What’s The Difference between
Thermal Imaging and Night Vision?
Let’s start with a little
background. Our eyes see reflected light. Daylight cameras, night vision
devices, and the human eye all work on the same basic principle: visible
light energy hits something and bounces off it, a detector then receives
it and turns it into an image.
Whether an eyeball, or in a
camera, these detectors must receive enough light or they can’t make an
image. Obviously, there isn’t any sunlight to bounce off anything at
night, so they’re limited to the light provided by starlight, moonlight
and artificial lights. If there isn’t enough, they won’t do much to help
Thermal Imaging Cameras
Thermal imagers are altogether
different. In fact, we call them “cameras” but they are really sensors.
To understand how they work, the first thing you have to do is forget
everything you thought you knew about how cameras make pictures.
FLIRs make pictures from heat, not
visible light. Heat (also called infrared, or thermal,
energy) and light are both parts of the electromagnetic spectrum, but a
camera that can detect visible light won’t see thermal energy, and vice
Thermal cameras detect more than
just heat though; they detect tiny differences in heat – as
small as 0.01°C – and display them as shades of grey in black and white
TV video. This can be a tricky idea to get across, and many people just
don’t understand the concept, so we’ll spend a little time explaining
Everything we encounter in our
day-to-day lives gives off thermal energy, even ice. The hotter
something is the more thermal energy it emits. This emitted thermal
energy is called a “heat signature.” When two objects next to one
another have even subtly different heat signatures, they show up quite
clearly to a FLIR regardless of lighting conditions.
Thermal energy comes from a
combination of sources, depending on what you are viewing at the time.
Some things – warm-blooded animals (including people!), engines, and
machinery, for example – create their own heat, either biologically or
mechanically. Other things – land, rocks, buoys, vegetation – absorb
heat from the sun during the day and radiate it off during the night.
Because different materials absorb
and radiate thermal energy at different rates, an area that we think of
as being one temperature is actually a mosaic of subtly different
temperatures. This is why a log that’s been in the water for days on end
will appear to be a different temperature than the water, and is
therefore visible to a thermal imager. FLIRs detect these temperature
differences and translate them into image detail.
While all this can seem rather
complex, the reality is that modern thermal cameras are extremely easy
to use. Their imagery is clear and easy to understand, requiring no
training or interpretation. If you can watch TV, you can use a FLIR
Night Vision Devices
Those greenish pictures we see in
the movies and on TV come from night vision goggles (NVGs) or other
devices that use the same core technologies. NVGs take in small amounts
of visible light, magnify it greatly, and project that on a display.
Cameras made from NVG technology
have the same limitations as the naked eye: if there isn’t enough
visible light available, they can’t see well. The imaging performance of
anything that relies on reflected light is limited by the amount and
strength of the light being reflected.
NVG and other lowlight cameras are
not very useful during twilight hours, when there is too much light for
them to work effectively, but not enough light for you to see with the
naked eye. Thermal cameras aren’t affected by visible light, so they can
give you clear pictures even when you are looking into the setting sun.
In fact, you can aim a spotlight at a FLIR and still get a perfect
Infrared Illuminated (I2)
I2 cameras try to
generate their own reflected light by projecting a beam of near-infrared
energy that their imager can see when it bounces off an object. This
works to a point, but I2 cameras still rely on reflected
light to make an image, so they have the same limitations as any other
night vision camera that depends on reflected light energy – short
range, and poor contrast.
All of these visible light cameras
– daylight cameras, NVG cameras, and I2 cameras – work by
detecting reflected light energy. But the amount of reflected light they
receive is not the only factor that determines whether or not you’ll be
able to see with these cameras: image contrast matters, too.
If you’re looking at something
with lots of contrast compared to its surroundings, you’ll have a better
chance of seeing it with a visible light camera. If it doesn’t have good
contrast, you won’t see it well, no matter how bright the sun is
shining. A white object seen against a dark background has lots of
contrast. A darker object, however, will be hard for these cameras to
see against a dark background. This is called having poor contrast. At
night, when the lack of visible light naturally decreases image
contrast, visible light camera performance suffers even more.
Thermal imagers don’t have any of
these shortcomings. First, they have nothing to do with reflected light
energy: they see heat. Everything you see in normal daily life has a
heat signature. This is why you have a much better chance of seeing
something at night with a thermal imager than you do with visible light
camera, even a night vision camera.
In fact, many of the objects you
could be looking for, like people, generate their own contrast because
they generate their own heat. Thermal imagers can see them well because
they don’t just make pictures from heat; they make pictures from the
minute differences in heat between objects.
Night vision devices have the same
drawbacks that daylight and lowlight TV cameras do: they need enough
light, and enough contrast to create usable images. Thermal imagers, on
the other hand, see clearly day and night, while creating their own
contrast. Without a doubt, thermal cameras are the best 24-hour imaging
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• AXIS Cameras • RDASS Survey Imaging Pack • ICI 9320 IR Camera • Specialty Cameras • Thermal Imaging vs Night Vision • BIZ-711 Export Application Regulations for Cameras •
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