Infrared radiation is a form of lightwave that is not visible to humans. It is above 700nm in the electromagnetic spectrum, and is located between visible light and radio waves. Visible light has wavelengths between 400nm and 700nm, ranging from violet to red light. Below 400nm is the spectrum known as ultraviolet light. Humans don’t see ultraviolet, but some birds and insects do.
You might be asking how I take photographs with invisible light? Photographers use film or their digital sensor (and I use the later) to capture infrared radiation, instead of visible light. My camera sensor can capture IR (infrared) waves in cloud cover and haze and “see” long-wavelength heat emitted by objects. It can make clouds look more interesting and an object off in the distance visible when a normal camera would not see them. IR photography makes leaves and foliage appear white/pink and glowing.
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 camera that was converted for me by LifePixel Infrared Camera Conversions. The manufacturer makes digital cameras with a hot mirror over the sensor. The hot mirror allows visible light to pass through but reflects infrared light. 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 wavelength. 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 waves.
Infrared light focuses at a different point than visible light. Using a mirror-less 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 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, and LifePixel calibrated it for me.
The 720nm filter is considered the standard for infrared. The 590nm is super false color, 665nm is extra color, 830nm is deep infrared and has dark skies and bright whites with little or 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. When I look at the photo taken with all but the 830nm filter I see mostly red, gold and some bluish colors. In post processing, I have to exchange what is red for what is blue 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 reddish result that comes out of the camera and turning it into a fine art photo with details that the human eye could not see. I can hardly contain my enthusiasm when I am out photographing with my infrared camera.