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Lenses and Infra-red
Much
like cameras, lenses vary in price and quality. Remember that the
"system" is only as good as the weakest link in the chain.
Lenses are a key element in good night time IR imaging. Lenses that
perform adequately during the day in good lighting conditions may
be appalling at night when used with IR.
Lens factors to bear in mind:
Prime or fixed focal length lenses:
These lenses are fine for applications where changes of image size
are not required and the available focal lengths suit the application.
The main test for these lenses is to open the aperture fully and
check the image is sharp across the whole picture area, often on
the cheaper lenses the focus falls off badly at the edges. Bear
in mind that auto iris lenses usually work at full aperture at night.
Some of the low cost lenses do not work well in IR light due to
the poor anti-reflection coatings. There are some lenses made which
will try to focus both photopic and IR light in the same place but
the cost is still high and the range limited. IR light will normally
focus behind the photopic light so requiring a focusing adjustment
at night when using IR lighting.
Varifocal lenses:
These are useful where intermediate focal lengths are required and
offer a zoom type feature but it is necessary to refocus at each
focal length. Some companies e.g. Fujinon are offering lenses with
special extended IR coatings to improve transmission in the near
IR.
Zoom lenses:
A zoom lens differs from a varifocal in that the lens is designed
to stay at the same focus throughout it's zoom range. It is essential
to adjust the "back focus" of the lens/camera to optimise
the "tracking" of the lens.
To correctly adjust the back focus is often an issue and using IR
can compound the problems. The best way to adjust the back focus
is to ensure the lens aperture is fully open. This can be done on
auto iris cameras by capping the lens to force the iris wide open
and then unplugging the iris drive cable. If back focus is done
in daylight you will need to increase the camera shutter speed or
put a neutral density filter over the lens to get a viewable picture.
Zoom the lens to its longest focal length (tele end), focus the
lens on an object some distance away (anywhere between infinity
and 10m). Zoom the lens to it's widest angle and adjust the back
focus to give the best picture. Repeat the process until the lens
tracks through its range correctly (or the best it can). The back
focus position is different for IR and various approaches are used
to get a compromise unless a motorised back focus is available,
this can then be preset to different positions for both daylight
and IR working. The compromise often used is to set the back focus
for IR working and use the fact that the lens will stop down during
the day thereby improving both the depth of field and focus sufficiently
to give good pictures.
Other factors to watch out for with zoom lenses - f. numbers! they
are not what they seem. The calculation for the f. number is derived
from the apparent size of the aperture relative to the focal length
of the lens, and should indicate how much light the lens is able
to pass. This formula holds reasonably true for prime lenses, however,
zoom lenses usually have a lot more glass and suffer large transmission
losses. The calculation may say a lens is f.1.8 but the real transmission
value could be f.2 or f.2.2 - more than a stop less. Many of the
bigger zoom lenses suffer "ramping" at the longer end
of the zoom range. This is where the maximum f. number is much reduced
at the tele end of the zoom range and the image darkens as you zoom
right in on an object. Ramping is usually more pronounced on the
cheaper lenses and should be taken into account when designing lighting
requirements for a system.
More....
Infra-red light, Cameras and
Infra-red
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