What is transmittance?
Transmittance refers to the amount of light that passes through the material, usually glass or plastic. It can be determined using a simple equation involving the angle of the ray of light, the index of refraction and the thickness of the material in question.
This equation is known as Snell’s Law and was created by a Danish scientist named Willebrord Snell in 1621 when he was investigating optics and lenses. Transmittance measurements are expressed in decimals ranging from 0 to 1 and are used primarily in the production of optical devices such as cameras, microscopes and eyeglasses where precision is paramount.
Light, through a given thickness of material. The amount of light that passes through something in a specific period of time (light transmission). For example, window glass has 99% transmittance. This means it allows 99% of all incoming light to pass through it. A good piece of opaque white paper has low transmittance because so little light passes through its thickness.
If you have one pane and double-pane windows, you can use them together to obtain even higher levels of heat loss protection. Do note that while high-transmissive windows do a fantastic job at keeping heat outside during colder months, they also let more heat out during warmer months than comparable levels of insulation found in low-transmissive windows or shades.
Types of Transmittance
1. Transmission (also called Transmittance or T)
2. Emittance (also called Emissivity or E)
3. Absorption (also called Absorbance or A)
4. Reflectance (also called Reflectivity or R)
5. Transmissivity (also called Transmit-t-t-tive or Tt)
11. Luminous Transmittance
13. Luminous Reflectance
15. Radiant Flux
16. Radiant Intensity
17. Photosynthetic Photon Flux Density
18. Photosynthetic Photon Flux
19. Photosynthetically Active Radiation
20. Photosynthetically Available Radiation
21. Photosynthetically Active Radiation
22. Photosynthetically Available Radiation
A device for measuring transmission, typically using a light source and an integrating sphere (note: you need to convert from % T to % T at 50 or 60 Hz).
To measure T we use a method similar to what you would use for reflectance or diffuse. However, there are differences due to ambient light conditions and the characteristics of integrating spheres. I’ve found that if you have white walls in your lab, it does not make much difference whether you place a black backing behind your sample. A higher than needed value of tT can result in lower values of Y (and conversely).
Remember also that if your color formula doesn’t include both X and Y then these terms can be eliminated as they will not add information.
Calculation of transmittance
When determining a material’s transmittance, you will first need to know how much power is being shone on it. This can be calculated by multiplying two values, one of which must be in watts and one of which must be in volts. The answer should also come out to be in watts.
To calculate a material’s transmittance, take that number and divide it by 100%. For example: if you have an object that has been hit with 100 watts of light, then divide 100 by 100 to get 1; your object has a transmittance of 1%.
If you have another object that has been hit with 20 watts of light, then divide 20 by 100 to get 0.2; your object has a transmittance of 0.2%. If these examples make sense to you, then congratulations—you understand what is transmittance!
Transmittance vs. opacity
It’s generally easier to know what an object or material will allow to pass through it than what it won’t. Transparent objects, such as glass and clear tape, let light (and other forms of energy) travel through them. Opaque objects, like dark-colored cardboard or paint, block light from traveling through them. A material that allows some light to pass through but also blocks some light is called translucent.
The amount of light that passes through a transparent, translucent or opaque object depends on its thickness—the thinner an object, the more likely it is to be transparent; thicker objects are more likely to be opaque.
What is the difference between transmittance and transmission?
In engineering, transmittance and transmission both describe how much light passes through a substance, such as glass or plastic. Because both terms refer to passing through, you might think they are interchangeable. They aren’t. Transmittance refers specifically to visible light; transmission refers to all wavelengths of electromagnetic radiation (e.g., infrared and ultraviolet).
If your device shows a measurement in one but not the other, chances are it’s measuring either light or energy that we can’t see. A window may be rated at 90% visible-light transmittance (meaning 90% of what enters will exit), but that doesn’t mean it lets 90% of energy—visible or not—through! It just means that if you’re looking at something on the other side, you’ll only see 10% of it.
Now let’s look at those numbers another way: If a material has 100% transmittance for visible light, then 0% of visible light is reflected and 100% passes through. So now we know: 100 – 10 = 90%. That makes sense!
Each material has a different absorption spectrum, which means that each material will selectively absorb and transmit specific wavelength ranges. It’s these selective absorptions that are responsible for transmitting light of different colors through filters.
When choosing an ND filter, you should pick a filter based on what area of your image you want to darken, not simply on how dark you want it to get. If your goal is to darken or blur part of your image so that it appears more dreamlike or ethereal (giving it a creamy/dreamy look), then choose one with less sharpness in that color range.
Conversely, if your goal is to show detail in a specific area while preserving clarity in others, then choose one with sharper details in other parts of its spectrum.