Picture framing glass
Encyclopedia
Picture framing glass usually refers to flat glass or acrylic ("plexi") used for framing artwork and for presenting art objects in a display box (also, "conservation framing").
, heat
, and soiling. Laminated glass
and some acrylic
may be used to protect against physical damage from glass breakage and to offer protection from a malicious attack. Regular glass as well as some glass surface treatments can also filter some of the damaging ultra-violet radiation (UV) and heat (NIR). Artworks that require protective glazing are those rendered on paper or fabrics (including photographs), which contain pigments and dyes that absorb UV and are susceptible to discoloration. In the case if the framed object or artwork is UV resistant, UV protection can still serve the purpose of preserving the integrity and colors of non-conservation grade framing materials susceptible to UV damage, such as mat board (passe partout).
Although protection is a primary purpose of glazing, displaying an artwork is the primary purpose of framing it. Therefore, the least visible glazing best displays the artwork behind it. Visible light transmission is the primary measure of glass’ invisibility, since the viewer actually sees the light, reflected from the artwork. Light transmission of glass is especially important in art framing, since light passes through the glass twice – once to illuminate the artwork, and then again, reflected from the artwork, as colors - before reaching the viewer.
Light transmission
(for this article, the perceivable visible spectrum
between 390 nm and 750 nm is considered) through glass is diminished either by light reflection or light absorption of the glazing material. The total light transferred through the glazing material (light transmission) is reduced by reflection and/or absorption. In art framing, light reflection causes glare
, while light absorption also may cause the transmitted colors to be dulled or distorted. While type of the glass substrate will affect the light absorption of the glazing, the surface treatment can affect light scattering
, light reflection and in some cases, light absorption. There are various glazing options to achieve this goal as explained in the following sections on Types of Picture Framing Glass.
is most commonly used for picture framing glass. Glass thicknesses typically range from thin 2.0 mm, to 2.5 mm. Clear glass has light transmission
of ~ 90%, absorption of ~ 2%, and reflection of ~ 8%. Whereas absorption can be reduced by using low-iron glass, reflection can only be reduced by an anti-reflective surface treatment.
, the light transmission will be significantly better vs. clear glass. Low iron glass has light transmission of ~91.5% and reflection of 8%.
offers shatter-resistance and protection from malicious breakage to art glazing. The most commonly used configuration is Glass + PVB Foil + Glass. Some variations of foils and glass thicknesses can offer shatter- and breakage-resistance or even bullet-resistance. The absorption of laminated glass depends on the glass substrates and foils used in the laminating process. Reflection of laminated glass is similar to monolithic glass, unless surface treatments are applied to reduce reflection.
can have the high light transmission and optical quality of glass. Acrylic is also light weight, compared to glass, and is shatter-resistant, making acrylic an attractive choice for framing large, over-sized works of art. In general, acrylic sheet scratches easily and retains a static charge, which can be problematic when framing pastels or charcoals. Some manufacturers, add dyes to acrylic glass
to filter the UV light transmittance, and its surface can also be treated with both anti-static and anti-reflective coating
s.
, as a light beam travels from air (refractive index
of ~1) into glass or acrylic (refractive index
of ~1.5) and then back into air, these transitions cause part of the light to be reflected. While "anti-glare" (a.k.a. "non-glare" or matte finish) glass treatments focus on scattering the light, "anti-reflective" coatings actually reduce the amount of light, which is reflected from each glazing surface, which has the benefit of increasing the amount of light transmitted through the glazing.
e glass is to transform the specular reflection
into reflection haze
. So-called "scattering
" of the reflected light renders reflected images blurry, so that distinct reflected shapes and sources of light do not distract from the art viewing experience. Scattering the light does not reduce the reflection
or absorption
, which remain at the level of the glass substrate. There are several ways of making the glass surface matt
e – from pressing the pattern when the glass is still soft to fine etching of the glass surface by acid. The quality of matt
e glass is usually determined by its gloss factor or haze factor.
or sol-gel process (or other processes, which can control the uniformity of deposition on nanometer-scale), and can reduce the light reflection to lower than 0.25% per side (0.5% total).
The definition of the upper limit of UV protection as 380 nm by the framing industry is not consistent with accepted standards above.
According to the Library of Congress Preservation department, the artwork damage does not stop at 380 nm though, and all radiation (UV, visible, IR) has the potential to damage art. Thus, calculating a simple average of all wavelengths between 300 nm and 380 nm does not account for the fact that different wavelengths have different artwork damage potential. At least two other methods exist, which provide a more holistic measurement of radiation damage, from both the UV and visible portions of the spectrum:
For picture framing purposes, it is not appropriate to use these methods for absolute ratings, since "Better" ratings are obtained with lower visible light transmission, which is not aesthetically desirable in a framing glazing. However, by including more art damaging factors than UV radiation between 300 nm and 380 nm, these methods provide a more holistic relative ranking tool. For example, comparing a 99% and 92% UV Blocking glazing, would translate to 44% and 41%, respectively, under the KDF.
filtering is necessary in art framing is complex and controversial, driven by conflicting corporate interests. There have so far been no independent organizations, not tied to corporate sponsors, which have presented scientifically verifiable and conclusive evidence to the amount of UV filtering necessary for a glazing to both display and at the same time protect an artwork. On one hand, the issue is complicated by the varying amount of damaging light actually present in an indoor environment (from low level indirect sources to direct daylight). On the other, by the fact that not only UV
, but also visible light damages an artwork. According to the National Fenestration Rating Council, only 40% of artwork fading is caused by UV radiation. The remaining damage comes from the visible light, heat, humidity and material chemistry. This means that increasing visible light transmission by an anti-reflective coating actually increases the amount of damaging radiation on an artwork.
One of the most thorough and independent studies was conducted by the US Library of Congress in an effort to display and preserve the US Declaration of Independence. At first, it was decided to use special yellow "Plexiglass UF3", which removes both the ultra-violet as well as the blue end of the visible spectrum, with significant, but acceptable interference for viewing. Sealing the display by a chemically inert gas such as nitrogen, argon or helium also aided its preservation. In 2001, the display of the US Declaration of Independence was revised to include a multi-laminated glazing for shatter-resistance, with sol-gel interference-based multi-layer Anti-Reflective coatings on outer surfaces to improve the visibility of the document.
From the above evidence, it can be concluded that if preservation were the only goal of glazing, then only a climate-controlled, dark space would offer the best possible protection for an artwork, which can be exhibited once in every several years, while no glass at all provides a perfect displaying option. Therefore, for those artworks, which are chosen to be displayed, the ideal amount of UV blocking should be as much as possible, without affecting the visible light transmission.
.
The relative amounts of damaging light in equal quantities of light:
The above indicates that the damage level of even direct sunlight coming from the vertical skylight is reduced to 36% by regular window glass. Due to the sun's changing position, even less direct light enters through side windows and hanging an artwork away from direct sunlight reduces the exposure to potentially damaging direct sunlight even further.
Indoor lighting, especially fluorescent lighting, is considered to contain some UV Light. GELighting.com asserts that "UV exposure from sitting indoors under fluorescent lights at typical office light levels for an eight-hour workday is equivalent to just over one minute of exposure to the sun in Washington D.C. on a clear day in July. Additionally, the relative damage of incandescent light is 3 times less than that of fluorescent light. Since UV filtering picture framing glass does not protect against all damage factors, it is important to display framed artwork in a well controlled environment to reduce the effects of heat, humidity, and visible light.
Purpose
The primary purpose of glazing in art framing is to physically protect the work of art from damaging factors such as humidityHumidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
, heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
, and soiling. Laminated glass
Laminated glass
Laminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, typically of polyvinyl butyral , between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high...
and some acrylic
Acrylic glass
Poly is a transparent thermoplastic, often used as a light or shatter-resistant alternative to glass. It is sometimes called acrylic glass. Chemically, it is the synthetic polymer of methyl methacrylate...
may be used to protect against physical damage from glass breakage and to offer protection from a malicious attack. Regular glass as well as some glass surface treatments can also filter some of the damaging ultra-violet radiation (UV) and heat (NIR). Artworks that require protective glazing are those rendered on paper or fabrics (including photographs), which contain pigments and dyes that absorb UV and are susceptible to discoloration. In the case if the framed object or artwork is UV resistant, UV protection can still serve the purpose of preserving the integrity and colors of non-conservation grade framing materials susceptible to UV damage, such as mat board (passe partout).
Although protection is a primary purpose of glazing, displaying an artwork is the primary purpose of framing it. Therefore, the least visible glazing best displays the artwork behind it. Visible light transmission is the primary measure of glass’ invisibility, since the viewer actually sees the light, reflected from the artwork. Light transmission of glass is especially important in art framing, since light passes through the glass twice – once to illuminate the artwork, and then again, reflected from the artwork, as colors - before reaching the viewer.
Light transmission
Transmittance
In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, which is the fraction of radiation absorbed by a sample at a specified wavelength...
(for this article, the perceivable visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
between 390 nm and 750 nm is considered) through glass is diminished either by light reflection or light absorption of the glazing material. The total light transferred through the glazing material (light transmission) is reduced by reflection and/or absorption. In art framing, light reflection causes glare
Glare
Glare may refer to:* Glare is difficulty seeing in the presence of very bright light* A glare is a facial expression of squinted eyes and look of contempt* A call collision in telecommunications* GLARE, an advanced aerospace material...
, while light absorption also may cause the transmitted colors to be dulled or distorted. While type of the glass substrate will affect the light absorption of the glazing, the surface treatment can affect light scattering
Light scattering
Light scattering is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media...
, light reflection and in some cases, light absorption. There are various glazing options to achieve this goal as explained in the following sections on Types of Picture Framing Glass.
Regular (or "Clear")
Due to widespread availability and low cost, Soda Lime GlassSoda-lime glass
Soda-lime glass, also called soda-lime-silica glass, is the most prevalent type of glass, used for windowpanes, and glass containers for beverages, food, and some commodity items...
is most commonly used for picture framing glass. Glass thicknesses typically range from thin 2.0 mm, to 2.5 mm. Clear glass has light transmission
Transmittance
In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, which is the fraction of radiation absorbed by a sample at a specified wavelength...
of ~ 90%, absorption of ~ 2%, and reflection of ~ 8%. Whereas absorption can be reduced by using low-iron glass, reflection can only be reduced by an anti-reflective surface treatment.
Low-iron (or "Extra-Clear," "Water White," etc)
Low-iron, or water white glass, is made using special iron free silica, and is generally only available in 2.0 mm thicknesses for picture framing applications. Because low iron glass light absorption can be as low as 0.5% vs. ~ 2% for clear glassFloat glass
Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and various low melting point alloys were used in the past. This method gives the sheet uniform thickness and very flat surfaces. Modern windows are made from float glass...
, the light transmission will be significantly better vs. clear glass. Low iron glass has light transmission of ~91.5% and reflection of 8%.
Laminated Glass
Laminated glassLaminated glass
Laminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, typically of polyvinyl butyral , between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high...
offers shatter-resistance and protection from malicious breakage to art glazing. The most commonly used configuration is Glass + PVB Foil + Glass. Some variations of foils and glass thicknesses can offer shatter- and breakage-resistance or even bullet-resistance. The absorption of laminated glass depends on the glass substrates and foils used in the laminating process. Reflection of laminated glass is similar to monolithic glass, unless surface treatments are applied to reduce reflection.
Acrylic
Some types of acrylic glassAcrylic glass
Poly is a transparent thermoplastic, often used as a light or shatter-resistant alternative to glass. It is sometimes called acrylic glass. Chemically, it is the synthetic polymer of methyl methacrylate...
can have the high light transmission and optical quality of glass. Acrylic is also light weight, compared to glass, and is shatter-resistant, making acrylic an attractive choice for framing large, over-sized works of art. In general, acrylic sheet scratches easily and retains a static charge, which can be problematic when framing pastels or charcoals. Some manufacturers, add dyes to acrylic glass
Acrylic glass
Poly is a transparent thermoplastic, often used as a light or shatter-resistant alternative to glass. It is sometimes called acrylic glass. Chemically, it is the synthetic polymer of methyl methacrylate...
to filter the UV light transmittance, and its surface can also be treated with both anti-static and anti-reflective coating
Anti-reflective coating
An antireflective or anti-reflection coating is a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. This improves the efficiency of the system since less light is lost. In complex systems such as a telescope, the reduction in reflections also...
s.
Glass Surface Treatments and Coatings
Due to the change in the refractive indexRefractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
, as a light beam travels from air (refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
of ~1) into glass or acrylic (refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
of ~1.5) and then back into air, these transitions cause part of the light to be reflected. While "anti-glare" (a.k.a. "non-glare" or matte finish) glass treatments focus on scattering the light, "anti-reflective" coatings actually reduce the amount of light, which is reflected from each glazing surface, which has the benefit of increasing the amount of light transmitted through the glazing.
Matte (Etched, "Non-Glare," or "Anti-Glare")
The main purpose of mattMatt
Matt may refer to:*In British English, of a surface: having a non-glossy finish, see gloss *Matt, Switzerland, a municipality*Matt , people with the given name Matt or Matthew, meaning "gift from god"...
e glass is to transform the specular reflection
Specular reflection
Specular reflection is the mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction...
into reflection haze
Visual appearance
The visual appearance of objects is given by the way in which they reflect and transmit light. The color of objects is determined by the parts of the spectrum of light that are reflected or transmitted without being absorbed...
. So-called "scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
" of the reflected light renders reflected images blurry, so that distinct reflected shapes and sources of light do not distract from the art viewing experience. Scattering the light does not reduce the reflection
Reflection
Reflection or reflection may refer to:-Science:* Mirror image, a reflection in a mirror or in water* Reflection nebula, a nebula that is extended and has no boundaries* Reflection , a common wave phenomenon...
or absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
, which remain at the level of the glass substrate. There are several ways of making the glass surface matt
Matt
Matt may refer to:*In British English, of a surface: having a non-glossy finish, see gloss *Matt, Switzerland, a municipality*Matt , people with the given name Matt or Matthew, meaning "gift from god"...
e – from pressing the pattern when the glass is still soft to fine etching of the glass surface by acid. The quality of matt
Matt
Matt may refer to:*In British English, of a surface: having a non-glossy finish, see gloss *Matt, Switzerland, a municipality*Matt , people with the given name Matt or Matthew, meaning "gift from god"...
e glass is usually determined by its gloss factor or haze factor.
Single-Layer
Single-layer anti-reflective coatings aim to achieve the refractive index of 1.25 (half-way between air and glass), and can be made either by single-layer micro-porous structures achieved by etching, hybrid materials and other processes suitable for producing large-area coatings for art framing purposes. Single layer coatings have been used as a lower-cost alternative to multi-layer anti-reflective coatings. Single-layer anti-reflective coatings can reduce light reflection to as low as 1.5%.Multi-Layer
The lowest reflection can be achieved with multi-layer anti-reflective coatings, which can be applied by either magnetron sputtering, evaporationEvaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
or sol-gel process (or other processes, which can control the uniformity of deposition on nanometer-scale), and can reduce the light reflection to lower than 0.25% per side (0.5% total).
Features of Anti-Reflective Coatings
- Light Reflection - the main goal of anti-reflective coatings is to reduce the light reflectionReflectionReflection or reflection may refer to:-Science:* Mirror image, a reflection in a mirror or in water* Reflection nebula, a nebula that is extended and has no boundaries* Reflection , a common wave phenomenon...
that causes the so-called glareGlareGlare may refer to:* Glare is difficulty seeing in the presence of very bright light* A glare is a facial expression of squinted eyes and look of contempt* A call collision in telecommunications* GLARE, an advanced aerospace material...
. Therefore, the lower the light reflectionReflectionReflection or reflection may refer to:-Science:* Mirror image, a reflection in a mirror or in water* Reflection nebula, a nebula that is extended and has no boundaries* Reflection , a common wave phenomenon...
, the less glareGlareGlare may refer to:* Glare is difficulty seeing in the presence of very bright light* A glare is a facial expression of squinted eyes and look of contempt* A call collision in telecommunications* GLARE, an advanced aerospace material...
reaches the viewer. The best anti-reflective products available for the picture framing market have light reflection of 0.5%. The seemingly small differences in light reflection are actually very important due to the logarithmic response of human eyes to signal intensity (Weber's law). In other words, under normal lighting conditions, the human eye perception of the intensity of a reflected light source in a 1% reflecting glass surface will be perceived as more than twice of the same light source in 0.5% reflecting glass. - Light Absorption - light absorptionAbsorption (electromagnetic radiation)In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
of glazing is the light that is neither transmitted nor reflected by the glazing. Since light is not necessarily absorbed uniformly, some wavelengths may be transmitted more than others, causing the transmitted color to be distorted. A good way to detect light absorption of glazing is so-called white paper test. This test, used to detect the transmission color of glazing, involves placing a piece of glazing on white paper, and comparing the paper’s color with and without the glass. A slight greenish tint will indicate the presence of iron oxide in the raw materials used to produce clear float glass. Additional transmitted colors may result from the absorption of any applied coatings.
- Light Transmission - the lower the light reflection and light absorption, the higher the light transmission, and therefore, the visibility of the objects displayed behind the glazing.
- Reflected Color - Uncoated glass reflects light uniformly and does not cause reflected light to be distorted (a white light source reflected in a non-coated glass pane will still appear white). However, anti-reflective coatingAnti-reflective coatingAn antireflective or anti-reflection coating is a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. This improves the efficiency of the system since less light is lost. In complex systems such as a telescope, the reduction in reflections also...
s typically cause some wavelengths of light to be reflected more than others, causing a shift in the reflected colorReflection (physics)Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
. This way, a white light source reflected in an anti-reflected glass surface may appear green or blue or red, depending on the wavelengths that are favored by a particular anti-reflective coating design.
- Intensity of Reflected Color - intensity of reflected color can be measured by its relative distance from the color neutral zone (i.e. white). Due to the variability of industrial processes, some producers design their anti-reflective coatings to have more intense colors so that the statistical deviation of outcomes falls within a specific color (green or blue, etc.). The tighter a manufacturer’s control of its processes, the closer the design can be to the color neutral zone, without crossing over from a designated color.
- Reflected Color under an Angle - as a reflected light source is reflected from the glazing under a shallow angle, some anti-reflective coatings may cause the reflected color to shift. Therefore in picture framing, a stable color under a wide viewing angleViewing angleIn display technology parlance, viewing angle is the maximum angle at which a display can be viewed with acceptable visual performance. In a technical context, this angular range is called viewing cone defined by a multitude of viewing directions....
is desirable.
- Cleaning - since anti-reflective coatings render the glass surface virtually invisible, the dirt or soiling of the surface is much more visible on an anti-reflective surface. This enhanced visibility of surface smudges results in an end-user difficulty to clean the AR-coated glass. Therefore some anti-reflective coatings have special surface treatments to improve cleanability, while others give special cleaning instructions to avoid damage to its coating.
- Handling - Some coatings are more durable than others. A scratch through an anti-reflective coating is also much more visible than a scratch through the surface of an uncoated glass due to the difference in the reflectivity of the scratched surface (glass ~8%) and the reflectivity of the anti-reflective surface around the scratch (~0.5%). Therefore, anti-reflective coatings with more scratch-resistance are preferred in art glazing. Magnetron-sputtered and sol-gel anti-reflective coatings are typically metal oxides with superior hardness compared to other application methods.
UV Filtering Coatings
In order to reduce the amount of damaging light radiation transmitted through glazing, some glass coatings are designed to either reflect or absorb the ultraviolet (UV) spectrum. The following technologies are used to reduce the amount of UV from reaching the artwork:- Organic UV Absorbers are added to an inert, inorganic silica-based coating to yield a UV absorbing layer on one side of the glass. Organic UV absorbers are capable of blocking almost 100% of UV radiation between 300 nm to 380 nm, but in an industrial environment it is difficult to make a sharp UV cut-off without affecting the visible spectrum, therefore UV absorbers tend to also increase the absorption of the visible light. Chemically-deposited UV absorbers also result in a less scratch resistant surface than magnetron-sputtered or Sol-gel UV blocking layers, as evidenced by the manufacturer’s recommendation for avoiding environmental and other contact with the UV-coated side.
- Interference UV Blockers are usually built into anti-reflective interference thin film stacks, and focus on maximizing the UV reflection below the visible light boundary. Industrially-available sol-gel processes offer up to 84% UV block, while magnetron-sputtered AR/UV-Blocking layers can block up to ~92% with no adverse effects on the transmission or absorption of visible light.
- UV Filtering of the Substrate is possible by adding UV filtering agents during the production of the substrate. While typical clear float glass blocks ~45% of UV radiation, the addition of CeOx to glass has been shown to further reduce UV transmission as well as widespread use of organic UV blocking dyes in the production of acrylic substrates. Most soda-lime glass completely absorbs short wavelength UV-B radiation below 300 nm. Low-iron glass typically blocks ~12% of UV radiation between 300 and 380 nm.
UV Definition in Art Framing
The most widely used definition of "UV Light" in the framing industry has been defined as non-weighted average transmittance between 300 nm and 380 nm, while the ISO-DIS-21348 standard for determining irradiances defines various UV light ranges:| Name | Abbreviation | Wavelength Wavelength In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a... range in nanometers | Energy per photon |
|---|---|---|---|
| Ultraviolet A, long wave, or black light Black light A black light, also referred to as a UV light, ultraviolet light, or Wood's lamp, is a lamp that emits ultraviolet radiation in the long-wave range, and little visible light... |
UVA | 400 nm–315 nm | 3.10–3.94 eV |
| Near | NUV | 400 nm–300 nm | 3.10–4.13 eV |
| Ultraviolet B or medium wave | UVB | 315 nm–280 nm | 3.94–4.43 eV |
The definition of the upper limit of UV protection as 380 nm by the framing industry is not consistent with accepted standards above.
According to the Library of Congress Preservation department, the artwork damage does not stop at 380 nm though, and all radiation (UV, visible, IR) has the potential to damage art. Thus, calculating a simple average of all wavelengths between 300 nm and 380 nm does not account for the fact that different wavelengths have different artwork damage potential. At least two other methods exist, which provide a more holistic measurement of radiation damage, from both the UV and visible portions of the spectrum:
- Krochmann Damage Function (KDF) is used to rate a glazing’s ability to limit fading potential. It expresses the percentage of both UV and of that portion of the visible spectrum from 300 nm to 600 nm that passes through the window and weights each wavelength in relation to the potential damage it can cause to typical materials. Lower numbers are better.
- ISO-CIE Damage-Weighted Transmission (ISO) uses a weighting function recommended by the International Commission on Illumination (CIE). Its spectral range is also weighted and extends from 300 nm to 700 nm.
For picture framing purposes, it is not appropriate to use these methods for absolute ratings, since "Better" ratings are obtained with lower visible light transmission, which is not aesthetically desirable in a framing glazing. However, by including more art damaging factors than UV radiation between 300 nm and 380 nm, these methods provide a more holistic relative ranking tool. For example, comparing a 99% and 92% UV Blocking glazing, would translate to 44% and 41%, respectively, under the KDF.
How much UV filtering should a glazing have
The discussion on how much UVUltraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
filtering is necessary in art framing is complex and controversial, driven by conflicting corporate interests. There have so far been no independent organizations, not tied to corporate sponsors, which have presented scientifically verifiable and conclusive evidence to the amount of UV filtering necessary for a glazing to both display and at the same time protect an artwork. On one hand, the issue is complicated by the varying amount of damaging light actually present in an indoor environment (from low level indirect sources to direct daylight). On the other, by the fact that not only UV
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
, but also visible light damages an artwork. According to the National Fenestration Rating Council, only 40% of artwork fading is caused by UV radiation. The remaining damage comes from the visible light, heat, humidity and material chemistry. This means that increasing visible light transmission by an anti-reflective coating actually increases the amount of damaging radiation on an artwork.
One of the most thorough and independent studies was conducted by the US Library of Congress in an effort to display and preserve the US Declaration of Independence. At first, it was decided to use special yellow "Plexiglass UF3", which removes both the ultra-violet as well as the blue end of the visible spectrum, with significant, but acceptable interference for viewing. Sealing the display by a chemically inert gas such as nitrogen, argon or helium also aided its preservation. In 2001, the display of the US Declaration of Independence was revised to include a multi-laminated glazing for shatter-resistance, with sol-gel interference-based multi-layer Anti-Reflective coatings on outer surfaces to improve the visibility of the document.
From the above evidence, it can be concluded that if preservation were the only goal of glazing, then only a climate-controlled, dark space would offer the best possible protection for an artwork, which can be exhibited once in every several years, while no glass at all provides a perfect displaying option. Therefore, for those artworks, which are chosen to be displayed, the ideal amount of UV blocking should be as much as possible, without affecting the visible light transmission.
Controlling UV Lighting Inside
While determining how much UV light should be filtered by art glazing, it may also important to consider the amount of UV light present inside a room or a building. Note that regular window glazing filters away a significant portion of the UV light, which originates from the sunSun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
The relative amounts of damaging light in equal quantities of light:
| Illumination | Relative Damage |
|---|---|
| Vertical skylight, open | 100% |
| Vertical skylight, window glass | 34% |
The above indicates that the damage level of even direct sunlight coming from the vertical skylight is reduced to 36% by regular window glass. Due to the sun's changing position, even less direct light enters through side windows and hanging an artwork away from direct sunlight reduces the exposure to potentially damaging direct sunlight even further.
Indoor lighting, especially fluorescent lighting, is considered to contain some UV Light. GELighting.com asserts that "UV exposure from sitting indoors under fluorescent lights at typical office light levels for an eight-hour workday is equivalent to just over one minute of exposure to the sun in Washington D.C. on a clear day in July. Additionally, the relative damage of incandescent light is 3 times less than that of fluorescent light. Since UV filtering picture framing glass does not protect against all damage factors, it is important to display framed artwork in a well controlled environment to reduce the effects of heat, humidity, and visible light.