Open your eyes, now close them - and see different.
Now we know that we are
only able to visually decode so much of the waveferno around us - and surely
not for the worst ;-)
Mentioning infrared photography most commonly brings the enthusiastic layman to respond with "Ahh, capturing heat signatures!" along with conspiratorial lifted eyebrows. First work to do there is to set this right where we stumble over one important differentiation - the whole infrared band usually gets divided into 3 scientific different ranges, for our purposes we only need two. The first range from about 780nm to 1400nm is called the "near" infrared whilst everything above is to be referred to as "far" infrared. Near means close to our visible spectrum. The far infrared cannot be decoded with optical means - sensors, usually cooled down to freezing ranges, need to be used to do thermal imaging there.
We photographers, as big devotees to the glass we need to use, need only to care about the near infrared then - and that´s exciting enough!
Basics about light and colors:
Light waves are being emitted
and hit objects on their course. Every object reflects or absorbs a certain
amount of them. Our built-in imager stands somewhere in space where reflected
rays can hit the photoreceptors. That´s the general setup. If an object
absorbs every incoming ray we get none ourselves from it - so we see "black"
as in nothing. There is no such thing as a perfect blackbody in our visible
spectrum, so an object is always bound to reflect something. If an objects reflects
as much as the entire spectrum we perceive it as white - the colors in between
are being the perception of partial reflectance of waves. The single channels
of our cone-based color system are put together by our brain to "show"
a color image.
Now if we would like to
perceive only one "color" or kind of wavelength - we need to get rid
of all the other waves. If we had a freely customizable"filter" where
we could easily set our starting and end wavelength the result would of course
be a completely monochrome image. Depending on the kind of imager used, an 8-bit
based system would record 256 different shades of luminance - zero being nothing
or black, 255 the highest as in white.
Based on this we can now take a look into the band between 780nm an 1200nm.
If there was a filter for the whole range here we´d get quite a dull image - 420nm of information is quite much, too much obviously for 256 shades of tonal difference. Good thing that the different available filters all have very narrow windows to look into the near infrared.
Because we want to see what
we can´t see. To start with it will prove most interesting for us to cut
all wavelengths below 780nm. The real infrared experience can only be had this
way. There is infrared-photography as a quick effect to make the own pictures
more interesting - fast. Too bad you´ll lose some valuable food for thoughts
along this way. So i strongly recommend to buy one of the "deep" filters
as the first one - you´ll not regret it.
There is quite a bunch of
companies offering different filters, manufacturers of the glass they use seem
not as numerous though.
IR filters are offered by B+W, Cokin, Heliopan, Hoya and Kodak Wratten. Every company (at least the professional ones) offers spectral charts for their filters, this is needed to choose the own filter.
These are the spectral charts for Heliopan filters:
As we see, this company only produces 3, to be really exact 2 filters that fulfill our wish to completely block the visible spectrum.
The unwanted waves are now
banned from reaching our imager, only "invisible" rays can pass -
we hold a small, deep black disc in our hand and wait for the narrator to go
Many road are leading towards Rome, most of them with holes and dips for which we now need a map.
Infrared photography can be achieved on 2 quite different imaging media - analog film in all sizes and digital sensors.
Before reaching the crossroads where we say goodbye to film-based media and concentrate on the digital world first some basic information.
The media of our choice
for recording has to be sensitive for the wavelengths we`d like to see.
To reproduce "natural" colors the best all manufacturers have found that infrared "pollution" is unwanted and counterproductive - so on every media there are different technical traps for this band.Your normal "fantastic colors" film stops being sensitive before the near infrared band as well as do most of the black and white films. You can screw on your filter and try it - but next to nothing won't be much else to see for you on the resulting picture.
The solution at hand is a few specialized films - a handful of black and white films and one color film.The most famous of them would be Kodak´s HIE (High speed Infrared Emulsion). Used for military aerial photography first and then entering into civilian use now at least for this use quite obsolete because there are enough high-tech laden satellites up there that do this job even better and quicker. Good that fine art photographers and enthusiastic amateurs took a liking of this technique so today stocks are still very wide available - but still at a considerable premium. Ilford, Maco, Konica and some others offer infrared-sensitive films today, the one looking into the band the deepest still is the Kodak HIE which peaks out at about 1000nm.
The offering for color infrared never came to roll really fast - the Kodak EIR could only convince a small crowd of photographers of it´s pictorial value.
The reason why infrared photography lives through a little renaissance these days is a technical "flaw" in modern photo-cells out of which CCD elements are comprised. With the very first digital cameras it seems to have escaped the designers that the elements are well sensitive up to about 1200nm. At this time no or very weak cut-filters were installed which led to "infrared-bleeding", manifested in many pictures as "ugly" skin and red tones.
Until 2002 this still posed as a major problem for many manufacturers - it looks as though a simple complete cutoff at 780nm was not enough and research was slower than marketing demands. 2004 this is being handled very well - color reproduction in general has never been better and infrared photography never been that hard ;-)
The "system-cameras" most often got a little optical block inserted into the ray-tube on which the Hot Mirror (for sending bad IR back to where it came from) was vaporized. Single Lens Reflex cameras had to incorporate other solutions - optical elements in front of the CCD are in a system with changeable lenses not possible. From quite nice detachable screens the manufacturers then started to get comfortable with vaporizing the Hot Mirror directly onto the Bayer grid on the CCD.
Using film the photographer
himself decides the sensitivity of his recording media - all he has to do then
is choose the right lens and the filter and expose for what it´s worth.
Digital cameras have a "built-in" sensitivity due to the photo cells on the CCD's, only the manufacturers had to work hard to give the people great colored images and cut the sensitivity. Depending on the model there are more or less hurdles to take before we can produce stunning pictures - more on that in the next chapter.