I was recently searching for film to buy and saw an auction for "infrared" film, however what was being auctioned wasn't true infrared film -it was infrared-like. So, I thought I would take this opportunity to talk about the properties that make infrared film infrared.
What does infrared mean? Well, light is electromagnetic radiation (EMR), a form of energy that is emitted through charged particles. EMR is divided into frequencies, which include visible light, infrared radiation, radio waves, and x-rays.
Visible light has a short wavelength of about 380 nm to 740 nm, which is the light that we see. The infrared spectrum falls above the visible red wavelength at about 700 nm and includes the far-infrared thermal radiation emitted by objects in thermal imaging. Infrared film is sensitive to the near-infrared spectrum.
Infrared-like film, or film that has extended infrared sensitivity, is not true infrared film, however its sensitivity reaches the bottom limit of the infrared range (at ~780 nm). This film can be handled and developed like regular film without special handling requirements. An example of infrared-like film is Ilford's SFX, which even the Lomography website incorrectly classifies as true infrared film.
True infrared film is sensitive to wavelengths up to 900 nm and requires special handling, filters, and even specific types of cameras. An example of true infrared film is Rollei IR. Unfortunately, many of the true infrared films are no longer manufactured.
Hopefully this helps to differentiate between the two types of film that produce infrared and infrared-like effects. Next time I will talk more about the special handling requirements of infrared film and the types of effects both of the film types offer.