. If a light source being tested has a temperature ranging from 2000K to 5000K, a black body radiator is used as the other common light source it is compared to; but, if the temperature is above 5000K, the light source being tested is compared to sunlight. The light source being tested is tested on eight different color samples. One by one, they are tested first with the artificial light source, and then either the black body radiator or sunlight. If the color samples did not change in the different light, the color rendering index of the light remains 100, the highest possible rating. If any change in the color's appearance occurs when the light source changes, the rating for the light source being tested goes down, and the rating continues to decrease the more each color sample appears to change. ("Colour Temperature Chart"; "Light Color Characteristics")
Full Spectrum Lighting, a manufacturer of full spectrum lighting, wrote an article on the color rendering index. In this article it is noted that a color rendering index of 100 does not mean the light source is ideal. A light bulb with a CRI of 100 and a color temperature of 3000K only means that the light bulb did not change the appearance of the color sample compared to a black body radiator heated to the same temperature of 3000K. With such a low color temperature the light bulb will give off a warm cast, and it will be hard to visually perceive differences in various shades of blue. The same goes for a light bulb with a color temperature of 6000K or above; it will have a bluish cast and make reds and oranges seem dull. ("Color Rendering Index (CRI) Explained")
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Debbie Zimmer, paint and color expert with the Rohm and Haas Paint Quality Institute, explained that "Sunlight is the purest light and provides the purest color from the spectrum standpoint of the perception of color."(qtd. in Brimo-Cox). But even sunlight changes within the day and with the seasons. In the morning the temperature of sunlight is lower and gives off a warmer yellowish glow. As the day goes on, sunlight's temperature rises. At midday, the temperature is its highest, and gives off a cool, bluish cast. As the sun sets, it starts to cool off again, and sunlight will have a reddish cast. Seasons affect the color temperature of light as well. Winter's sunlight has a bluer cast than in summer. Since sunlight varies so much, a standard of 5000 Kelvin was set, which is the color temperature on a cloudless day at 12 noon. (Brimo-Cox)
There is also artificial light. In a household, incandescent, fluorescent, and halogen are the most common lights sources used. Incandescent and halogen lighting has a cooler temperature, and warms up reds and yellows. Incandescent bulbs have a color temperature of about 2800K, and halogens are a little warmer at 3300K. Fluorescent lights have varying color temperature, but are usually warmer, and give off a bluish tint. Fluorescent lights generally enhance blues and greens, but the latest technology has enabled us to change the color make- up of fluorescent lights.
When light is shown on an object, some wavelengths reflect off of the object to our eyes while others are absorbed by the object. The wavelengths reflected back to our eyes are what our retinas perceive as color. The American Science Dictionary defines color as "the sensation produced by the effect of light waves striking the retina of the eye." The part of our eye called the retina contains four types of light sensors, namely rods and three kinds of cones. These three cones are what enable us to see color. Each set of cones is designed to absorb a different spectrum of visible light. The first type of cones absorbs long wavelengths, or the reds. The second set of cones absorbs mid-length wavelengths, which are greens. The final group of cones is designed to absorb short wavelengths, the blues. These cones then send the information to our brain, and it is processed as color. The color of something depends mainly on which wavelengths it emits, reflects or transmits. Take a blue crayon, for example. The crayon is not actually blue, but when it is illuminated by a white light, it reflects the blue wavelengths and absorbs nearly all of the other color wavelengths. The crayon reflects the blue wavelengths to our retinas, which send the signal to our brain, where the color is perceived as blue. ("Color and Light Relationships"; "What Is Color?"; "What is Color?")
Paint colors also have factors that change the way we will perceive a color. Paint colors have a rating called light reflectance value. Light reflectance value (LRV) is a measurement that measures how much light a color reflects and in turn how much light the color absorbs. The LRV of a paint color deals with the lightness or darkness of a color. The scale runs from 0% to 100% light reflectance. Black, because it is the absence of color, has a LRV of 0%, and no light wavelengths are being reflected; instead they are all absorbed. Absolute white has a LRV of nearly 100% because white is nearly equal amounts of every color, so nearly every wavelength is being reflected to our eyes. Paint colors also have sheens which also affect the reflectance of the paint. There are four sheens: flat, eggshell, semi-gloss, and gloss, with flat being the least reflective and gloss being the most. ("LRV-Light Reflectance Value of Paint"; "Using Light Reflectance Value")
It is important to know the effects different color temperatures and color rendering indexes of a light source will have on our eyes' perception of a paint color. A color can change quite a bit when illuminated by different color temperatures and color rendering indexes of light. A higher Kelvin temperature of light will change the appearance of a color because it is sending out more blue wavelengths. Therefore, if you have, say, a wall painted green, and shine a lower temperature bulb on it, it will appear more yellow. Conversely, if you illuminate the wall with a higher temperature light, it will appear bluer. This is because it is reflecting more of the yellow or blue wavelengths, respectively, and less of the other color wavelengths. The CRI of a light bulb is also very important; it can change the appearance of a color greatly. A color might look fine lit by the black body radiator, or sunlight of a given color temperature, but that does not mean it will look fine illuminated by an artificial light source of that same color temperature. It is also important to note the LRV of paint, because it will cause the color to appear lighter or darker; too low of a value might cause and the color to appear dark and dull. It is also important to choose the best sheen, to increase or decrease the brightness of a color. As we see, there are many things that need to be taken into consideration when choosing the best paint color. Not one of these things alone will ensure you the best results. All the factors which determine the way a paint color is affected by light must be taken into account.
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