Due to the change in the
refractive index, as a
light beam travels from air (
refractive index of about 1) into glass or acrylic (
refractive index of about 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 that 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
matte glass is to transform the
specular reflection into
reflection haze.
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 matte, from pressing the pattern when the glass is still soft to fine
etching of the glass surface by acid. The quality of matte glass is usually determined by its gloss factor or haze factor.
Anti-reflective coatings Single-layer Single-layer anti-reflective coatings aim to achieve the refractive index of 1.25 (halfway between air and glass), and can be made 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%.
Features of anti-reflective coatings •
Light reflection – the main goal of
anti-reflective coatings is to reduce the light
reflection that causes the
glare. Therefore, the lower the light
reflection, the less
glare 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
absorption 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 the
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 coatings typically cause some wavelengths of light to be reflected more than others, causing a
shift in the
reflected color. 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 – the intensity of a 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 angle 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 presents difficulties when cleaning anti-reflective-coated glass. Therefore, some anti-reflective coatings have special surface treatments to improve cleanability, while others give special cleaning instructions to avoid damage to their coatings. •
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 uncoated glass due to the difference in the reflectivity of the scratched surface (for glass, about 8%) and the reflectivity of the anti-reflective surface around the scratch (about 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 cutoff without affecting the visible spectrum; therefore, UV absorbers tend to also increase the absorption of 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 approximately 45% of UV radiation, the addition of
CeO2 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 approximately 12% of UV radiation between 300 and 380 nm. ==UV protection in art glazing==