Examining the Chemistry of Light and Color

The year 2019 is the 150th anniversary of the periodic table of elements. In 1869, Russian chemistry professor Dmitri Mendeleev published his version of the periodic table, which acquired wide-ranging acceptance for two reasons:

  1. He left spaces in the table where it appeared that an as-yet-undiscovered element would be placed. Developments in the extant periodic table allowed him to forecast the characteristics of such absent elements, such as gallium (Ga).
  2. He didn’t arrange his elements according to their atomic weight, but rather ordered them by chemical families (grouping elements with similar properties).

Mendeleev’s periodic table of the elements, ca. 1871.

Mendeleev’s periodic table of the elements, ca. 1871.

Mendeleev’s original table possessed a mere 63 elements; continued research and discovery have increased the total on contemporary periodic tables to typically 108-109 elements. To commemorate the 150th anniversary, the U.S. Department of Energy’s Idaho National Laboratory has produced an interactive periodic table.

The Appearance of Color

Light (both visible and invisible) represents an electromagnetic phenomenon generated by the sun’s radiation. Humans see color as a consequence of the interaction of light with our eyes’ rods and cones, as well as the additional biological components of our visual system. When light interacts with an object, the physical features of that object (which include its chemical composition) decide how it absorbs, reflects, and/or emits light, influencing our visual perception.

Visible light comprises every color from red to violet. An object obtains its color when electrons absorb energy from the light and undergo “excitation” (elevation to a state of increased energy). The excited electrons absorb particular wavelengths of light. What humans perceive is the complementary color of the absorbed wavelengths, i.e. the residual wavelengths of light that have not been absorbed. For instance, if an object absorbs the red wavelengths of light, humans will see green (red’s complementary color).

The corresponding colors of the absorbed wavelengths and the complementary color (what we see). (Image Source: LibreTexts™ Chemistry).

The corresponding colors of the absorbed wavelengths and the complementary color (what we see). (Image Source: LibreTexts™ Chemistry).

The Color of Chemicals

Numerous chemicals and chemical compounds appear colorless because they absorb UV or additional wavelengths of light that do not form part of the visible spectrum. Chemicals that appear colored absorb wavelengths in the visible spectrum; such colored chemicals are known as chromophores. The color perceived by humans, as well as its brightness and its intensity, is dependent on the shape of the absorption spectrum of the substance, which is ultimately derived from that substance’s chemical structure.

The absorption spectrum of the chemical compound chlorophyll a (C55H72MgN4O5). Because it absorbs primarily the violet/blue and orange/red wavelengths, chlorophyll a—the substance that is essential to photosynthesis in plants—appears green to our eyes, giving plants their green hue. (Image Source: NASA)

The absorption spectrum of the chemical compound chlorophyll a (C55H72MgN4O5). Because it absorbs primarily the violet/blue and orange/red wavelengths, chlorophyll a—the substance that is essential to photosynthesis in plants—appears green to our eyes, giving plants their green hue. (Image Source: NASA)

Chemical Emissive Properties

Every substance possesses its absorption spectrum, which is the exact inverse of its corresponding emission spectrum. Whereas absorption is brought about by excitation of the electrons, which moves them from a lower to a higher energy level, the emission is stimulated by the electrons falling back down to a lower energy state (“relaxation”), causing the release of a photon—a unit of electromagnetic radiation. Traveling at divergent wavelengths, the released photons generate a signature for each substance, which is expressed in terms of the visible spectrum – that is, in a sort of color map.

Scientists have a methodology for determining the signature of a substance as a means of evaluating its elemental makeup. Whenever a substance’s emitted light is passed through a prism, it undergoes diffraction into its frequencies. This creates a signature pattern of colored lines, known as the atomic emission spectrum, which is unique to every element. By examining a substance’s signature (atomic emission spectrum), it becomes possible to identify which elements are present.

The atomic emission spectra of hydrogen (H), neon (Ne) and iron (Fe) (Image Source: Mathematica. Stackexchange.com)

The atomic emission spectra of hydrogen (H), neon (Ne) and iron (Fe) (Image Source: Mathematica. Stackexchange.com)

Luminescence (also known as cold-body radiation) refers to the emission of visible light by a substance as a result of electron excitation and photon release. Excitation is most typically a consequence of the absorption of light, although alternative stimuli, including chemical reactions, physical agitation, or an electrical current, might also bring about the emission of photons. Certain substances emit visible light only following exposure to light as a means of exciting their atoms; others, such as phosphorus (P), glow as a consequence of chemiluminescence: the chemical reaction that takes place when phosphorus interacts with oxygen (O).

Gaseous elements may emit light following heating or when electrical energy is applied to excite their atoms. The latter excitation approach is how neon signs function (the name is technically inaccurate, as they do not all contain neon gas). Glass tubes comprising divergent gases are utilized for creating an array of colors. For instance, helium (He) glows pink, neon (Ne) generates red-orange light, argon (Ar) is blue, krypton (Kr) is pale green, and xenon (Xe) glows pale blue.

A multi-hued neon sign includes glass tubes bent into shapes, filled with various gases to which an electrical current is applied to excite the electrons and produce a steady glow.

A multi-hued neon sign includes glass tubes bent into shapes, filled with various gases to which an electrical current is applied to excite the electrons and produce a steady glow.

The Chemistry of LEDs

LEDs (light-emitting diodes) utilize the chemical and electromagnetic characteristics of light and color. LEDs are produced using materials that conduct electricity under certain conditions, known as semiconductor materials. Elements located in the middle of the periodic table are typically insulators which inhibit the flow of electrical current. However, a chemical process referred to as “doping” (integrating additional materials) converts them into semiconductors.

For instance, silicon (Si) is typically an insulator, but the incorporation of a small number of atoms of the element antimony (Sb) augments the number of free electrons, thus creating an “n-type” (negative-type) semiconductor. Likewise, if atoms of boron (B) are mixed with silicon, they effectively extract electrons from the silicon, exposing "holes" where there should be electrons. This category of silicon is known as p-type (positive type) as the holes, which are also mobile, embody a positive electric charge.

Conventionally, LED semiconductors utilize gallium (Ga)-based materials—for instance, gallium nitride (GaN) or gallium phosphide (GaPO4). The color of light emitted by an LED is regulated by the material that has been utilized.

LEDs are constituted by two layers of semiconducting material which undergo doping to produce an n-type and p-type layer. Whenever electrical current is applied, the electrons in the n-type layer and the electron holes in the p-type layer are simultaneously pushed to an active layer (or conduction layer) sandwiched between the two semiconductor layers. The free electrons subsequently fit into the holes, causing the release energy in the form of photons or visible light.

Simplified schematic of an LED. (Image courtesy of www.ucusa.org)

Simplified schematic of an LED. (Image courtesy of www.ucusa.org)

The energy divergence between the n-type and p-type layers is referred to as the band gap. The size of the band gap regulates the color generated by the LED. The larger the band gap, the shorter the wavelength of generated light. Therefore, for a red LED (red possesses a long wavelength), only a small band gap is needed. For blue LEDs, a larger band gap is required.

It is more straightforward to generate LEDs with smaller band gaps, and it thus took significant time for researchers to determine the appropriate chemical mix of materials to generate the large band gap required for blue LEDs. Blue LEDs were finally produced in the 1990s utilizing gallium nitride. This milestone allowed color mixing for LED-based electronics such as lights and displays because every color of LED (red, green, and blue) is required to generate an array of colors, including white light.

Infographic depicting the chemical composition of different colored LED lights. (Image Source: Compound Interest)

Infographic depicting the chemical composition of different colored LED lights. (Image Source: Compound Interest)

Measuring LED Luminance and Color

Radiant Vision Systems has developed integrated solutions for both R&D and production-line quantification of illuminance, luminance, intensity, and chromaticity of multiple lighting sources, comprising LEDs, LED arrays & display modules, LED light strips, and more.

It is possible to find out more about Radiant solutions for LED and lighting measurement utilizing its ProMetric® selection of scientific-grade imaging photometers and colorimeters. To learn about the criteria for choosing the optimal methodology of LED and light source testing, please refer to Radiant’s white paper “Choosing a Measurement System for LED Sources, Luminaires, and Displays.”

Acknowledgments

Produced from materials originally authored by Anne Corning from Radiant Vision Systems.

This information has been sourced, reviewed and adapted from materials provided by Radiant Vision Systems.

For more information on this source, please visit Radiant Vision Systems.

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