As discussed earlier in this Ebook, in this section we are mainly speaking about filters which allow infrared light to pass through, rather than blocking it. The filter you choose must block all light except that in the 700- 1200nm range of the electromagnetic spectrum.
The cutoff point on the electromagnetic spectrum at which a filter blocks 50% of light measures its effectiveness. For example, while a Wratten #25 filter blocks 50% of illumination at 600nm, a Wratten #87A blocks 50% of light at 1050nm.
The higher the cutoff point, the more visible light is blocked. However, as the cutoff point increases, so does price; A Wratten #87A gel filter will set a buyer back around $200.00. For this reason, the Wratten #89B filter is a very popular choice for most buyers – it provides suitable visible light blockage for infrared photography, and is much cheaper at around $30.00. Filters with a cutoff point below 700nm are no longer true infrared filters as they pass some visible red as well. Filters bellow 700nm can be used if you are looking to capture a mix of infrared and visible light.