Tuesday, May 31, 2005

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).

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

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

Sunday, May 22, 2005

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
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)

Tuesday, May 17, 2005

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.


Tuesday, May 03, 2005

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.