By Patricia Daukantas

You’ve seen her portrait: the doelike eyes, the Mona Lisa smile, the jaunty feathered hat tipping down over a bare shoulder. Since the early days of computer networks, her image has become one of the standards for image calibration, manipulation and transmission within the optical imaging community. Her portrait has graced hundreds of image-processing papers over the past 36 years and will doubtless contribute to many more. But who is she, and where did she come from?

Her name is Lena Sjööblom Söderberg, and today is her 57^th birthday. The head-and-shoulders portrait of Lena (sometimes spelled “Lenna” in English) is cropped from a far more revealing image in the November 1972 issue of Playboy magazine. Researchers at the University of Southern California created a digital file of the picture as an alternative to the more boring test patterns of the era. As the former editor-in-chief of an IEEE journal once noted, the photo has a good mix of details for testing image-processing algorithms – and her face had a certain appeal to the then-mostly-male research community.

It took many years for Lena to learn that her face had helped bring about the JPEG and MPEG standards and other imaging technology that we take for granted today. Since her days of modeling in Chicago, she returned to her native Sweden, married and had three children. Various Internet sources have her working either for the Swedish national liquor monopoly or as a teacher of computer skills to adults with disabilities. According to one of Lena's fans at Carnegie Mellon University, she was a special guest at a 1997 conference of the Society for Imaging Science and Technology.

2008-03 March, Optics and pop culture optics and pop culture, photography, imaging

By Patricia Daukantas

If you’ve watched TV lately, you may have noticed an ad from a domestic beer company that made an extraordinary claim involving lasers. The company said it would project its logo on the near side of the moon during its full phase last Friday.

It didn’t take much Googling for me to debunk the “moonvertising” plan as a hoax dreamed up by an advertising agency team. But it raises a fascinating question: Is it possible to shine a laser beam onto the moon and see the light from Earth?

One entertainment executive says that a soft-drink company had similar moon-lighting plans for New Year’s eve in 2000, but the Federal Aviation Administration nixed the idea out of concern for aviation safety. Although the executive claims that scientists had done the math and found that such a feat was possible, he doesn’t present the research to back up the claim.

So I asked Tony Campillo, OSA’s senior director of science policy, to work out some back-of-the-envelope calculations. Tony has more than 40 years of experience in optics and photonics, as both an optical scientist with the Naval Research Labs and other institutions and as the former editor of the journal Optics Letters. His verdict: Making a light spot on the moon that people could see without assistance would likely require continuous laser power beyond anything we have on Earth right now.

Let’s imagine a single beam of green laser light originating on Earth’s surface and aimed at the moon. (The human eye is most sensitive to light of about 555 nm in wavelength, and green just happens to be the favorite color of the beer company that started this whole thing.) Assume that the outgoing beam is 3.5 m wide, as in the Apache Point Observatory laser-ranging program. After atmospheric distortion, the beam width would be 2 km at the moon, and about 90 percent of the light would reach the lunar surface.

But wait! The moon reflects only about 10 percent of the light that hits its surface. And even that, according to Tony, is scattered into a Lambertian pattern that covers an area that is 100 times the size of the Earth by the time it returns.

We know that the light-gathering part of the human eye—the dark-adapted pupil—is 1 cm wide at best, and let’s make a rough assumption that the diameter of the scattered beam is 1 million km wide when it hits the Earth. Thus, the naked-eye observer is catching only about 1 photon in every 10²².

How much light is needed for the human eye to see? In other words, what’s the threshold of human vision? I know more about astronomy than biology, so I’m a little shaky on that answer. Although some say that, in principle, the human eye should be sensitive to single photons of visible frequencies, in practice noise from both the visual field and the observer’s own neurological system gets in the way. Astronomers must subtract out background light when they are trying to measure the brightness of heavenly objects with their telescopes.

In Optics InfoBase, I found a 1919 (!) report by P.G. Nutting, OSA’s very first president. On the next-to-last page of the 25-pp. document, there’s a table that provides visual detection thresholds for sources of various areas; for the smallest source, the threshold light energy is 17.1 × 10^-10 erg/s, or 0.17 femtowatt (fW). (You could quibble that the table heading should be “power entering eye” instead of “energy entering eye” because the erg is the CGS unit for energy, not power. However, maybe unit accounting was different in 1919.)

Tony also guessed that an input of at least 1 fW from a continuous-wave (cw) laser would be needed to trigger sight in the human eye. Extrapolating to the originating laser, he guesstimated that a 100-GW cw laser would be needed to produce a visible spot on the moon.

Another way of thinking about the sensitivity of human vision involves the astronomers’ system of apparent magnitudes for measuring the brightness of objects in the night sky.

The apparent-magnitude scale is a logarithmic scale dating back to ancient days. The faintest stars that the human eye can see have a magnitude of 6, while stars with a magnitude of 1 are 100 times brighter than their 6^th-magnitude cousins. Nowadays, you might still be able to see 6^th-magnitude stars from a high desert on a clear night. However, from the typical suburb of a brightly lit American city, you would probably see only 3^rd-magnitude and brighter stars.

Suppose that the brightness of the one-pixel lunar display is 1^st magnitude. With the assumption that the pupil of the dark-adapted eye is 7 mm in diameter, Tony calculated that the eye will receive 200 photons/s from a 6^th-magnitude star and 20,000 photons/s from a 1^st-magnitude star. According to Tony, 1 W of green light corresponds to 2 × 10¹⁸ photons/s. By this line of reasoning, 0.01 fW of light from a 1^st-magnitude star hits the retina, and thus only a 1-GW cw laser would be required to make a visible dot on the moon.

In this case, Tony adds, his estimate assumes that the laser is painting a single 1^st-magnitude pixel on the darkened portion of a first-quarter or last-quarter moon. The beer commercial calls for shining an image on the full moon, which would require a lot more energy.

How does this estimate compare with existing lasers? The petawatt laser-fusion projects at the University of Rochester and Lawrence Livermore National Laboratory will generate huge energies – but over pulses in the nanosecond to picosecond range, not cw.

What would it take to build and fire a 100-GW cw laser? My best guess (from U.S. Energy Information Administration data) is that the United States generates roughly 4,000 GW of electricity every hour, but I could be way off. Would it really take 2.5 percent of the U.S. electrical supply to make a visible green dot on the surface of the moon?

Here’s where you come in. We’d like to ask for your feedback on our calculations and estimates. Many of the assumptions that Tony and I have made need to be refined, especially the size of the Lambertian scattered beam returned to the Earth. A better calculation of that beam size would have to take into account the cosine-square nature of the scattering to estimate the peak flux; that could change the guesstimate by an order of magnitude or more. Some questions to ask yourself:

§  What is the threshold at which a human on Earth could detect a bright spot on the moon? What is the optimal size of a pixel on the moon?

§  How much different would the experiment be if the bright spot was on the face of the full moon (which has an apparent magnitude of roughly −12) or a darkened region of the moon (between the quarter-moon and new-moon phases)? What is the required pixel brightness in each case?

§  What kind of a laser would be required to make this stunt work?

§  What would be the technical challenges involved in making such a laser?

Once you’ve tackled these yourself, you might pose these questions to your students as a thought experiment.

If any of your friends mention that they were disappointed about not seeing a logo on the moon, at least you’ll be able to explain why.

Image of the near side of the moon taken by the Clementine mission.

2008-03 March, Astronomy, Lasers, Optics history astronomy, lasers, optics history, optics and pop culture, laser ranging, moon

Posted by Christina Folz, OPN Managing Editor

I can still remember how excited I was when I saw the March 1984 issue of my father’s National Geographic magazine, which featured the laser-sculpted image of an eagle on its cover. That was the first time I ever saw a hologram. (After doing a quick Google search, I’ve learned that that particular issue is now selling for close to $200; it may be time to revisit my parents’ stash of old magazines!)

Now there is reason to believe that the eagle may one day take flight. OSA Fellow Nasser Peyghambarian and his colleagues at the University of Arizona, along with engineers from Nitto Denko Technical Corporation, in Oceanside, Calif., have produced a prototype of the first photorefractive polymer film in which 3D images can be captured, erased and re-recorded. Similar to a paper “flip book” of images taken in rapid succession, a series of these holograms—when captured quickly—can create the illusion of 3D motion.

For more details and images of the holographic film, check out the recent news pieces in IEEE Spectrum and Scientific American.

And stay tuned for an OPN feature article on this technology by Peyghambarian himself in a summer issue of OPN. We probably won’t feature a flying eagle on our cover, but a girl can dream, can’t she?

2008-03 March, Miscellaneous Optics, Optics and pop culture holography, imaging, 3d

By Patricia Daukantas

When OPN described the Large Binocular Telescope (LBT) in an article last September, only one of the twin telescopes was up and running. Last week, the LBT team announced that the 8.4-m telescopes, mounted side by side, have achieved “first binocular light,” meaning that they took their first image as a working pair.

And what an image! The LBT looked at a spiral galaxy known as NGC 2770 and produced a photo of a sight that would be simply impossible to see with bare human eyes. One of the LBT’s scopes is optimized for ultraviolet and short-visible wavelengths of light; the other, for red and infrared. Astronomers photographed the galaxy with both instruments and produced a false-color image of wavelengths ranging from 324 nm to 0.9 µm.

Why bother to look at all these wavelengths? Different bands allow astronomers to see different objects—from young stars much hotter than the Sun to cool gas clouds. Many of the objects that emit in the ultraviolet or infrared are either very young or very old (well, by astronomical standards).

Much more detail about the LBT and the optical science behind the image, plus a gallery of photos of the twin telescopes, is in this press release.

Using both LBT mirrors, this First Binocular Light image shows a false-color rendition of the spiral galaxy, NGC 2770. The galaxy lies 102 million light years from our Milky Way (a relatively close neighbor), and has a flat disk of stars and glowing gas, tipped slightly toward our line of sight. The image displays ultraviolet, green and deep red light in the same composite, showing the detailed structure of hot, moderate and cool stars in the galaxy.

2008-03 March, Astronomy, Astrophysics astronomy, astrophysics, telescopes, imaging

Posted by Christina Folz, OPN Managing Editor

Last Sunday night, the CBS news program 60 Minutes ran a fascinating story about a non-lethal “ray gun” that the Pentagon has developed. The weapon is a flat-dish antenna that shoots a 100,000-watt electromagnetic beam of high-frequency radio waves, hitting anything in its path with an intense blast of heat. Unlike in Buck Rogers, however, the beam is invisible unless viewed with an infrared camera. It does not inflict any lasting damage and just barely penetrates the body’s tissues. (It is absorbed only in the top 1/64 of an inch on the skin, which is where the pain receptors are located.) The weapon is chiefly intended as a crowd-control device. 

The report reminded me of Steve Wilk’s March 2005 Light Touch article in OPN about “How Ray Guns Got their Zap,” which describes the real science behind the ray guns used by fictional heroes Flash Gordon and Buck Rogers. The article explains the origins of the word “ray” and how it came to refer specifically to directional electromagnetic radiation.

2008-03 March, Biomedical optics, Optics and pop culture biomedical optics, optics and pop culture