why my christmas shopping isn’t done

Most people are scurrying around malls this time of year looking for those last-minute presents.  My colleague Yoda, however, got sidetracked during a visit to an outdoor mall called Watter's Creek at Montgomery Farms in Allen, TX.  He described a tree with "decorations on it that look like lights are falling.  They're plastic tubes with falling lights inside.  Tell me how that works."

He was so enthusiastic about the decorations that I made a trip Tuesday night up to the Mall to see them.  I normally avoid malls like The Plague this time of year; however, I'd just returned from some dental work that left me not feeling like getting much work done, the temperature has been in the sixties and it's a short trek up the freeway.  I was skeptical:  the decorations on our house consist of candles and red bows, with some greenery over the fireplace and on the stair railings.  And the mall in question is very close to Frisco, which was recently documented in a really interesting book about how a couple of Texas families celebrate Christmas.  If you look on the Amazon website for the book Tinsel: A Search for America's Christmas Present, you can see an example of an understated Texas yard at Christmas time.  I'm not a big fan of the 'bust the electric bill' mode of decorating, so I wasn't expecting that the effect would be impressive.  But Yoda was right: it is pretty cool.

I've shown two pictures at different times on the right.  It would be easier to see if they didn't have the blue globes on the tree (I told you I'm a decorating minimalist) , but I guess they wanted something you could see during the daytime as well.  I've highlighted one device with a faint yellow ellipse and pointed to it with a grey arrow.  See how the light is positioned at different places with respect to the static blue globes (So those blue globes are good for something – they are constants in my tree experiment.)

It didn't take long to figure out how they worked.  I was actually hoping they would be a little more science-y, but they work on the simple principle of persistence of vision — the same thing that allows us to view movies. 

Or so I thought.  In film school, many years before I decided to stick with physics because I wouldn't have to spend my life begging for money and listening to idiots criticize my work, I remember learning the theory of 'persistence of vision'.  You've probably seen those little books that have drawings in the upper right hand corner.  You flip the pages fast enough and it looks like the drawing is in motion.  In film, the minimum rate to make things appear to move is 16 frames per second, although most modern films run at 24 fps.

Your eye works like a movie projector.  The image comes in through the lens and is projected on your retina, which is like the movie screen.  (It's projected upside down and your brain is smart enough to account for that, but that's another blog.)  Persistence of vision says that there is an afterimage on your retina – the image remains on your retina for another 40 milliseconds after the object creating the image is gone.  The entry for afterimage on wikipedia has an example of a negative afterimage.  If you stare at a word written in red on a green background for 20-60 seconds, then look at a blank piece of white paper, you'll see the word in cyan on a magenta background.  (cyan in the complement of red and magenta the complement of green.

The original theory of persistence of vision claimed that the reason people see motion from still pictures is because the eye sees an afterimage, which persists long enough for the next frame to appear and take its place, giving the eye a continuous image where there actually isn't one.  I learned this in the early 1980's, although apparently back in 1912's, people realized that the ability to see motion from still pictures is not due to the eye's mechanical persistence of vision.  It's due instead to complex phenomena in the brain called 'phi phenomenon' and 'beta movement' that psychologists and film critics are still somewhat confused about. 

A website from Purdue compares the beta and phi effects.  If you go to their website and click the 'phi' button, see if you can make the image transition from two separate dots blinking on and off to a single dot moving left and right.  I can see both, but one is easier for me to see if I move back from the computer screen and sort of stare.  (I'm still digesting the detailed difference between beta and phi, but if you're interested, ignore how annoying it is to have to click for each line of text in their presentation and look at the numerous neat demos toward the end of the presentation.)

Even if you look at the photos I took really quickly, you still won't see any motion, so take a look at the video below that shows the same kinds of lights as those I saw at the mall.  (You really only have to look at the first 45 seconds or so to get the idea, but it's a nice video.)

The first thing you notice when you look at the lights close up is that they are LEDs.  The cool color is the giveaway, since LEDs emit a very different spectrum of light than incandescent lights.  Incandescent lights are much warmer in color.  The fancy dropping light fixtures are called Snowflake LED lights or LED Motion lights or similar names and they are expensive, which is why you probably won't see them on houses this year.  LEDs or Light Emitting Diodes are gaining popularity with consumers because they don't get hot and use a lot less energy than incandescent bulbs.  They also respond much faster.  An incandescent light bulb produces light because electrons moving through a high-resistance filament make the filament hot.  It takes time to raise the temperature of the incandescent light's filament to 2500 degrees Celsius (more than 5000 degrees Fahrenheit), which is how hot it needs to be to provide light.

In contrast, LEDs are semiconductor diodes that produce light when electrons combine with holes.  A hole is a place where there could be an electron, but there isn't.&#01
60; The combination electron/hole pair has less energy than the electron and the hole have by themselves. They excess energy is emitted in the form of light.

The electron-hole recombination process happens much more quickly (and much more efficiently) than heating a filament.  LEDs turn on almost instantaneously, while incandescent bulbs may take 200 milliseconds to start producing light.  That may not sound like a lot of time, but if you're going 70 mph, you travel 20 feet in 200 milliseconds.  That gives you twenty extra feet to avoid the car in front of you when it slams on its brakes.

The fast response time allows the LED snowfall lights to work.  The diagram below shows the sequence of lights with time going to the right.  Groups of lights are lit at different times and your brain interprets that sequence as motion, much in the same way the racing lights around a cinema marquee give the impression of motion.  That's much easier to make happen with LEDs than with incandescent bulbs.  

As I mentioned, the lights are a little pricy.  Five double-sided tubes, each about 39 inches long have 240 LEDs each and are between $700 and $800.  The tree was probably 50 feet tall and there were at least 25 LED fixtures, so we're talking about a whole lot more than the price of a dozen red velvet bows at Michaels.

So that's my excuse for why my Christmas shopping isn't done.  Between the lights and the convection currents in the pond that were made visible by a thick film on the water's surface, I didn't make it to a single store before closing time. 

I guess one might say being perennially behind is the price you pay for being easily distracted. On the other hand, it's sort of nice that the wonder and amazement of Christmas that one had as a child can be recreated (at least for a few moments) by something as mundane as a bunch of electrons and holes pairing up in a brilliant, well-orchestrated process.