High visibility in the context of our dye marker technique means that we want good contrast between the dyed fluid and the surrounding ambient fluid. The ambient fluid (often water) is transparent, meaning light passes through it with little interaction. For contrast, the dyed fluid should have a maximum interaction with light.
So, let’s review the different ways that light can interact with matter; both with the ambient fluid and the dyed fluid. We might as well include what happens to solids too, to keep it all straight.
Minute paper: List the ways that dye (or any molecule) can interact with light (from an external source; later we’ll talk about emitted light)
Take a few minutes to think for yourself before reading these answers from class:
We will cover these interactions here, with the exception of refraction. Refraction is a big one, and we’ll devote a whole chapter to that, and the related flow vis techniques.
When light can pass through a collection of molecules, and come out the other side in some form we call it transmission of light. If the direction of the light remains unchanged, we call it simple transmission through a medium (for flow vis, the medium is usually air or water).
Another type of transmission is diffuse transmission. Light passes through the diffusing material, but emerges spread over a wide range of directions. You’ve seen this effect in white lampshades. Materials that diffuse light can be called translucent or semitransparent. Professional lighting equipment most always includes a translucent diffuser to place in front of a light source. Thin white plastic bags can make nice diffusers, but thick ones won’t work; they absorb too much light. Glass with a roughened surface can diffuse light too, and is called ‘ground glass’ because it’s made by grinding one of the surfaces. Sheets of ground glass are available from any optics supplier, and an ordinary window glass shop may even stock it for use in bathroom windows. Or you can just sand transparent acrylic sheet with a fine grit if you don’t need the durability of glass. If you need to diffuse a laser, try a small container of milk; it does a great job making the light incoherent and can handle quite a bit of power.
Diffuse light sources are very valuable as backgrounds for boundary techniques. You can use one of the methods above to diffuse a small light source such as an LED or light bulb, but integrated diffuse LED panels are now available, with very bright, even light. A 2 foot by 4 foot unit is $80, and small units can be $10 or less.
Or instead, if you don’t need a huge amount of light, just shine light on a white wall or sheet of paper, and get diffuse reflectance.
Reflectance is mostly how we see the world. Light strikes an object, and bounces into our eyes. There are two types of reflectance: 1) diffuse or scatter, and 2) specular. Most objects reflect with some of both types. The object could be anything: a solid surface, a solid or liquid particle, or even a molecule, but typically we think of solid surfaces. Anything shiny will have a specular component; mirrors are an extreme example. Dull or matte surfaces are examples of diffuse reflection (AKA scattering), which results in the light that strikes an object being reflected in a wide range of directions due to small surface irregularities, as shown in Figure 3 .
Specular reflection, in contrast to diffuse reflection, is when most of the incoming light is bounced in a specific direction: the angle of reflectance that the light leaves at is the same as the angle of incidence, but on the other side of the normal (the direction perpendicular to the surface), as shown in Figure 4. This requires a smooth surface.
Any shiny surface will generate some specular reflection. A first surface mirror is usually a thin metal layer on a glass substrate layer; the metal does the reflection. The metal layer is easily scratched, so first surface mirrors are only used for specialized optics, like telescopes and the mirrors in DSLRs. Instead, mirrors in everyday use are second surface mirrors; a glass or acrylic substrate with metallic backing, as shown in Figure 5. Second surface mirrors are more durable, but generate unwanted extra reflections. The incoming light encounters a change in index of refraction at the glass (first) surface, and around 10 percent of the light is immediately reflected. The rest passes through the glass to the metallized back surface of the glass, and is reflected there. Upon reaching the glass surface again, a small fraction is reflected back into the glass, and the primary reflection emerges from the glass. When you look in a bathroom mirror; look closely and you can see at least two images; one will be strong and the other will be faint, and slightly offset. Light continues to bounce inside the glass, with some emerging and a little remaining with each bounce at the glass surface.
Your mirror choice will depend on the quality of the reflection you need. Second surface mirrors are cheap and common, and you won’t notice the extra reflections unless you are close to the mirror. First surface mirrors are also not very expensive; less than $15/sq ft on acrylic for a low quality mirror. The quality is determined by how smooth the reflective surface is. If the flatness is specified in fractions of a wavelength of light then the mirror will be much more expensive. Applications like schlieren and laser interferometry require higher quality mirrors. First surface mirrors are almost impossible to clean without damage, but the metal can be re-coated; there is a small cottage industry that does this for the DIY telescope community .
Light that strikes a surface that is not reflected or transmitted is generally absorbed; the photons are converted to heat energy. In the world around us that we see, most of the light hitting objects is absorbed. White light, i.e. light from the sun and from our common light sources are broad-band, and contain a range of wavelengths. When an object has color that means that it has absorbed all the light hitting it except for the wavelengths corresponding to that color, so an object that looks red will have absorbed all the wavelengths except for the red ones, which are then reflected back into our eyes. White objects reflect the whole visible spectrum, and black objects absorb across the whole spectrum, but even then, not all the light that strikes a black object is absorbed; some is reflected or we wouldn’t be able to see it at all.
It takes a very special black to be fully absorptive. Vantablack is a paint first developed by Surrey NanoSystems in 2014 that can absorb over 99% of the visible light that strikes it. The original formulation used tightly packed carbon nanotubes to trap incoming photons. Alternative versions are coming out as nanotechnology continues to develop. Vantablack got a lot of publicity when artist Anish Kapoor got an exclusive license for all artistic applications. This enraged the art community, Stuart Semple in particular. He teamed up with paint makers and other artists to develop the “world’s pinkest pink” and made it available to everybody except Kapoor . Semple has continued work and his group has released “Black 3.0” which absorbs a respectable 95% of incoming light, and is available online at $26 for 10 ml .
Black backgrounds for flow vis purposes are even easier to come by. A square of black velvet (but not crushed velvet) can be purchased from any fabric store, and reflects very little light unless directly lit. I’ve found that black paper or woven fabric backdrops sold by photographic suppliers are not as effective. One drawback to velvet is that if the velvet becomes wet the nap (the fibers that stick up) is damaged and there is no good way to fix it; you’ll end up with varying light absorbance across the fabric. Still, if there is good contrast between your subject and a dark background, it’s easy to take the background to full black in post-processing as well.
One more use of absorption is a laser light trap or dump. A laser spread into a sheet for flow vis purposes is usually low enough in power that sheet metal painted matte black can safely stop the beam. A high power laser beam is a different story. Simply blocking the beam with a flat surface will still result in a lot of dangerous backscatter, and a high power beam can melt or ignite the surface. Commercial beam dumps are not very expensive or, for lasers of a few watts or less, you can build your own from matte black sheet metal. Figure 6 shows a design that uses multiple surfaces to absorb the beam while minimizing backscatter.
The fourth major type of light-matter interaction is diffraction. We discussed it briefly in the context of lenses and small sensors. Diffraction is the tiny bending of light around corners and spreading through small holes (on the order of the wavelength of light) provided we think of light as an EM wave. Diffraction can be explained by the Huygen-Fresnel principle . Huygen proposed that each point on a wavefront acts like a spherical source of the wave and Fresnel added the concept of interference, that high amplitude areas will add together and could be canceled by a low amplitude area. Figure 7 shows a wave passing through a hole larger than the wavelength. The wave diffracts around the edges of the hole, while in the middle the wave fronts from each portion add up (constructively interfere) to make a new planar wave front about as wide as the hole; in other words, a beam.
The smaller the hole gets, the smaller the beam width, until there is only the diffraction from the edges, as shown in Figure 8. So, for a given hole size, short wavelengths will form a beam as they pass through and long wavelengths will spread widely.
You may have noticed diffraction effects in sound waves more than in light. Consider a loudspeaker (bigger than earbuds) that has a tweeter and a woofer. The tweeter emits high frequencies whose wavelengths are short compared to the size of the speaker. You’ll hear the high frequencies ( like Sss or cymbal sounds) best if you are on the axis of the speaker and not much off to the sides. Low (bass) frequencies are large compared to speaker sizes, and spread out, so you can hear bass well all over the room.
The Huygen-Fresnel principal can explain refraction and reflection, as well as diffraction effects. When a light wave hits a denser medium, it slows down, and the wavelength shortens, as shown in Figure 9. The frequency doesn’t change, so the colors stay the same.
Light waves, or any kind of wave, get diffracted when they interact with objects that are close in size to the wavelength of the light. In flow vis, we encounter diffraction of light in cameras as mentioned, but also when water waves go through holes in a breakwater, or hit an object of a size with the wavelength.
Before we leave light-matter interactions, there is one more important phenomenon that can change what happens with any of the previous four types: dispersion. Dispersion is when the interaction varies with wavelength or frequency. For example, a prism will refract (bend) light at an angle dependent on the color, and therefore on the wavelength. Dispersion leads to chromatic aberration in lenses, but it can also produce beautiful effects when water is doing the dispersion, such as in a rainbow, or by a wedge of water acting as a prism, as in Figure 10.
The moral of this story is to be aware of the light-matter interactions when using dye as a marker, and choose lighting and background to maximize contrast. In Figure 11 light is scattered by the white marker back into the camera, and the more the better, while the background is kept as dark as possible.
In contrast (heh) Figure 12 shows the opposite circumstance: with a darker dye, a bright background is needed. The dye – India ink – absorbs and blocks light from the background, creating the contrast. With a black dye no amount of light shone on the dye will improve its visibility. In general, the situation will depend on how transparent the dye is. Is light being blocked/absorbed, transmitted or scattered by the dye?
Light may be colored as it is partially absorbed coming through the dye from a bright background as shown in Figure 13. If it’s opaque like the acrylic paint in Figure 14, you’ll want some light on the dye so that the colored light will be reflected (scattered), and the background could be dark or light, whichever provides the best contrast.
Minute paper: Which surface texture is better for a background, glossy or matte?
Take a few minutes to think for yourself before reading these answers from class:
A glossy surface can work well if the specular reflection from the room can be controlled. But usually, unwanted reflections will creep in, so a matte surface that scatters in all directions is better. Try it!