February 27, 2018

Expert Interview

Video Transcript

My name is Sarah Stoll, I’m an associate professor of chemistry, I’ve been here since 2002, and my research group works on magnetic nanomaterials.

Q1: Which chemical gases form inside the high pressure sodium bulb?
Dominantly it’s Sodium and Mercury, are the gases that are formed in the High Pressure Sodium bulb.

Q2: How do these chemicals interact to produce light?
You’re putting a large voltage across these. The Sodium and Mercury make an amalgam, which means that the Sodium dissolves into the Mercury at any kind of combination. And when you put the voltage across it, it causes that to evaporate. And once it’s evaporated, and then- so it’s in the gas phase- and it’s been ionized, those ions have electronic energy states that when the electrons get excited and then relax, they’ll emit specific frequencies of light.

Q3: What are the chemicals’ light spectrum outputs?
Sodium is 589 nanometers and for Mercury- probably the most important one- is the 435 nanometers. So the 589 gives it a very yellow color, and the 435 is more of a bluish.

Q4: Why does light spectrum matter for lighting technologies?
It depends on what the application is for, but if you’re trying to reproduce something close to sunlight, then you’ll want a certain range of wavelengths and intensities that would produce something that looks analogous.

Q5: Why do plants like the HPS bulb?
It is connected to the wavelengths of light, but also the intensity of the light match well with the absorption of chlorophyll.

Q6: What is Lucalox ceramic and how does it facilitate chemical reactions?
The ceramic is Aluminum Oxide, so that’s where the “luc” L-U-C part of it comes from- translucent- “A-L” is from Aluminum and “O-X” from Oxide. So Al2O3 is a very common material used in a lot of things. It’s the ceramic that you put in your catalytic converter, for example. The material that’s translucent is polycristallin, but it lacks pores which I guess maybe refracting light and influencing its ability to transmit light. And it’s important because the Sodium, more so than Mercury, is a pretty reactive element, and if it gets any moisture in it, it will react quite energetically to be oxidized, so to keep these in the elemental form is a challenge.

Q7: How does the HPS compare to other lighting systems?
The sort of traditional, old-fashioned light bulb used to have a little wire on the inside- it was a little Tungsten filament- and you were putting a voltage across that and same kind of idea that you were exciting electrons and then when they relax down they emit certain frequencies of light. And the downside of that is that those wires get hot over time, and if they break the wire, then the lightbulb becomes inactive. So the advantage of these High Pressure Sodium lamps is that they’re pretty long-lasting and efficient. People are moving more and more towards what are called “Light Emitting Diodes” which depend on semiconductor technology, so they involve a PN junction where you put a voltage across it and get light emitted that way. And that also has advantages that it’s very energy efficient. It has the disadvantage actually that there’s no heat so they’ve had problems where you actually need the streetlight to melt the snow when it falls on it, and LEDs don’t do that. So, there’s some unintended effects.