Advanced UV for Life
What Color Is UV Light?
Ultraviolet radiation, commonly referred to as “UV light,” surrounds us almost everywhere – it shines from the sun, reflects off surfaces, and even illuminates the world for some of our animal and plant neighbors.
For us humans, however, UV remains a realm of complete invisibility. Our eyes, tuned to the familiar rainbow spectrum of visible light in the wavelength range of approximately 380–750 nm, simply cannot detect the higher-energy UV photons below 380 nm. This is why physicists often refer to it not as UV light, but as UV radiation.
But what if we could see UV radiation? What would its “colors” look like?
On This Page
UV - An Introduction
The (In)visible UV Color Palette
In print and on screens, color is our universal language for conveying information. From warning signs to weather maps, we rely on a shared understanding of color to communicate. But when it comes to UV, there is no such consensus – because biologically speaking, there is no color for UV as far as humans are concerned. Our eyes’ three types of cone cells (trichromacy) are not sensitive to these shorter wavelengths, so the “true color” of UV remains a blank canvas to us (1).
In everyday life, we usually encounter UV light indirectly: certain materials and colors start to glow under blacklight or UV lamps. We owe this to the physical phenomenon of fluorescence. In this process, invisible UV light is absorbed by special substances and re-emitted as visible light of a particular color. This creates glowing effects, like those seen in clubs, blacklight theaters, or the security features of banknotes (2)(3). So-called daylight fluorescent colors also convert the invisible UV portion of sunlight into visible, highly intense hues known as neon colors (4).
False-Color Representation in the UV (up to approx. 400 nm) and IR (from approx. 750 nm) ranges.
Only visible light (approx. 400–750 nm) is perceived by the human eye as “true color.”
UV through Animal’s Eyes
For some animals, UV light is not only visible – it’s vital. Birds, bees, some fish, reptiles, and even mammals such as reindeer can perceive UV light thanks to specialized photoreceptors in their eyes (5)(6). Some birds and insects are tetrachromats, meaning they have a fourth type of cone cell sensitive to UV. This allows them to perceive a color spectrum that is literally invisible to humans (5)(7).
Reindeer, for example, have eyes that allow UV light to reach the retina, helping them detect food and predators in the snowy Arctic. Scientists believe that reindeer may perceive their winter world in hues that could be described as “violet” or “blue” – but these are merely analogies. The actual color experience of UV for a reindeer or a bee is fundamentally unknowable to us, as their brains process these signals in ways with no human equivalent (8).
The Limits of Human Vision
If we wanted to assign “colors” to UV light for practical purposes – for example, in scientific imaging or design – we would have to do so arbitrarily. Often UV false-color images use shades of violet, purple, or blue to represent UV, but these are just conventions and do not reflect true perception.
And even though a few people with certain eye diseases (such as aphakia, the absence of the eye lens) report perceiving UV as a kind of bluish-white glow, it is not a true UV color, but an encroachment on the visible color spectrum (9).
The color of the Advanced UV for Life
As Advanced UV for Life association, we are dedicated to the development and application of UV technologies – from UV LEDs and UV sensors to applications in medicine, environmental technology and production. The association’s current logo is designed in the main color #431b5e, a deep, saturated purple. This colour was chosen according to aesthetic and design criteria and is, of course, in the visible spectrum.
What would happen if the logo is printed on white paper with a wavelength of 300 nm?
If the Advanced UV for Life logo were printed on white paper with a wavelength of 300 nm – i.e. in the UV range – it would be invisible to the human eye. Our retina does not contain photoreceptors that react to such short-wave radiation. A logo that only reflects UV in the range of, say, 300 nm would appear invisible on white paper – the paper would appear blank.
However, under special conditions, such as the use of fluorescent fabrics, UV radiation could be converted into visible light by fluorescence, making the logo indirectly visible. Without this effect, however, a pure UV color logo remains invisible.
Neon Colors and Fluorescence – The Conversion of Invisible UV into Visible Light
Special fluorescent colours, such as neon colours, have the ability to convert the invisible UV component of daylight into visible, particularly intense neon colours. This physical effect is called fluorescence. Fluorescent dyes or pigments absorb UV radiation and convert it into visible light with a longer wavelength. The result is a particularly bright and intense colour that shines through the emission of the transformed light. These colours are visible both in daylight with a certain UV component and under targeted UV lamps.
Logo of the Advanced UV for Life: 1st image: Colour violet #431b5e for the screen display. 2nd image: The logo in a UV color on white paper. 3rd image: The logo in “UV color” on a standard RGB screen with additional (theoretically existing) UV-emitting pixels.
What would we see on a screen with UV-emitting pixels?
Similarly to printed UV inks, screens are equipped with color pixels that emit light in the UV range (e.g., at 300 nm), we wouldn’t be able to see those pixels either. Our eyes are not sensitive to UV light, and even modern displays only use light sources in the visible spectrum, as only these are perceptible to humans. A screen that displays a logo with UV pixels would look the same to us as a switched off or black screen – regardless of how much UV radiation is actually emitted.
ICULTA 2026 logo in the original and in neon colors. In the printing area, fluorescent dyes are used that absorb UV radiation and convert it into visible light. The fluorescent effect is not feasible on screens. The impression of a neon effect is achieved here by a high contrast between a signal color – such as orange and green – and the black background.
Only when UV radiation hits materials that are able to convert them into visible light – such as special fluorescent substances – could we perceive an indirect effect.
Choosing the color #431b5e for the Advanced UV for Life or ICULTA logo is a conscious choice to create a brand image that is recognizable to humans. However, pure UV light, whether on paper or on a screen, remains invisible to the human eye.
The universal reference system for colour in technology, science and design is the CIE scale. It ensures that colors can be clearly communicated and reproduced as faithfully as possible, regardless of the device or medium used. It also defines how, for example, the logo color is #431b5e perceived by an “average” observer.
Important note: The term “UV” in UV inks, UV varnishes or UV adhesives used in the printing industry or in material finishing (e.g. wood) refers to the fact that these products are cured by UV radiation. This “curing” plays a central role in the application of UV radiation. UV curing enables extremely fast curing (or drying) of varnishes, inks and adhesives, often within seconds.
UV curing requires less energy compared to conventional curing processes such as heat, which makes UV curing the energy-efficient standard in many industrial sectors.
UV “light” and the CIE color diagram
The CIE color spaces, such as the CIE XY system, are mathematical models that define the relationship between the visible spectrum and human color perception. They are based on the reactions of an idealized “norm observer” who idealizes the color perception of an average person and only includes wavelengths that stimulate the human eye – i.e. approximately from 380 nm (violet) to 750 nm (red). The outer edge of the CIE chromaticity diagram, the so-called spectral locus, is defined exclusively for these visible wavelengths (10).
UV and infrared are not included in the CIE chromaticity diagram. The CIE system is fundamentally tied to human perception, not just physical wavelengths (10).
If the CIE chromaticity diagram were theoretically extended to UV wavelengths up to 10 nm, artificial color matching functions (CMFs) would have to be defined for these areas. However, the existing CMFs are zero for UV and IR because the human eye has no sensitivity in these areas (10). If UV wavelengths were to be entered in the diagram, they would not correspond to any visible color and would have no position within the classical color space. If one were to artificially assign non-zero values, “imaginary colors” would emerge – points outside the human perceptual spectrum that have no physical or perceptible meaning.
The CIE XY color space already contains ranges that correspond to so-called “imaginary” or “theoretical colors” (e.g. through the combinations of tri-stimulus values that are not produced by any real light spectrum). UV wavelengths – if forcibly inserted – would extend the spectral locus into the range of these imaginary colors, without this corresponding to a real or visible color for humans.
Mathematical representation of CIE-xy color spaces (standard color chart, without brightness representation). The lowest color – which is practically still visible – is given in the graphic as 380 nm (bottom left). Colors below this wavelength are no longer perceptible to humans.
Depiction by Torge Anders, https://de.m.wikipedia.org/wiki/Datei:CIE-Normfarbtafel.png
Qualification and quantification in the UV range
Even though UV radiation remains invisible to the human eye, the precise qualification and quantification of these wavelengths is of enormous importance. Precise measurements in the UV range are indispensable, especially in scientific, technical and industrial applications.
The international standard ISO 21348 plays a key role. This standard defines and classifies the different wavelength ranges of solar-related radiation – especially in the aerospace sector – and includes the ultraviolet spectrum. It establishes standards for the description of radiation, thus providing a uniform basis on which comparable and reliable data on solar irradiance can be collected worldwide (11). This is particularly important for areas such as research, climatology, aerospace, medicine, materials testing and numerous technical applications where precise information on UV radiation and its effect on materials, biological systems or technical processes is required.
In order to meet these requirements, there are specialized manufacturers of measuring devices that focus on UV measurement technology. Examples include the members of Advanced UV for Life: → Gigahertz Optik GmbH and → Opsytec Dr. Gröbel GmbH (12)(13). These companies develop and produce high-precision UV measurement solutions, including UV sensors, UV meters, spectrometers, UV light sources, UV (VIS-NIR) radiometers, spectroradiometers, and spectral lux meters. These devices can be used to precisely record, characterize and document UV radiation.
In UV technology, precise measurement is particularly critical: it ensures the safe application of UV radiation in areas such as medicine (e.g., phototherapy), water treatment, plastic curing, forensics, quality control, manufacturing, and scientific experiments. Without reliable UV measurement methods, many of these applications would either not be possible or would be associated with significant risks. They thus form the foundation for safety, efficiency and innovation in the entire UV technology.
Summarized
The “true colors” of the UV remain a mystery – not because they are hidden, but because they simply do not exist visibly for us humans. As we continue to explore the UV world with science and technology, we can only guess what it looks like for the creatures that live in it. For now, the colors of the UV belong to bees, birds, and reindeer – and to the limitless possibilities of our imagination. The theoretical integration of UV into color systems such as the CIE diagram remains a mathematical gimmick, without reference to human perception – the colors of UV are and remain an invisible secret for us. But the effects of UV radiation are invaluable to us.
| Wavelength range | Representation in the CIE diagram | Perceptual significance |
|---|---|---|
| 380–750 nm | Spectral locus (visible) | Real, visible colors |
| <400 nm (UV) | Not Shown | No Human Color; “imaginary” in the case of forced representation |
| >750 nm (IR) | Not Shown | No Human Color; “imaginary” in the case of forced representation |
Sources
CIE 1931 Color Space – Wikipedia. Accessed 25 May 2025. https://de.wikipedia.org/wiki/CIE-Normvalenzsystem
Invisible Colours – A. Haussmann GmbH. Accessed on 22 May 2025. https://www.ahaussmann.com/praxisloesungen/unsichtbare-farben/
Ultraviolet Light – Making the Invisible Visible – VCP Blog. Accessed 28 May 2025. https://vcp.de/pfadfinden/auf-neuem-pfad/ultraviolettes-licht-violettes-licht-unsichtbares-sichtbar-machen/
DAYLIGHT FLUORESCENT PAINTS – Coates. Accessed 29 May 2025. https://www.coates.de/images/Service-Support/Fachartikel/fachartikel_pdf/farbmetrik/agesleuchtfarben.pdf
Animals See a World That’s Completely Invisible to Our Eyes – All About Vision. Accessed 27 May 2025. https://www.allaboutvision.com/eye-care/pets-animals/how-animals-see/
How Science Came To See Ultraviolet Light In Animals – Science Friday. Accessed 24 May 2025. https://www.sciencefriday.com/articles/ultraviolet-light-animals/
Animals see the world in different colours than humans – The Conversation. Accessed 21 May 2025. https://theconversation.com/animals-see-the-world-in-different-colours-than-humans-do-110973
Tests show Arctic reindeer “see in UV” – BBC News. Accessed 31 May 2025. https://www.bbc.com/news/science-environment-11314429
Why do some people see ultraviolet light? – BBC Future. Accessed 26 May 2025. https://www.bbc.com/future/article/20150727-the-people-who-see-ultraviolet
CIE color spaces and the spectral locus – All About Vision. Accessed 27 May 2025. https://www.allaboutvision.com/eye-care/pets-animals/how-animals-see/
ISO 21348: Definitions of Solar Irradiance Spectral Categories – ISO. Accessed 27 May 2025. https://www.iso.org/standard/54564.html
Gigahertz Optik GmbH – UV measurement technology. Accessed May 30, 2025. https://www.gigahertz-optik.de/deutsch/produkte/uv-messtechnik/
Opsytec Dr. Gröbel GmbH – UV measuring devices. Accessed 28 May 2025. https://www.opsytec.com/produkte/uv-messgeraete/
