The sun as a microwave source

The Sun is a convenient source of radio waves. You can see this yourself by watching weather radar around sunrise or sunset. Radar works by emitting a very short pulse of microwave radiation. Then it listens for the weak returns for a short time (since a radar beam is line of sight, there is no point is listening for returns past 200 miles or so because of the curvature of the earth, or roughly 1 millisecond). The time it takes to hear the return gives you the "range" or distance to the object that reflected the beam. When the sun is low on the horizon, the radar receiver picks up the radiation as if it were a continuous return, and it's visible on all of the weather radars.

The image is of the radar from St. Louis last night at 8:28 PM CDT. You can see how the weather radar interprets a continuous return as if there were stronger reflectors further out, and how far north the sun is at this time of year near the summer solstice.

Colin is here

Colin is spending the summer and next year in Santiago, Chile. I pulled up a recent TERRA image of Chile & surrounding areas:

http://rapidfire.sci.gsfc.nasa.gov/realtime/single.php?2005175/crefl1_143.A2005175145500-2005175150000.2km.jpg

If you have a computer with a lot of memory and a great Internet connection, try this one: http://rapidfire.sci.gsfc.nasa.gov/realtime/single.php?2005175/crefl1_143.A2005175145500-2005175150000.250m.jpg

These images were taken on 06/24/05 at 14:55 UTC, about three days ago.

As usual, wikipedia has a link to great canonical satellite image of the city: http://eol.jsc.nasa.gov/sseop/images/city/lowres/ISS004/ISS004-E-6990.JPG

A really high-res image of part of the city is here: http://www.landinfo.com/GalSatIkonosSantiago.htm

Planetary conjunction

Mercury, Saturn, and Venus bunch up low in the evening sky.
http://skyandtelescope.com/observing/article_1534_1.asp

Don't always assume

For some reason I trust newspapers' web sites to provide me with news that is important to me--like important local news that may affect me personally. Like a big transformer fire, or such, that kills power to the local Internet POP and destabilizes routing. But, the Chicago Tribune didn't do that yesterday evening; instead I first saw the story on dslreports.com and linked there was a local story from a TV station.

Time to melt some bricks

Sam bought a surplus fresnel lens for me. It's June, mostly clear, and it's time to melt some bricks.

Plants in the near infrared

I always had thought that the bright white appearance of foliage in the near infrared was due to chlorophyll fluorescence--indeed, in a college class on plant structure and function, I recall examining a large beaker filled with chlorophyll dissolved in some alcohol, illuminated with a strong beam of light, that was deep red when viewed to the side.
You can see a spectrum of various vegetation here. Note how strongly plants begin to reflect at 700nm, which is near the far-red limit of vision. (You can see light beyond 800nm, if it's bright enough, but that's another post of OSHA violations and laser regs).

However, this remote sensing page shows that the high albedo of plants in the near infrared is actually just due to the cellular structure of the leaf. Plant structures are essentially transparent in the near-infrared, and the light is efficiently scattered in the air spaces beyond the first layer of cells. This has been compared to the mechanism that causes snow to be so white and reflective for a substance that is made up of transparent pieces. Newer leaves are not as thick and full of air gaps as mature leaves, and so should reflect less. The spongy mesophyll (what a great term, in my opinion), this interior area, allows for proper interchange of gases, and access to it is controlled by the stomata. Here is a (large) photomicrograph of a leaf cross-section: http://koning.ecsu.ctstateu.edu/Plants_Human/images/leafcslabel.jpg

http://www.storm.uni.edu/rs/2001/vh3.html

I took most of these photos over a month ago, when Chicago was experiencing spring, so the trees had small leaves. In many cases, the wind is blowing the trees around and ruining the multi-second exposures!

Visual/Infrared comparisons

This is a split screen image of Yerkes Observatory in the visual and near-infrared. I moved the camera in between images and wasn't planning in advance, but the comparison still works. You can see the pine tree turns white, the blue sky turns very dark, and the tones of the brickwork and dome sheeting subtly change.

Car Windows
Automobile windows, we are all taught, are wonderful examples of the greenhouse effect. Visible light streams in, is absorbed by the car's interior, and converted to heat. The thermal infrared light is then reradiated by the interior but unable to escape the glass, and the car heats up.

Manufacturers are well aware of this fact, and today's cars have a higher glass to surface ratio than older ones. Given this, why aren't we melting the plastics in the car?

We could tint the windows, to reduce the total light into the car, but this is dangerous at night, and is completely impractical for the windshield.

The answer is in the glass itself. Automotive glass contains a special additive to absorb near-infrared light. NIR is worthless to human vision, but can contribute significantly to the heat load of a car. (I can't remember the reference, but I've read it can be 50%).

Here is an image of a car that visually had no tinted windows; indeed, no one can opaque their windshield legally:


A spectral graph of a glass like that is here. (Link used to go to a nice graph but it's missing).

Currency
You can see that American currency has some sort of blank stripe code on the back of the new bills: http://www.lib.uchicago.edu/~dean/infrared/slides/IMG_4784.html.
I saw this at here; and then a simple search shows that it is no secret but a great anti-counterfeiting technique: http://www.accubanker.com/support/irmappings.phtml

Plants
These need a separate post.
P.S. Here's an image of a Linne statue with a false-color mapping.
My infrared gallery
IR hosta
Bushes in the near-IR

Infrared gallery

I have placed most of the near infrared images I've taken at http://www.lib.uchicago.edu/~dean/infrared/.
All of the images were normalized with "Auto Levels" in Photoshop and desaturated to greyscale. The raw images are very purple and underexposed.



The filter material
was color negative film, fully exposed and processed, stacked
up in three. This sandwich is opaque to the eye except for
bright filaments. There is leakage around the sides of the filter when I place it on the camera lens, and since it's just negative film, there are significant reflections between the three pieces, creating a hotspot in the center of the frame.

In the camera, all three channels respond,
although the green is weaker than red and blue. The response
from all three channels must be the residual near-IR leakage
through the color filters, and scenes appear to be the same if
you compare each color channel. With a two layer sandwich you
begin to see more red light come through, and this creates a
neat contrast between near-infrared-red and near infrared:
http://www.lib.uchicago.edu/~dean/blog/ir2-auto.jpg. There is
a normal visual comparison at
http://www.lib.uchicago.edu/~dean/blog/ir-normal.jpg.
The exposures are roughly 1000x longer with the filter in bright sunlight.




In the next post, I'll discuss some interesting details about near-infrared light: plants, automotive windows, and currency.

And split screen comparisons with normal visual images.

Visual Infrared Comparisons
Plants in the near infrared
Linne Statue in the near infrared
IR bushes
IR hosta (or plantain lily)

Aurora photos from Yerkes Observatory

This Saturday members of the Ryerson Astronomical Society visited Yerkes Observatory to use the 41-inch reflector. Luckily the clouds cleared for observing until midnight or so, when the clouds reappeared. While taking a break, I noticed a faint glow low to the north, and assumed it was scattered light from the setting first quarter moon, but then I realized the moon was too far to the west for that to happen. Then the glow turned green, and I knew we had an aurora. The clouds cleared out, and we were treated to a fantastic display of the northern lights. It started out green and in the north, and spread overhead, where it suddenly changed color and we saw light pastel blue and violets in addition to the green. The display continued until dawn at 4AM.

http://astro.uchicago.edu/RAS/rchive/yerkes-aurora/index.html

IR bushes

Plus, a graph of the transmission of fully exposed and developed color negative film, from recorded by here:

This helps explain how dust removal scanners with ICE work--the film dyes are transparent in the infrared, but dust isn't. An infrared scan of the negative shows a clear negative with only the dust visible. Silver grains in black and white film are uniformly opaque to light and so ICE won't work with it.

Ryerson office background radiation

Steven conveniently graphed the data from my geiger counter, covering a week in the Ryerson Astronomical Society office. http://gateway.spc.uchicago.edu/~stevenl/cosmic/

Another airline plot

This was a trip to Las Vegas. Maximum altitude was at 39,000ft, and the flightpath was a relatively northern one across Nebraska and Colorado, then turned south in Utah. We flew for a while in southern Utah at a lower altitude while getting into the inbound approach, and you can see that on the graph.

http://www.lib.uchicago.edu/~dean/blog/mdwtolas.jpg

more near-IR tests

I added a third layer of color negative to the filter, and this makes the sandwich completely opaque to the naked eye. The sun is barely visible (but not at all safe to look at) through it. I can get a reasonable if slightly underexposed image of open shaded areas with an exposure of 4 seconds at f/5.6 at ASA 400. With the third layer there is no difference between the red and blue filters, so I believe whatever I am getting is residual IR leakage.

http://www.lib.uchicago.edu/~dean/blog/reynoldsclub-ir.jpg


That isn't snow, it's grass.

A clue to what wavelengths we are seeing is the fact that the streetlight happened to be on, and we can see it, and HPS lights emit a very intense 819.3nm and 819.5nm doublet.

near-IR world

http://www.lib.uchicago.edu/~dean/blog/ir1.jpg
http://www.lib.uchicago.edu/~dean/blog/ir2.jpg
http://www.lib.uchicago.edu/~dean/blog/ir-normal.jpg
These were shot with a near-IR filter over my camera
(a near infrared filter being two pieces of fully exposed color negative film).

If you swap the RGB colors around, you can eventually get something almost, but not quite entirely unlike, normal. http://www.lib.uchicago.edu/~dean/ir2-rgb-bgr.jpg

Some more (post-processing):
http://www.lib.uchicago.edu/~dean/blog/ir1-auto.jpg



http://www.lib.uchicago.edu/~dean/blog/ir2-colorswap.jpg

From looking at the RGB histograms out of the camera, it appears that the red and blue filters let the most near-IR through. Traditionally this is the case. The red lets very near-IR through, and the blue lets in IR at double the blue wavelength (a 450nm blue filter will let in 900nm IR, e.g.). The blue channel shows the most chlorophyll fluorescence too.

Lunar maria colors

A nice image of exaggerated maria colors on the Moon: http://www.rc-astro.com/img/moon_colors_2005-04-18_web.jpg.

Visually, you can see this, especially between Mare Serenitatis and Mare Tranquilatis (wow I can't believe I spelled that right the first time). They are the two prominent, quasi-circular touching maria on the left side. The color difference for maria basalts is the difference in composition of TiO2.

Tranquilatis, the bluer one, is very rich in TiO2.

Giguere, Thomas A.; Taylor, G. Jeffrey; Hawke, B. Ray; Lucey, Paul G.
The titanium contents of lunar mare basalts
Meteoritics & Planetary Science, vol. 35, no. 1, pp. 193-200 (2000).