Let us begin with a brief background. Human eyes see reflected light. The human eye, thermal cameras, night vision devices, and daylight cameras, all work on the same principle: visible light energy strikes an object, bounces off it, a sensor receives it and forms an image.
Whether an eyeball or in-camera sensors, these detectors must receive enough light, or they can't make an image.
Obviously, there is no sunlight during the night, so they are confined to the light supplied by the moon, stars and artificial sources. They won't do much to help you see if there is not enough light.
Thermal Imaging Cameras
Thermal imagers are entirely different. We call these "cameras," but they're actually sensors. To know how they work, the first thing you have to do is forget everything you thought you knew about how cameras make images.
FLIR's make pictures from heat, the not visible light. Heat (also known as infrared, or thermal, energy) and light are both portions of the electromagnetic spectrum. However, a camera that can detect visible light will not see thermal energy and vice versa.
Thermal cameras identify more than heat; they can sense little variations in heat -- as small as 0.01°C -- and display them as shades of gray or with different colors. This can be a complex idea to get across, and many people just don't know the concept, so we'll spend a little time describing it.
Everything we experience in our everyday lives emits thermal energy, even ice. The hotter something is, the more it gives off the thermal energy. This emitted thermal energy is known as a "heat signature." When two objects alongside one another have different heat signatures, FLIR can clearly see them, whatever be the lighting conditions.
Thermal energy arises from many sources, on what you are seeing at the time. Some things – engines, warm-blooded animals and people, and machinery, for example – generate their own heat, either mechanically or biologically. Other things – rocks, land, buoys, vegetation – absorb heat from the sun during the day and radiate it off during the night.
As different materials radiate and absorb energy at various speeds, a place that we consider as being one temperature is a mosaic of subtly varying temperatures. This is the reason a log that has been in the water for days on end will seem to be a different temperature than the water and is hence visible to a thermal imaging device. FLIR's detect these temperature variations and convert them into the image detail.
The truth is that while this may seem somewhat complicated, the reality is that advanced thermal cameras are remarkably easy to use. Their imagery is simple and very clear to comprehend, requiring no interpretation or training.
Night Vision Devices
The greenish images that we see in the videos and on TV actually come from night vision goggles (NVGs) or other systems that use the same underlying technologies. NVGs take in little amounts of visible light, magnify it considerably, and present that on display.
Cameras made with NVG technology have the same constraints as the human eye: they can not see well when there is not sufficient visible light available. The imaging performance of something that depends on reflected light is limited by the amount and strength of the visible light.
NVG and low light cameras are not very helpful during twilight hours when there is too much light for them to operate but not enough light that you see with the naked eye. Thermal cameras aren't influenced by visible light, so they can provide you clear pictures even when you are looking into the setting sun. In fact, you can project a spotlight at a FLIR and still get a perfect image.
Infrared Illuminated Cameras
Infrared Illuminated (I2) cameras try to produce their own reflected light through projecting a ray of near-infrared energy their imager can observe when it reflects an object.
This works to a point, but I2 cameras rely on reflected light to generate a picture so that they have the same limitations as any other night vision camera that relies on reflected light energy or poor contrast, and short-range.
All these visible-light cameras -- daylight cameras, NVG I2 cameras, and other cameras -- operate by detecting reflected light energy.
However, the amount of reflected light they receive is not the only factor that defines whether or not you can see with these cameras: image contrast also matters.
If you are looking with lots of contrast in comparison to its surroundings, you will have a better opportunity of seeing it with a visible light camera.
You won't see it, no matter how bright the sun is shining. A white object seen against a backdrop has lots of contrast. A darker thing will be challenging for these cameras to see against a dark background. This is known as having poor contrast.
During the night, when the lack of visible light naturally lowers image contrast, visible light camera performance suffers more.
Thermal imagers do not have any of these shortcomings. First, they don't have anything to do with the reflected light energy; that is: they see heat. Whatever you see in normal daily life has a heat signature. This is the reason you have a much better chance of seeing something at nightlight using a thermal imager than you do with the visible light camera, even a night vision camera.
Many of the items you could be looking for, like folks, create their own contrast because they make their own heat. Thermal imaging devices can see them well since they do not just create images from heat; they create images in the moment differences in temperature between objects.
Night vision devices have the same shortcomings that daylight and lowlight TV cameras do: they want sufficient light, and enough contrast to generate usable images.
On the other hand, thermal imagers, see clearly day and night, while making their own contrast. Undoubtedly, thermal cameras are the best 24-hour imaging option.