Dr. Murzy Jhabvala, Chief Engineer of the Instrument Systems and Technology Division
- Saturday, 18 November 2006
So what the array does is convert infrared radiation, photons, into electrons. And silicon chips can all understand electrons. They have no understanding of photons. We take those electrons and convert them into voltage, and the voltage that we get is proportional to the incoming radiation, the quantity of the photons that came in.
NTB: How does this model differ from other infrared sensors or past QWIP arrays?
Jhabvala: For past QWIP arrays, I think, this is the largest format. It has the largest number of pixels, in our case, a million, that have been made in this broad band, in this 8 to 12 micron region. We don't think anybody has done that. That is the first difference.
The second is, we are not sure if there are competing technologies. Can't say for sure one way or the other, but my search shows that there are not other technologies. For example, MerCad Telluride, indium antimonide, and a whole range of silicon-based detectors that respond to the infrared, but we're not aware of any of them that have been made in a 1 K-by-1 K 8 to 12 micron responsivity format. So that is how it differs from other technologies.
It differs from what we have done in the past by being the first time we've had a broad-band detector. We have made a 1 K-by-1 K narrow-band detector on the way to doing this. It was responsive from about 8 to 8.5 microns. Very narrow range. So you can only get one spectral point, basically, if you are looking at an object. This one, 8 to 12 microns, if you put on the appropriate filters, you can get many different spectral points of information on the same object with the same detector.
If you are looking at a star, a bright star, and you're looking at it at 8 to 8.4 microns, you'll see a nice picture of what that star looks like, a photograph. But that is all you'll be able to see. You won't be able to tell all the different compounds in that star, all the different molecules. You don't have enough spectral information, you aren't looking across enough of the spectrum. So it'd make a very pretty picture in that one wavelength, but if you wanted to know all the different compounds, all the different atoms, you'd have to look at it from 8 to 8.1, then 8.1 to 8.2, and then, say, 8.2 to 8.3 and then, what do you see? What do you see from 8.3 to 8.8? What do you see from 8 to 9? And so on and so forth, all the way up you'd be able to get a whole lot of different information because you are looking at a broad spectral region. That's the spectroscopy of it. And that is where the information is. It'll still make a nice photograph, but what you really want is to understand what the constituents are of that star.
If you want to look at an object on the Earth and try to ascertain it's temperature, you need at least two different wavelengths to do that, because you have two variables when you are trying to determine temperature of an object. So in this sense, this QWIP is far more versatile than the single-wavelength QWIPs.
NTB: What applications does infrared have?
Jhabvala: Any object in the entire Universe will emit radiation as the function of the temperature of that body. The hotter the body, the more radiation it will emit. Your eyeball is sensitive to only a very narrow portion of the spectrum, the visible spectrum. Objects have to get pretty darn hot before you can see it with your eyeball. You stick a poker in the fire-until that poker reaches about 900° C, it looks black. And then it starts to turn red. That color red that you see is the visible radiation coming off the poker because it is that hot.
But most objects aren't that hot. Now, that poker, when it is only 400° C, will be very hot to your finger, but to your eyeball, it looks just as if it were ice-cold. You can't tell the difference. Detectors, infrared detectors, can. They can tell the difference. It is because of that characteristic, that objects as a function of their temperature, will emit more and more radiation, infrared radiation, and then visible radiation, and then if you get is so hot, like the sun or even hotter, you get x-ray radiation. Infrared sensors are particularly sensitive to objects in what we call the thermal region, which is room temperature and a little hotter, they can tell the temperature of things around that your eyeball cannot.