The principle behind this technique
What is infrared? How do you take an infrared photograph?
The term “infrared” means very simply that electromagnetic frequency range that is placed below the frequency of the Red color. If we consider what is called the “visible spectrum” represented below, we see that the part called “infrared” is the one outside the visible range of our eye.
The purpose of infrared photography is to eliminate all the part that is visible to our eyes and to “visualize” only that of that small part called infrared. To do this, special filters are used which are affixed in front of the lens and which block the entire part of the visible spectrum, letting only that relating to infrared frequencies pass through.
It is possible to speak of frequency, measured in Hz or for greater convenience with multiples KHz, MHz, GHz, etc … or of wavelength, measured in m (meters) and in the case of frequencies of this type, submultiples are always used as nm (nanometer) which corresponds to 1 · 10-9 m or the ångström which has the symbol of Å which corresponds to 1 · 10-10 m. The relationship that binds the frequency and the wavelength is:
λ = c/f
Where λ is the wavelength, c is the speed of light, and f is the frequency.
There are various types of infrared filters, the differences can be those relating to the bandwidth, i.e. how wide is the frequency range that crosses it and that of the maximum transfer point, i.e. the wavelength in which the filter becomes more “transparent”. Filters for infrared photography typically appear as in the photo opposite.
Apparently they are black and looking through us you can see practically nothing precisely because the function is to “block” the visible light, letting only the infrared light pass through, which obviously we cannot perceive.
By imagining the visible spectrum as shown above, and expanding only the visible part, it is possible to imagine the effect of the filter that is applied to the lens as indicated in the following diagram:
In practice, the two areas, the red one and the violet one, indicate the part of the frequencies that are passed through by the filter. As you can see, each filter has its maximum transfer peak at different wavelengths. The filter in the previous figure is marked as IR850. This means that it has the maximum transfer point at 850 nm (nanometers). Considering that the visible band ranges from about 390nm, corresponding to the violet color, up to about 700nm, corresponding to the red color, a transfer point of 850nm corresponds to a wavelength that is well beyond red.
Filters that have a maximum transparency peak at 850nm or higher can be considered a filter for the “deep” or “far” infrared while filters that have a peak transparency of less than 800nm are called “near” infrared filters. The example is that of a famous filter, called R72, which has a maximum transparency peak at 720nm.
Poiché questi filtri non hanno una finestra molto strettissima, ovvero non fanno “passare” solo le frequenze che hanno una lunghezza d’onda pari al valore Since these filters do not have a very narrow window, ie they do not “pass” only the frequencies that have a wavelength equal to the indicated value but rather have several nanometers of aperture (sometimes up to +/- 50 nm or more – as you can see in the previous figure), with an R72 filter there is a nice portion of red color. This type of near infrared photography, due to the presence of a more or less significant Red in the photo, is very suitable for the various artistic interpretations in post production in which each photographer chooses one or more particular processing techniques, a little ‘as if it were an author’s signature.
Did you like the explanation, do you still have doubts or something that is not clear to you? you can contact me by email and I will be happy to give you an answer or to help you in some way.