Wednesday, August 29, 2018

Making Invisible Waves Visible, Near Infrared Imaging - Part 1

Simone models her scary giraffe costume in visible and near IR
The human eye is not capable of detecting wavelengths much larger than 700 nm. However, digital cameras have the capability to detect near infrared radiation a little beyond 1000 nm in wavelength.  This is almost as large a range in wavelength as the entire visible spectrum! Digital cameras are able to detect this electromagnetic radiation and reveal it as images and movies. This allows many creative and novel ways to explore IR radiation in the classroom and beyond. For example, clothing looks different in near IR as the above Halloween costume reveals. In this post I will give examples of near IR images I have made. In Part 2 I will describe how to make your own inexpensive near IR camera.

There are many things that can be explored with a near IR camera. One of the more surprising things to observe is paper money. The back of $10, $20, and $100 bills have strips of near IR reflecting pigment that obscure the printing behind it.
The reverse side of a $20 bill in visible and near IR
I remember showing this to a laser physicist and he got so excited he went home and made his own near IR camera, and he worked every day at the National Ignition Facility at LLNL! The patterns are different allowing scanners to quickly and accurately differentiate between denominations as these images of other denominations show.
The reverse side of a $10 and $100 bill in near IR, notice almost invisible 100 on right
The counterfeiting measures on the front of the $100 bill look very different in near IR. The inkwell disappears revealing the Liberty Bell. This happens in visible if you tilt the bill. The fountain pen and phrases from the Declaration of Independence disappear and the security ribbon becomes very faint.
Obverse of $100 bill in visible and near IR
It is interesting to watch appliances like hotplates, toasters, hairdryers, and a range top burners warming up. These are all easily visible in near IR even when there is no red glow yet detectable to the eye. The range of brightness is much greater than seen in visible. Below is video of me touching a hotplate when it was just warm but glowing in near IR.
Near IR passes through black plastic bags. You can shine an IR LED through it or place IR emitters like this space heater behind it and see right through the bag.
Visible and near IR image of a black plastic bag draped in from of space heater
Fluorescent light sources are dimly visible, the warmer ends showing up brighter. Compact fluorescent lights show up brighter than standard fluorescent tubes. Reflected fluorescent light is mostly invisible. A room illuminated by only fluorescent light is dark.
My classroom with all the fluorescent lights on in visible and near IR on a foggy morning
Incandescent lights are very bright and can light up the whole room with near IR. The filament can be seen for a few seconds after they have been turned off. Different color LEDs are interesting. Red shows up brighter than other colors. The red LED has a peak wavelength in the visible but still emits light on either side of this peak, some in the near IR. Most of the near IR from the red and other LEDs is probably from thermal radiation. IR LEDs show up as intense lights. Remote controls have near IR LEDs and can be used as near IR flashlights to “see” objects in an otherwise dark room.
The red LEDs of my binary clock and shining a remote control on my face in the dark with a remote control shining on it
Smoke and dust particles scatter visible light better than near IR. As a result, near IR reveals distant objects that are obscured in visible light.
South San Fransisco Bay Area from Saratoga, distant mountains only seen in near IR include Mission Peak
There was a small wildfire in North Cascades National Park during our trip in July of 2018. I took several photos of the mountains in visible and near IR. This shot of Colonial Peak turned out the best. Notice the extra details visible in near IR and the smoke is less of a problem. The shadows are more stark, showing that near IR does not scatter as much as visible. Trees reflect near IR more than visible, showing up brighter.
Colonial Peak looms over Diablo Lake in North Cascades National Park
The Los Gatos High School pool looked very different in IR. Very little IR is coming from the bottom of the pool so the lane markings on the bottom can't be seen. The sky reflected from the surface looks dark making it look like a senior prank involving ink has been perpetrated. Other reflections off the pool are more visible like the flags and light poles in near IR.
Los Gatos High School pool in visible and near IR
There are many other things to check out, I won't show them all here. LCD monitors show up as a dull glow with nothing showing from the display. Some have a bright corner where the back light is located. Red lasers are absolutely dark unless you shine it directly into the camera. The resulting dim glow is probably thermal radiation. Different items of black clothing that looks identical in visible can have dramatically different shades in near IR. Plants tend to reflect a lot of near IR but this varies by species.

I hope I have piqued your interest in near IR imaging. Near IR cameras can be made by modifying a webcam. All you need to do is replace the IR cut filter with an IR pass filter. Another option is to purchase a Sony Nightshot camera and place a near IR pass filter over the lens. You can find used Nightshot cameras on eBay for less than $40. I will describe both of these options in detail in my next post. If you can't wait, there is a lot already posted on the Internet, search "DIY infrared webcam".

No comments: