You might be asking how I take photographs with invisible light? Infrared photographers use film or their digital sensor (and I use the later) to capture infrared radiation. My camera sensor can capture infrared waves in cloud cover and haze and “see” long-wavelength heat emitted by objects. It can make clouds look more interesting and make objects off in the distance visible when a normal camera would not see them. Infrared photography makes leaves and foliage white/pink and glowing.
Infrared is electromagnetic radiation with longer wavelengths than visible light. It is located on the electromagnetic spectrum between visible light and radio waves above 700nm. If your eyes just glazed over at 700nm, this is just a measurement of the wavelength in nanometers. Visible light has wavelengths between 390nm and 700nm, ranging from violet to red light. Below 400nm in the spectrum is known as ultraviolet light. Infrared radiation is used in medical, industrial and scientific applications, as well as photography.
The light that I work with in my digital infrared photography is in the spectrum between 590nm and 900nm. It is only slightly longer than visible light and sometimes includes some visible light. I use a Sony Alpha A7II full-spectrum camera that was converted for me by LifePixel Infrared Camera Conversions. Full-spectrum means that my camera has the ability to capture ultra-violet, visible and infrared images on its sensor. Sony and other manufacturers make digital cameras with a hot mirror over the sensor in order to block (reflect) infrared and UV light but allow visible light to pass through. LifePixel removed the hot mirror from my camera and replaced it with a clear filter that allows UV, visible and infrared light to pass through. In order to use my camera for infrared, I must add an Infrared filter in front of my lens. I have filters of 590nm, 665nm, 720nm and 830nm. Those filters restrict the amount of visible and infrared light that passes through with individual cut-off points that restrict the wavelengths, thus yielding the different infrared looks. If I want to use my camera as a visible light camera, I add a hot mirror filter back in front of my lens, which restricts the infrared and ultraviolet waves.
Infrared light focuses at a different point than visible light. Using a mirrorless camera with an electronic viewfinder makes it possible for me to make exact focus decisions. In the days when photographers only had infrared film to use, some lenses had marks that showed the infrared infinity focus, but it was hit and miss and not as accurate as what digital photographers now have available. Even with the electronic viewfinder, lenses need focus calibration on some systems in order to autofocus. My modern lenses do not need calibration, but one of my favorites is an older lens which I had LifePixel calibrate to my camera. I will mention here that a regular infrared conversion usually just consists of removing the hot mirror and replacing it with an infrared filter over the sensor. This might be a 720nm, but 590nm, 665nm, and 830nm are also available. This type of camera does not require a filter on the lens, but it can no longer be used for visible light photography. I've included the link to LifePixel below. They explain in more detail what you would expect from each conversion and have sample images.
The 720nm filter is considered the standard for infrared. The red wavelength cuts off at 750nm, so it only lets a little visible light in. The 590nm produces super false color (leaving in the wavelengths of orange and red, blue, and some green), 665nm filter produces extra color (leaving in much of the red wavelength), 830nm filter produces deep infrared and has dark skies and bright whites with no color.
Once the filter is in place on the front of my lens, a custom white balance is set in the camera before the shot is taken. The colors violet, blue, green and yellow have wavelengths shorter than 590nm. The colors captured are orange and red and must be manipulated. I have to exchange red and blue color channels in post-processing in order to have blue skies and pink/white foliage. This is called false color. Many times I opt to convert my photos to black and white.
Why do I do this? I love the surreal look and dramatic skies in my infrared photos. I love the surprise of delving into the bland result that comes out of the camera and turning it into a fine art photo with details that the human eye can not see. I can hardly contain my enthusiasm when I am out photographing with my infrared camera.