When I received my converted Fuji XT3 back at Christmas, transformed into an infrared camera, the first thing I did was to walk around the neighborhood with it looking at the world in this new light.
What first strikes you, looking through through the viewfinder, are the classic telltale signs of an infrared image—that white vegetation and those dark, dark skies.
The sky goes dark with an infrared-only camera because the atmosphere scatters light, but not equally. Longer wavelengths (like red, and yes, infrared) don’t get scattered that much while shorter wavelengths (like blue) does–thus the blue sky (scattered light) and the yellow Sun (not so scattered light). Also, water (and water in the atmosphere) absorbs infrared (lakes and oceans tend to look very dark to black in infrared). Which one dominates I have no clue.
Why the vegetation goes white on the other hand is much more interesting. Leaves are, of course, green, which is right in the middle of the visible spectrum, ands if you look at a graph of the spectral reflectance of plants in the visible light range you’ll generally see a sort of little hill or bump right there at green.
The higher the bump, the brighter that part of the spectrum and so, while the plant bounces back some blue colors (on the left side) and some yellow and red (on the right), most of the reflected light is green, thus the plant looks green to our eyes. The red rectangle (the infrared part of the graph), is hiding something surprising.
If you remove the red rectangle to show the part of the spectrum that is invisible to our eyes but not to the infrared camera you see a dramatic increase in reflectivity.
Why this sudden jump in brightness occurs just outside the range of the spectrum invisible to humans (but visible to some other living things) I do not know. It has the feel of an evolutionary adaptation of some sort where seeing the infrared would be disadvantageous to us or advantageous to plants. But I’m not a biologist!
In any event, vegetation, which is reflecting a greater percentage of light than many of the other things in the scene, registers as white in an infrared image.
Clothes can show unexpected tones as well. Here is an image (taken by my daughter, Lizz) of a pair of black leather boots. They render as white, though her pants and jacket remain dark.
And here is a picture (also taken by my daughter, Lizz) of some apprehensive-looking dude, probably some control freak who feels nervous having someone else use his camera. His shoes look about right—they are bright orange in color—but his dark blue jeans have moved up the tonal scale and his flannel shirt with the standard plaid pattern has gone white, with no hint of the dyes at all.
Obviously some dyes are highly reflective of infrared and some are not. Or perhaps it has more to do with the fabric whiteners —although whiteners tend to operate at the blue end of the spectrum. A mystery!
Here’s another mystery. I was waiting for a train at the Denver Union Station last month and was looking up at the ceiling and at the lights and decided it might make for a good picture. But when I looked through the viewfinder I could see what appeared to be large shadows on the ceiling. Only there was nothing casting the shadows.
Here’s a daytime shot (made by Flickr user JCSullivan24 to get you oriented.
And here is my image, looking straight up at the ceiling, made on a cold January night:
I have various theories, none of them good.
One theory has to do with heat. Perhaps there is a big difference in the temperature of the ceiling areas, with the dark areas much colder than the rest? Heat does register on the image in infrared as this outdoor heater image demonstrates:
It looks a little blinding but in fact there were only dim red coils inside. So maybe we are seeing something to do with plumbing? Or exposure to the cold January air?
You can make unexpected discoveries in infrared.
If you point an iPhone at the infrared camera the camera’s display freaks out, flashing and behaving as if it is experiencing some major electronic failure. The phenomenon apparently has something to do with the iPhone facial recognition system.
Don’t do that to your poor camera.
You can also make other sorts of discoveries with infrared. Here is a painting credited to the “Master of Frankfurt and Workshop,” part of the rewarding exhibit at the Denver Art Museum, Saints, Sinners, Lovers, and Fools. It’s a traveling show and the real name of the exhibit is the decidedly boring and academic “From Memling to Rubens: The Golden Age of Flanders,” which doesn’t do the show justice at all. Denver has wise curators.
The painting, a nativity scene commissioned by Emperor Maximillian, features on its right panel a portrait of…wait for it….Emperor Maximillian!. He is adorned in a wonderfully flamboyant array of leather boots, fur-trimmed coat, and all of the rest.
The painting looks to be in near-perfect condition but if you look again in infrared light you can easily see where it has been damaged (by water?) and has been repaired. This may be the first time this painting has been imaged in IR.
The natural world holds the promise of discoveries as well, for infrared imaging.
If I had been asked the question, are rainbows visible in infrared light?, I would have chuckled in a sort on condescending way at the naiveté of the question, and replied that, of course not, rainbows are made by the splitting of sunlight into their separate bands of the spectrum and so you get the whole visible range up there, arcing through the sky. But just writing it down, putting the reasoning into words, gives the feeling that something it wrong with that logic. And so there is:
The little droplets of rain do indeed spread the incoming sunlight into separate frequencies—thus the rainbow—but it doesn’t just do it to the visible spectrum (somehow knowing what frequencies humans can see and which we cannot see), it does it to all of the light from the Sun. Of course. In infrared you can see the portion of the rainbow that is in the infrared part of the spectrum and if you could see (or had a camera set up to image in) the ultraviolet part of the spectrum you’d see that portion of the rainbow there, too.
Imaging in infrared isn’t like photographing in a whole new world but it is like photographing in our own world plus a little bit more.