Cyfrowa fotografia w podczerwieni.pdf

Permanently IR-modified Canon Rebel XT. Stitched panorama,

260Mpixel

Digital Infrared Photography

Part One

Marco Annaratone Email: RLph3oto@res3onantlink.com

Claudio Ruscello Email: IR3vision@li3bero.it

To email to one of the authors please remove first both digits 'three' from either email address above.

LIGHT AND PHOTOGRAPHY

Any radiation can be characterized by its wavelength , and light is of course no exception. What

we consider as “light” is in fact visible light , i.e., that part of the radiation spectrum that the

eye-brain system can indeed capture. We see various color hues, going from violet through

blue, and then green, then yellow, then orange, and finally down to red and deep red. A

wavelength in this area of the spectrum is typically measured in nanometers, where one

nanometer is equal to one billionth of a meter. The wavelength of violet is about 400nm; going

down to deep red the wavelength increases, from about 460nm (blue) to 540nm (green) and

then yellow (600nm) and finally deep red (750nm). Beyond violet and below deep red there

are radiations the human eye cannot see, i.e., ultraviolet (UV) and infrared (IR). Although the

human eye cannot see these radiations, the human body can certainly feel and react to them.

Infrared is often perceived as heat and we know how harmful certain UV rays can be to the

skin.

In what follows we will focus on infrared photography, a technique that can capture on film or

on a digital sensor the IR radiation reflected by the scene we frame. IR radiation does start

below a wavelength of 750nm 1 and continues down to 20,000nm or more. However, both film

and sensor are seriously limited in their capability to record IR radiation. Digital sensors can go

as far as 1300nm. Commercial IR films are unable to record radiations below about 900nm.

What we call “IR photography” then is the technique to capture IR radiation in the limited

range between 750nm and 1300nm (in the case of digital sensors) and even less in the case of

IR film.

IR photography is certainly not a new concept. On the contrary, it has been around for a long

time: the 1935 “Leica Manual” by Morgan and Lester has a section dedicated to IR

photography. IR photography was already carried out in the nineteenth century, but its appeal

1 Note that all these numbers are approximations. It does not mean that, say, yellow stops at 600nm and that a

wavelength of 601nm is ‘another color.’ The transition is not so abrupt! Some conventional threshold (albeit

approximate) needs to be agreed upon to simplify the discussion.

to the general public grew significantly after 1931, when technology made it possible to shoot

IR pictures using easy to handle infrared sensitive plates 2 .

The reason why IR photography is undergoing a small Renaissance nowadays is that digital

sensors are quite sensitive to IR radiation. This opens up, as we shall see below, interesting

opportunities in utilizing again this old tool.

WHY INFRARED (IR) PHOTOGRAPHY?

IR photography opens up a new visual dimension for the photographer, a somewhat 'different'

way of looking at the world around us. The spectrum of light is much wider than what the

human eye can capture and, until CCD sensors became affordable to the general public, the

only way to capture infrared radiation was to use special film, indeed sensitive to this part of

the spectrum. Many are the applications of IR photography, from criminology to

photomicrography and celestial photography. We shall focus here on landscape photography,

but the reader interested in experimenting in other fields should be well aware that what is

discussed here is the classical tip of the iceberg.

One of the fascinating features of IR photography is its ability to penetrate haze and light fog.

As we have discussed above, infrared radiation has a longer wavelength than visible light and

can 'go through haze’ more easily. This is becoming, unfortunately, more and more important

as the level of pollution in the air increases. Some even go as far as theorizing that 'moderate'

monochrome IR photography 3 may become the de facto standard in landscape black and white

photography, as finding truly crisp and clear days is getting more and more difficult.

Permanently IR-modified Canon Rebel XT. False color mode.

Stitched panorama, 490 Mpixel

At any rate, if we use black and white (BN) IR film we obtain a black and white negative, while

if we do the same (using methods we will see below) with a digital camera we will obtain an

image in 'false colors' that can be then manipulated and modified as we wish (see above).

When film was still the only game in town one could also buy 'false color' IR film. Nowadays,

color IR film has all but disappeared.

In the past, IR pictures where often hand-painted to deliver “quasi natural colors”. Here is an

excerpt from “Leica Manual” by Morgan and Lester (1943): “A very interesting application of

2 Handbook of Photography by Henney and Dudley, Whittlesey House, 1939.

3 By ‘moderate’ we mean a way to do IR photography without the strong “IR fingerprint.” This can be accomplished in

the digital domain by reducing or taming the “IR look” in post-production.

infra-red to landscape photography is to enlarge the photograph and tone the enlargement

blue. If properly composed and toned the photograph will then show white clouds against a

deep blue sky, white trees and grass, and various gray tones for buildings and pavements. The

addition of oil coloring to the trees and grass and other parts of the picture will produce a sur-

prisingly good imitation of a natural color photograph.”

In order to really appreciate the beauty of IR photography, we need to limit ourselves to the

part of the IR spectrum that the sensor (or film) can capture, i.e., we have to block visible

light. This is accomplished by using an appropriate filter in front the lens. This filter will let the

infrared portion of the spectrum go through while blocking at the same time the visible light

part of the spectrum. This filter looks like any other filter used in black and white photography

with one significant difference: it looks completely black , i.e., totally opaque, to the human

eye! 4

All the most important companies producing photographic filters offer at least one IR filter. The

availability of IR filters may be in some geographies somewhat difficult though, especially if the

diameter of the filter is not a common one (67mm or 72mm are relatively common diameters

for filters, for instance). One may want to consider (also to contain costs) to buy one large-

diameter IR filter (say, 77mm) and then use step-up rings to put it on lenses of different

diameter. An IR filter that seems to be easier to find than others if the Hoya R72. This filter is

a high-pass filter, i.e., it blocks all wavelength that are below a certain 'cut-off' wavelength . In

the case of the Hoya R72 this wavelength is about 700nm (nanometers, a nanometer is one

billionth of a meter). All the wavelengths below 700nm (i.e., all the visible light, ultraviolet

light, and so on) will be blocked by the filter and will not go through it. Going back to the

wavelength numbers presented in the introduction the reader will immediately notice that this

filter lets also some visible light (deep red) go through. In fact, the filter does not look totally

opaque to the eye but rather very dark red.

In the next page we show the same identical scene taken in three different ways, i.e., in color,

in black and white visible light, and in black and white infrared radiation. These shots have

been taken with a stock Nikon E5000 digital cameras and for the IR picture a Hoya filter has

been used in front of the non-interchangeable lens.

It is quite easy to notice that, besides the obvious 'whitening' of the leaves (a classical effect of

infrared photography due to the strong IR reflective power that the chlorophyll contained in

plants has), the infrared shot suffers from almost no haze at all (see the mountain range on

the extreme left of the picture) when compared to either the color picture or the visible light

BN one. Note also the dramatic effect that capturing IR radiation has on the sky.

4 Referring to the wavelength numbers presented above we will block all the radiation below 750nm and let that above

750nm go through.

Color picture in visible light

Black and white picture in visible light

Black and white IR picture with Hoya R72 filter

ANALOG INFRARED PHOTOGRAPHY

The only way to do ‘analog’ IR photography is by using a film such as Kodak HIE Infrared or

MACO IR820C (now Rollei 820c). These films tend to be quite sensitive to stray light, so it is

better to load a 35mm film into the camera in the dark. This is instead a must and not just a

recommendation with 120 film, where a changing bag is certainly a necessary accessory to

carry around. Another thing to check is whether the camera has a mechanical frame counter;

some cameras have this feature implemented via infrared light hitting an infrared sensor and

this may in fact slightly fog the film around the area where the IR ray hits the film.

IR film: Maco 820c, now Rollei IR820c

One of the authors of this paper has been using the Maco 820c for years with a great deal of

satisfaction, in spite of the constraints imposed by this film (that are quite typical of IR film

and not at all unique to the Maco 820c, though). The need for long exposures is one of them:

while the film is rated at around 200 ISO, the presence of the dark filter in front of the lens

results in exposure times between 1/2s and 2s even in bright sunny days for reasonable

f/stops (e.g., f/8) 5 . The tripod is therefore a must. Excellent results have been obtained from

this film with abundant pre-wash to eliminate the anti-halation coating and by developing it in

Kodak XTOL 1:2 for about 13 minutes at 20 o C.

This film maintains its sensitivity, as the name suggests, up to around 820nm. Beyond 820nm

its sensitivity drops rather abruptly. It is therefore a film sensitive to dark red and just the

onset of infrared. For this reason the film should not be used in conjunction with a ‘true’ IR

filter, but rather with ‘very dark red’ filters. Using it with IR filters results in a completely

unexposed frame. While IR filters tend to block wavelengths below about 900nm, the Maco

820c is sensitive up to about 820nm. There is no intersection between the film sensitivity and

the filter cut-off wavelength: where the film is sensitive the filter blocks the radiation from

reaching the film; where the filter lets the radiation go through, the film is no longer sensitive.

The result is therefore a completely unexposed piece of film!

An excellent match with the Maco 820c film is instead the 89B (092) dark red B&W filter. With

this combination one can obtain interesting images like the ones in the next page.

5 In the past, film speed could be increased by hypersensitizing the film by mercury vapor treatment. This could deliver

at most a 100% improvement in film speed, though. And we are now well aware of how terribly dangerous the exposure

to mercury vapors can be.

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