Corning's Gorilla Glass Is Optically Superior To Sapphire

| About: Corning Inc. (GLW)


Due to its scratch resistance, sapphire may be used more widely in the future for touchscreens.

However, sapphire's larger refractive index makes it more reflective than products like Gorilla Glass.

Here I calculate the single-surface reflectance of perpendicularly incident light to be over 83% greater for single-crystal sapphire than for Gorilla Glass.

This larger reflectance may negatively impact the user experience or require additional anti-reflective coatings.

The message for Corning investors is that sapphire is not suitable for applications where optical clarity is more important than scratch resistance. Gorilla Glass will continue to have a market.

The fact that Apple (NASDAQ:AAPL) has contracted with GT Advanced Technologies (GTAT) to build a factory to manufacture crystalline sapphire has many speculating that Apple intends to use highly scratch resistant sapphire as cover glass for some future iDevice instead of Corning's (NYSE:GLW) Gorilla Glass. I recently wrote about the mechanical differences between sapphire and Gorilla Glass and was asked by a reader to speak about the optical differences between these two materials. Some general statements about the differences are available, but I wanted a more thorough understanding. So, I got out the spec sheets and some textbooks and performed some calculations. While my previous article concluded that each material has mechanical advantages in certain situations, when it comes to optics, my conclusion is that Gorilla Glass is unquestionably superior to sapphire for touchscreen applications

Sapphire and Gorilla Glass Both Have Excellent Transparency to Visible Light

When it comes to the transmission of visible light, both Gorilla Glass and sapphire are exceptionally transparent. The following figures show the transmission spectra of Gorilla Glass 2 (top, I don't know the thickness of this sample) and single crystal aluminum oxide, i.e., sapphire (bottom, 1 mm thick). Click on the figures to go to the sources. In each of these figures, the horizontal axis is wavelength and the vertical axis is percent transmission. Note that both materials transmit quite well across the visible spectrum (wavelengths from 390 nm to 700 nm, 0.39 microns to 0.7 microns). The overall transmission of the sapphire sample across these wavelength is about 5-10% less, but since I don't know the thickness of the Gorilla Glass sample, I can't make any conclusions about this. The bottom line is that both materials transmit visible light quite well. However, the situation is much different when it comes to reflectance.

gorilla glass 2 transmission spectrum

(Source: Gorilla Glass 2 specification sheet via )

single crystal sapphire

(Source: sapphire transmission spectrum via

Gorilla Glass is Significantly Less Reflective Than Single-Crystal Sapphire

Light has a schizophrenic nature, sometimes behaving like a wave and sometimes behaves like a particle, depending on the experiment conducted. The truth is that light is neither of these things, but they are useful analogies to help us understand how light behaves. Light's wave-like nature is evident in the way it reflects off of boundaries between two media. Anytime a wave encounters such a boundary, a portion of the wave's energy is reflected. The amount of energy reflected is a function of the change in the wave's velocity as it passes from the first medium into the second and the angle of incidence relative to the line perpendicular to the boundary (the angle of incidence is illustrated by the theta in the following diagram).


(Source: angle of incidence via

Although Einstein famously theorized (a theory subsequently proven correct) that the speed of light in a vacuum is constant, light does slow down when traveling through transparent materials. The ratio of this speed relative to the speed of light in a vacuum is called the refractive index of the material. For example, water's refractive index of 1.33 means that light travels 1.33 times slower in water than it does in a vacuum. The refractive index of air is about 1.00 meaning that air negligibly affects the speed of light. The refractive index does dependent on wavelength (this is why prisms create rainbows from white light) but this dependence is small enough to ignore for the current discussion.

In summary, the amount of reflection off of a boundary between air and a transparent medium is proportional to that medium's refractive index and the angle of incidence. The table below taken from a GT Advanced Technologies specification sheet lists several mechanical and optical properties of single crystal sapphire versus Corning's Gorilla Glass 2. Click the table to go to the full specification sheet.

sapphire vs gorilla glass

(Source: Gorilla Glass 2 specification sheet via )

Note that sapphire has a refractive index that is about 16.5% larger than that of Gorilla Glass. One might expect that this means sapphire will be 16.5% more reflective than Gorilla Glass for a given angle of incidence:

(1.76-1.51)/1.51 = 0.165

However, the actual difference in reflectance is quite a bit more. For cover-screen applications, the reflection of interest happens at the boundary between air and the sapphire or glass. So one might expect the reflectance to depend on the proportional difference between each of these material's refractive index and that of air. Based on this assumption, one would estimate that sapphire is 49.0% more reflective than Gorilla Glass for a given angle of incidence:

[(1.76-1.00)-(1.51-1.00)]/(1.51-1.00) = 0.490

However, even this underestimates sapphire's reflectance. The suspense is building. The correct approach to take involves using reflectance equations worked out in the early 1800s by the French physicist Augustin-Jean Fresnel. Fresnel also contributed to our understanding of the diffraction of light and invented the Fresnel lens used widely in lighthouses. The Fresnel reflectance equations take into account that the light energy reflected from a surface is proportional to the square of the light-wave's amplitude.

Using the Fresnel equations, the reflectance of sapphire is an astonishing 83.7% greater than that of Gorilla Glass when the incident light strikes the surface perpendicularly (the angle of incidence is zero). This is illustrated by the following figure summarizing my calculations, which shows the reflectance of sapphire and Gorilla Glass as a function of angle of incidence assuming unpolarized light (like sunlight). The vertical axis uses a log scale for clarity. At an angle of incidence of zero degrees, Gorilla Glass reflects 4.13% of the light energy striking its surface while sapphire reflects 7.58%.

reflectance of sapphire vs gorilla glass


While both sapphire and Gorilla Glass have exceptional transparency to visible light, sapphire's larger refractive index makes it much more reflective. It is worth pointing out that the analysis here considers reflection off of a single surface. Any light transmitted through the top surface of a sheet of sapphire or Gorilla Glass will be partially reflected at the bottom surface too. If the screen is sufficiently thin (on the order of a few microns thick), the light that bounces around between the top and bottom surfaces can also destructively or constructively interfere with itself canceling some of the reflection or accentuating certain wavelength. This thin-film interference is the reason soap bubbles in sunlight exhibit colorful striations and is also the principle on which many anti-reflective coatings are based. Of course, sapphire could be laminated with an anti-reflective coating but if the sapphire isn't the top layer of the screen, that defeats the purpose of using this exceptionally hard material to begin with. If the sapphire itself could be manufactured to a thinness of a few microns (not yet possible), then it could be its own anti-reflective coating. However, it is doubtful that a sapphire layer a few microns thick would retain much durability.

The bottom line for Corning investors is that both sapphire and Gorilla Glass have advantages when used as a screen cover. Neither is perfectly suited for all applications so neither Corning nor sapphire equipment manufacturers like GTAT will be squeezed out of the market. On the contrary, the growing need for high-tech materials like sapphire and Gorilla Glass will benefit both companies.

Disclosure: I am long GLW, GTAT, AAPL. I wrote this article myself, and it expresses my own opinions. I am not receiving compensation for it (other than from Seeking Alpha). I have no business relationship with any company whose stock is mentioned in this article.

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