Near Infrared Digital Photography: A Tutorial
by Eric H Cheng
June 03, 1999
Near infrared light consists of light just beyond visible red light (wavelengths greater than 780nm). Contrary to popular thought, near infrared photography does not allow the recording of thermal radiation (heat). Far-infrared thermal imaging requires more specialized equipment, and is not the subject of this tutorial. Infrared images exhibit a few distinct effects that give them an exotic, antique look. Plant life looks completely white because it reflects almost all infrared light (because of this effect, infrared photography is commonly used in aerial photography to analyze crop yields, pest control, etc.) The sky is a stark black because no infrared light is scattered. Human skin looks pale and ghostly. I have a photograph of a friend's black shirt, which looks white in infrared. My dark sunglasses all but disappear in infrared because they don't block any infrared light, and it's said that you can capture the near infrared emissions of a common iron.
Infrared photography has been around for at least 70 years, but until recently has not been easily accessible to those not versed in traditional photographic processes. Since the charge-coupled devices (CCDs) used in digital cameras and camcorders are sensitive to near-infrared light, they can be used to capture infrared photos. With a filter that blocks out all visible light (also frequently called a "cold mirror" filter), most modern digital cameras and camcorders can capture photographs in infrared. In addition, they have LCD screens, which can be used to preview the resulting image in real-time, a tool unavailable in traditional photography without using filters that allow some visible (red) light through.
This tutorial will take you through all of the steps necessary to procure and manipulate near infrared photographs through the use of a digital camera (or camcorder). You will need:
Before we begin, you will need to check to see if your camera has an infrared blocking filter (also called a "hot mirror" filter) installed. A quick way to test this is to see if your camera can see beams emitted from a common infrared remote control. These beams seem to be fairly bright and are easily seen in a camera's LCD preview area. If you don't have an LCD on your camera, your only hope is to try to capture the beam by taking a picture of your remote (while depressing a button on it). If you cannot see the infrared beam, you may have to remove the infrared blocking filter by disassembling the camera. Note that this may void the warranty. Most brands of digital cameras are known to be sensitive to near infrared light.
Get a few different types of filters to play with. A #87C filter will filter out all visible light, but since these filters gradually filter out more and more light as the wavelength increases, the #87C will also filter out a good amount of the infrared light you're trying to capture! I use the #87 filter because it filters out all visible light, but still lets in enough of the infrared spectrum for clear crisp images. The #25 filter lets in a significant amount of red light, and is often used in traditional photography because it allows image previewing through the viewfinder. These filters do not work well with digital cameras. Supposedly, you can buy a visible light filter from Edmund Scientific for $5, but I'm guessing that it's not of very high optical quality. (609-573-6250, ask for a 1" diameter circular filter, part no. H43948)
Here's a table of % light transmission at different wavelengths for a few of the filters specified above. You should be able to figure out the approximate behavior of the other filters by comparing them to this table.
A tripod is absolutely essential for digital infrared photography. Even though the newest consumer or prosumer digital cameras can be pushed to ISO 320 or more, shutter speeds in daylight will still be on the order of 1/15-1/30 of a second.
You will also need a way to attach the filter to your camera. If your camera is threaded, then it's easy. Buy a gelatin filter holder and some step-up rings, or buy a threaded glass filter (expensive!). If your camera is not threaded, then you'll have to be creative.
My own equipment list includes: (for purchasing information, see the digital ir faq)
The Agfa produces grainy, dim photos with the #87C and #87 filters because of the effectiveness of its hot-mirror filter and limitations on how long the shutter can be left open, but the Nikon 950 produces wonderfully bright, clear images. In any case, you'll probably need an image editor to "pull-out" the image from the output of some of the lower-end digital cameras. Auto-leveling in Adobe Photoshop (Shift-Ctrl-L) usually does the trick. If you don't have Photoshop, find an image editor that allows you similar functionality (one that allows you to expand the variation between the darkest and brightest levels in the photo for more contrast).
Acquiring Infrared Images
Taking infrared photographs is simple, but there are a few things you should always be thinking about. First attach your filter to your camera. Check to see that no light can leak through whatever attachment mechanism you choose to use! Make sure you turn off the flash on your digital camera. Since very little light will be penetrating the visible light filter, your camera most likely underexpose if it tries to use the flash. Set your camera to its highest ISO rating. Once these precautions have been taken, you're ready to shoot! Take some test photographs to make sure that your set-up works. Remember to try to use your tripod.
If the images come out very dark, your camera is underexposing. My old Agfa ePhoto 1280 took pictures so dark that I initially thought that my experiment had failed (see below for an example). A dark exposure can be caused by any number of things, but most likely, it means that the combination of your camera's maximum aperture setting and minimum shutter speed still isn't letting in enough infrared light. Although this situation is not ideal, you can use Adobe Photoshop's "Levels" feature to expand the dynamic range of the picture. Try this by opening the image in Photoshop, and hitting Shift-Ctrl-L to "Auto Level" your image. To make sure the maximum allowable light is being captured by your camera, you can also try manually setting the aperture to its maximum setting and the shutter speed to its slowest setting. You can also try switching to a filter that lets in more visible light (like the Kodak Wratten #89B). If the images are still dark, then you'll have to work with what you have, and use the dark images -- the JPEG artifacting due to the low dynamic range of the Agfa ePhoto 1280's images seems to create a sense of nostalgia due to the unique texture of the resulting images!
In some of your infrared images, you may notice faint streaks of color. This is normal, and probably arises from strange CCD responses to this spectra of light. It may also come from the trace amount of red light that the filters may let through. As an option, you may want to desaturate your image (Shft-Ctrl-U) or force your image to grayscale (Image->Mode->Grayscale). In practice, I've found that forcing the image to grayscale preserves smooth textures better than desaturation of color. You may also notice small, brightly colored spots in your image if your shutter speed was very slow and/or you've pushed your camera to higher ISO ratings. These spots are the result of CCD noise, and can be removed by manual retouching, or with software like Qimage Pro or Camera Bits' Quantum Mechanic(TM).
Tutorial - Creating a view into the world of infrared
This tutorial will take you through the steps of creating an image that serves as a good illustration of a view into the world of infrared. You will need one infrared photograph and one full color photograph taken from the exact same place (again, a tripod is ideal), a photograph of a filter, frame, or similar object, and Adobe Photoshop. For the purposes of this tutorial, I will be using Adobe Photoshop 5.0. The filter or frame will be used to frame the infrared portion of the image in the final composite. If you don't want to deal with finding, photographing, and masking the frame, you can use the one that I used in the above image.
First, open the image of the filter, or frame in Photoshop. You'll need to cut out the entire filter or frame, not including the center portion. If you do not know how to do this, use the filter included with this tutorial, and read up on masking later -- it's a good skill to have in your arsenal of image manipulation tricks.
Now, open both the color and infrared images. You'll probably want to auto-level the infrared image in Photoshop (Shift-Ctrl-L), or manipulate both images to make them look perfect before proceeding to the next step.
Our goal is to merge all three images into our final product. First, click on the window with the color photo to make it active. We're going to copy its contents into a new Photoshop document. Hit Ctrl-A to select the entire image, and then Ctrl-C to copy it to the clipboard. Now, create a new Photoshop document (Ctrl-N), accept the defaults in the window by clicking "OK" (it will automatically size to the size of the image you're copying), and paste the image into the new windows (Ctrl-V). You will see a copy of the color image in your new window. If you don't see the layers control window, select the menu item Windows->Show Layers to make sure it is visible. Double click the "Layer 1" layer and rename it to something more descriptive, like "Color Image". Now, save the new document and continue. I'm going to refer to this new document as the composite image from now on.
Now we need to copy the entire contents of the infrared photo into the composite image's window as a new layer. Copy the contents of the infrared photo into the new window by following the same steps above. Click on the infrared image's window, hit Ctrl-A and then Ctrl-C. Click the composite image's window, and hit Ctrl-V to paste in the new layer. You'll see a new layer labeled "Layer 1" appear in the Layers tab in one of the control windows. Double-click on "Layer 1" to change the name to something more descriptive, such as ""Infrared Image".
Do the exact same thing with the filter image, and rename the layer to "Filter".
Now we have a document with three layers: a color image, an infrared version of the same image, and a filter. In the composite image's window, you should only see the infrared image with a filter on top of it because the color layer is being blocked by the ones above it in the list of layers.
Our next goal is to size the filter to an appropriate size, and then apply a mask to the infrared layer to let the color image through everywhere except for the area inside the filter. Select the "Filter" layer so that it is hilighted in the layers window. Now, select Edit->Transform->Scale from your menu bar. Click on one of the corner rectangles, and drag the cursor around while holding the mouse button down. When you let go of the mouse button, the filter will resize itself to fit in the bounding box you just specified. Note that you can hold down the Shift key while resizing to force the filter to stay circular. You can also move the filter around by clicking anywhere inside the box and dragging the cursor while holding the mouse button down. Adjust the size and position of the filter until you are happy. When you are done, double click anywhere in the box to commit your changes.
With the quality of the current crop of new digital cameras, exploring infrared photography is finally a possibility for those of us who don't want to deal with the hassles of traditional infrared photography. While it is by no means a replacement for photography in the realm of visible light, it certainly adds a unique, creative twist to digital imaging.