100 Nano-Stories: What’s Hemispherical Transmittance?
Episode #83: Transmittance | Refractive Index!
It’s your favorite material science & nanotechnology enthusiast! To finalize our rabbit hole on the optical properties of aerogels, I want to discuss hemispherical transmittance.
There is no article to prepare your mind for this brain dump! Let’s dive right into today’s topic! 😄
Hemispherical Transmittance Explained! 💡
Hemispherical Transmittance is a combination of three words:
“Hemisphere” is broken down into two words.
Hemisphere is defined as 1/2 of a sphere.
A sphere is defined as a round solid/liquid figure (ball, marble, planets, etc.)
Transmittance is defined as the light that passes through a material.
With this conclusion, we can define hemispherical transmittance as the direct transmittance plus the transmittance that has been scattered and transmitted throughout the hemisphere of the aerogel particles (diffuse transmittance).
In other words, the hemispherical transmittance is the sum of the direct and diffuse transmittance of light in the particle of an aerogel. If you’re confused by that definition, here is a photo to demonstrate the example given below:
However, the photo demonstrates both the direct and hemispherical transmittance, which is not as accurate as of the definition we gave for hemispherical transmittance (Remember the photo for later in the article, reader!)😉
So how can we adjust the definition to fit the concepts of aerogels?
Hemispherical Transmittance x Aerogels! 🔑
As I have talked before in discussing the optical properties of aerogels, the transmittance is defined as the amount of light that passes from one medium to another (an example can be from air to aerogel.)
The transmittance ratio can measure the amount of light that the surface of the aerogel can transmit from one side to the other.
Thanks to a research paper from The Royal Society of Chemistry, we can measure the transmittance ratio with a spectrophotometer that can produce the hemispherical transmittance (direct + diffuse transmittance.) The transmittance in the aerogel can be calculated in the Ultraviolet (UV), Visible Light, and Infrared (IR).
A spectrophotometer is measuring the transmittance of light in the form of wavelengths (UV Light, Green Light, etc.)
Direct Transmission is when light passes through the aerogel without changing direction.
Diffuse Transmission is when light passes through the aerogel and is scattered throughout the network of pores. The light will leave the aerogel at an angle rather than straight through the aerogel.
Direct-Hemispherical Equation! 🔑
Remember the photo above describing hemispherical transmittance? Now we can use an equation to properly describe the transmission of light/radiation passing through a material!
We can calculate the direct-hemispherical transmission via this equation:
T.R. → Direct-Hemispherical Ratio
τ(nh) → Normal-Hemispherical Transmittance
τ(nd) → Normal-Diffuse Transmittance
But some of you may be asking: What does the hemispherical transmittance have to do with anything related to aerogels?
Aerogel Intersections! 🔑
The Direct-Hemispherical Transmittance Ratio can define the total transmittance of light that that passes directly through the aerogel without the light being scattered (according to Rayleigh Scattering.)
This scattering creates this phenomenon called haze, which causes blurriness when trying to see the other side of the aerogel. You know, that blurry blue haze you can see on the aerogel.
But if the Direct-Hemispherical Transmittance Ratio equals one, this means that we have total transparency of the material. However, this has not been accomplished yet, but the closer the ratio gets to 1, the more transparent the aerogel gets.
This is where the refractive index can play a role in the aerogel.
The Refractive Index is the reduction of speed of the light and the bending of the light as it passes from one material to another. The closer the refractive index is to 1, the more transparent the material becomes.
The reason why is that there is less bending of the light; the light will directly go through the aerogel without scattering with the aerogel particles and because of the non-absorbance of light in the aerogel, which further reduces the refractive index closer and closer to 1.
The range of refraction in a silica aerogel is around 1.005 - 1.26. The lower the number, the less scattering will occur at the surface of the aerogel (where the air and the aerogel interact as an intersection for the light.)
Closing Thoughts! 💭
That is the importance of the direct-hemispherical transmittance ratio and the refractive index!
The refractive index can tell us how much of the light will bend at the surface of the material and when it travels through the material, which can tell us a bit more about the direct and diffuse transmittance of light in the aerogel.
The direct-hemispherical transmittance ratio can tell us a ratio/percentage of how much transmittance we can receive, which can tell us more about the transparency of the aerogel.
See you tomorrow to explain how to change some of the optical properties of aerogels! ✌🏽
Refractive Index → The reduction of speed of the light and the bending of the light as it passes from one material to another.
Incident Light → The light that is going to enter a material.
Hemisphere → 1/2 a sphere.
Transmittance → The light that passes through a material.
The Transmittance Ratio → The amount of light that the surface of the aerogel can transmit from one side to the other.
Direct Transmission → Light passes through the aerogel without changing direction.
Diffuse Transmission → Light passes through the aerogel and is scattered throughout the network of pores. The light will leave the aerogel at an angle rather than straight through the aerogel.
Hemispherical Transmittance Ratio → The sum of the direct and diffuse transmittance of light in the particle of an aerogel.
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