Fluorescence explained and why it is used and needed in SMV emblems

The American Society of Agricultural (and Biological) Engineers, the “ASAE” of the referenced standard for these emblems… whose standards have been codified into law in the 50 states, Canada and many foreign countries, has since the beginning of the establishment of this emblem indicated that the center area is to be “…a triangular-shaped emblem with a 12-inch-high fluorescent orange center… also referred to in some documents as “red-orange fluorescent.”

Repeated here first is something from the DayGlo Color Corp’s website is this handy refresher. After their piece is material from Wikipedia’s entries on fluorescence and black light paint as they appeared in June, 2015.

“At DayGlo we understand color – especially the Science behind it.

So why is it fluorescents appear so bright and seem to jump out at you? To understand this, we first need to look at how normal colors work.

The Mechanism of Color

The color spectrum is much larger than we can perceive! The range moves from invisible, low-energy infrared rays to high-energy ultraviolet rays. The ‘visible light spectrum,’ or the colors that we see, are actually in the middle of the range.

Normal color absorbs and re-emits a portion of the visible spectrum that matches its principal wavelength, while the remaining colors are absorbed and dissipated as heat. In this way, a regular orange color re-emits back to your eye just the narrow orange band of the visible light spectrum. Therefore, if there is little or no orange light around (such as early dawn or dusk) the orange surface would appear dark or even black.

Fluorescent Colors

How do fluorescent colors behave differently?

Fluorescent colors use a larger amount of both the visible spectrum and the lower wavelengths compared to conventional colors. They not only absorb and convert light energy of the dominant wavelength, but also the wavelengths of ultraviolet rays and other colors lower in the visible spectrum. As a result, your eye perceives a far more intense color.

Where a clean, bright conventional color is able to reflect a maximum of 90% of a color present in the spectrum; a fluorescent color can reflect as much as 200% to 300%.

The Fluorescent Color Theory

So how exactly does a fluorescent color fluoresce? It’s all about energy!

A fluorescent dye molecule begins at the lowest state of energy possible – the Ground State. When light shines on it, a photon of light is absorbed by the molecule. In effect, energy is absorbed. This excites the electrons inside the molecule to higher energy states – known as the Excited State. The excited electrons soon lose some of their energy to their surroundings, which prompts them to transition back to their unexcited state – the Ground State. In order to transition back, the electrons need to emit the photon of light absorbed. This flash of light (the emission of the photon) is the fluorescence we see.

And, that’s how fluorescent colors look so much brighter than conventional ones!”

(from http://www.dayglo.com/who-we-are/fluorescent-color-theory/
and please buy some new psychedelic black light posters at your local independent record store to send this fine company some business)  More information after the pictures below*.

blacklight posters
Safety Psychographics… Proudly Powered by the Spirit of the 1960s. Even our phone number is on board —1969! 🙂

From Wikipedia, the free online encyclopedia: (excerpts from: https://en.wikipedia.org/Fluorescence, https://en.wikipedia.org/wiki/Blacklight_paint)

The word ‘dayglo’ has become a genericized trademark, as it is used as an ordinary noun while ‘Dayglo’ is a registered trademark of the DayGlo Color Corporation.

The invention of fluorescent paints is attributed to Robert Switzer, who was confined to a dark room after a fall, and his brother Joseph, who was a chemistry major at UC Berkeley, in 1934. They took a black light into the storeroom of their father’s drugstore looking for naturally fluorescent organic compounds and from that developed paints.

Blacklight paints and inks are commonly used in the production of blacklight posters. Under daylight, the ultraviolet light ordinarily present makes the colors especially vivid. Under blacklight (with little or no visible light present), the effect produced can be psychedelic. The inks are normally highly sensitive to direct sunlight and other powerful light sources. The fluorescent dyes cause a chemical reaction when exposed to high intensity light sources (HILS) and the visual result is a fading in the colors of the inks. With paper, significant visible change in the color saturation can typically be observed within 45 minutes to one hour of exposure to the HILS. To date, there is no absolute method to prevent this phenomenon, although certain laminations, lacquer coatings and glass or plastic protective sheets can effectively slow the fading characteristics of the dyes.

{Note: the fluorescent dyes, coatings and films used in making quality SMV emblems and highway signs have significantly longer lives under bright +/or UV light exposure than the short paper life mentioned here. Emblem and sign life is now typically measured in years. However, no fluorescent material will stay bright forever; hence, it behooves the SMV sign users to follow published guidelines and cover their emblems when not actively being used on roadways. The guidelines state this because of the “dilution” effect when emblems are inadvertently viewed while stationary. The added benefit is that the life of the fluorescent colors are prolonged significantly and therefore the usable life of the emblems are also prolonged, giving a better return on investment.}

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, and the emitted light is in the visible region.

An early observation of fluorescence was described in 1560 by Bernardino de Sahagún and in 1565 by Nicolás Monardes in the infusion known as lignum nephriticum (Latin for “kidney wood”). Strongly fluorescent pigments often have an unusual appearance which is often described colloquially as a “neon color.” This phenomenon was termed “Farbenglut” by Hermann von Helmholtz and “fluorence” by Ralph M. Evans. It is generally thought to be related to the high brightness of the color relative to what it would be as a component of white. Fluorescence shifts energy in the incident illumination from shorter wavelengths to longer (such as blue to yellow) and thus can make the fluorescent color appear brighter (more saturated) than it could possibly be by reflection alone.

Fluorescence occurs when an orbital electron of a molecule, atom or nanostructure relaxes to its ground state by emitting a photon of light after being excited to a higher quantum state by some type of energy. (*alert: potential TMI ahead)

Excitation:  S_0 + h \nu_{ex} \to S_1

Fluorescence (emission):  S_1 \to S_0 + h \nu_{em} + heat

Here  h\nu is a generic term for photon energy with h = Planck’s constant and \nu = frequency of light. The specific frequencies of exciting and emitted light are dependent on the particular system.

State S0 is called the ground state of the fluorophore (fluorescent molecule) and S1 is its first (electronically) excited state.

A molecule, S1, can relax by various competing pathways. It can undergo non-radiative relaxation in which the excitation energy is dissipated as heat (vibrations) to the solvent. Excited organic molecules can also relax via conversion to a triplet state, which may subsequently relax via phosphorescence or by a secondary non-radiative relaxation step.

Relaxation of an S1 state can also occur through interaction with a second molecule through fluorescence quenching. Molecular oxygen (O2) is an extremely efficient quencher of fluorescence just because of its unusual triplet ground state.

In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation. The emitted radiation may also be of the same wavelength as the absorbed radiation, termed “resonance fluorescence”.

Molecules that are excited through light absorption or via a different process (e.g. as the product of a reaction) can transfer energy to a second ‘sensitized’ molecule, which is converted to its excited state and can then fluoresce.

The fluorescence quantum yield gives the efficiency of the fluorescence process. It is defined as the ratio of the number of photons emitted to the number of photons absorbed.

 \Phi = \frac {\text{Number of photons emitted}} {\text{Number of photons absorbed}}

The maximum fluorescence quantum yield is 1.0 (100%); each photon absorbed results in a photon emitted. Compounds with quantum yields of 0.10 are still considered quite fluorescent. Another way to define the quantum yield of fluorescence, is by the rate of excited state decay:

 \Phi = \frac{ { k}_{ f} }{ \sum_{i}{ k}_{i } }

where { k}_{ f} is the rate constant of spontaneous emission of radiation and

 \sum_{i}{ k}_{i }

is the sum of all rates of excited state decay. Other rates of excited state decay are caused by mechanisms other than photon emission and are, therefore, often called “non-radiative rates”, which can include: dynamic collisional quenching, near-field dipole-dipole interaction (or resonance energy transfer), internal conversion, and intersystem crossing. Thus, if the rate of any pathway changes, both the excited state lifetime and the fluorescence quantum yield will be affected.

Fluorescence quantum yields are measured by comparison to a standard. The quinine salt quinine sulfate in a sulfuric acid solution is a common fluorescence standard. Quiz to follow next week.

 

 

 

 

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