Color and Color Matching :

Color is all around us and affects how we feel. This is not new. Early humans likely felt sad under a grey sky, happy under a blue one, and amazed by a rainbow. They used colors from the earth and plants for body painting and decorating their caves. Color quickly became important in trade. Items like cloth, glass, stones, and pottery were traded for food because they were attractive. Today, almost everything in our lives has color: books, movies, TV shows, packages, and even many newspapers.

Marketers have realized that color helps sell products. Brands often have specific colors, and manufacturers demand precise color matching. This puts pressure on ink makers and printers. The science of color has been understood for over 100 years, and some of the math for color measurement was explained 50 years ago. However, earlier we couldn't measure color accurately or process the data fast enough to use these advancements effectively.

Today, we have instruments that measure color more precisely than the human eye, and these can be connected to fast, affordable computers.

The Physical Nature of Color :

Lightness, Hue, and Saturation

Lightness: Lightness describes how bright or dark a color appears. It ranges from black, which has no lightness, to white, which has maximum lightness. Grey shades fall between black and white and can be measured based on their level of lightness. For example, a matt black object has 0% lightness, while a piece of white chalk has 100% lightness.

Hue: Hue is what we typically think of as "color." It refers to the distinct characteristic that distinguishes one color from another, such as red, blue, yellow, etc. The hues can be arranged in a circle, forming a spectrum that includes colors like red, orange, yellow, green, blue, and violet. Newton demonstrated that white light can be split into these hues using a prism.

Saturation: Saturation, also known as intensity or purity, measures the vividness of a color. It indicates how much a color is diluted with white. A color with high saturation looks rich and vibrant, while a color with low saturation appears washed out or pale. For instance, adding blue paint to white paint creates a range of colors from pure white to pure blue, with each step showing increasing saturation of blue.

Together, these three properties—lightness, hue, and saturation—define any color.

As children, we learn to recognize colors like yellow, red, and blue. Black, grey, and white are not typically considered colors, but scientists include them. These colors differ in hue. Black, grey, and white lack hue but differ in lightness. For example, black velvet has no lightness, while white chalk has maximum lightness. Scientists use pure barium sulfate as the standard for 100% white, with black as 0%. Grey objects can be rated based on their lightness between black and white.

Colors also have hue, which Newton showed in 1730 when he split white light into red, orange, yellow, green, blue, and violet using a prism. These hues form a visible spectrum that can be arranged in a circle. Colors also differ in intensity or saturation. For example, adding blue paint to white paint creates a range from pure white to pure blue. This can be done with all hues, creating an almost infinite range of colors. Thus, color is described by three variables: lightness, hue, and saturation.

Most objects do not emit colored light but appear colored when lit. An object looks white if it reflects most light, black if it absorbs all light, and colored if it absorbs some light more than others. For example, an object that absorbs blue light appears yellow, while one that absorbs red light appears cyan.

Reflectance needs careful definition. A mirror reflects light but also images, known as specular reflectance. Matt white paint reflects light without images, called diffuse reflectance. Glossy white paint shows both types of reflectance. Light itself can also be colored. For instance, sodium vapor street lamps emit yellow light, making blue objects look almost black.

Newton observed that light rays are not inherently colored; color perception happens when light interacts with objects or the eye. Thus, the color we see depends on the object's nature and the light illuminating it.

Light Sources

When a black object like a carbon rod is heated, it emits light that changes color with increasing temperature. This is described by its color temperature in Kelvin (K). A tungsten filament lamp has a color temperature of about 2800 K, while north daylight is about 6700 K. The Commission Internationale de l’Eclairage (CIE) set illumination standards in 1931. Illuminant A represents a tungsten lamp at 2800 K, B is sunlight at 4900 K, and C is north daylight at 6700 K. Most color matching uses north daylight, but CIE illuminant C lacks some UV light. A new standard, D65, with a color temperature of 6500 K, closely matches true daylight and is used for more accurate color matching.

Fluorescent tube lamps, more common in shops and factories, emit light through mercury atom collisions. These lamps use phosphors that fluoresce to produce light, with various phosphor blends creating different color temperatures. High-efficiency lamps using rare earth phosphors, like those in color TV tubes, have been introduced by Philips as the Colour 80 series. These lamps emit light in three sharp bands, rather than a continuous spectrum.

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