It was a dark night with just a small sliver of moon shining through the trees. The mouse nosed its way through the underbrush using its nose to find small nuts and berries.  Unknown to the mouse there was something watching from a tree limb high above. The owl was able to see the mouse clearly even though there was hardly any light.  Poor mouse.  He had no idea he was in trouble. 

How does the owl see in the dark?  This article describes the various ways that animals, and more importantly, video cameras can see in the dark. We discuss how cameras make use of the limited light available, cameras that use IR illumination and cameras that use thermal imaging technologies.

Owls and other nocturnal creatures have very large eyes and have many more light receptors in their eyes.  This dramatically improves their night vision, but they do require some light.  To better understand how cameras see at night let’s first review how we measure light. 

Measuring Light

Foot-Candles
A foot-candle measures how much light is generated from a light source. For example, if you take a birthday cake with only one candle (Happy Birthday) and stand one foot away from it you will see one foot-candle of light. Also note that if you stand further away you see less light and if you stand closer you see more light.  The power of the light is inversely proportional to the distance from the light source.
 
Lumens
A LUMEN is also a unit of measurement of light. While a foot-candle is how bright the light is, a lumen is a way of measuring how much light gets to what you want to light up! One LUMEN is equal to one foot-candle falling on one square foot of area.
For example if we take the candle and place a book one foot away from the candle, and if that book happens to be one foot by one foot (one square foot), then all the light falling on that book equals one LUMEN!
 
LUX
Illuminace is measured in Lux.  It’s the light reflected from the object rather than the light hitting the object.  One Lux equals one lumen per square meter, so this is measured in metric units rather than English units of measure. To reinforce this, LUX is the measurement of actual light available at a given distance from an object.

Illuminace is what results from the use of light. You turn your flashlight on in a dark room, and you light something up. That's illuminace. Turning on a light in a dark room to make the burglar visible gives you illuminace. It also gives you another problem when you note the burglar is pointing your duck gun at your bellybutton.

Here are some practical examples.  Typical illuminance values are:

1 lx	Full moon
10 lx	Street lighting
100-1,000 lx	Workspace lighting
10,000 lx	Surgery lighting
100,000 lx	Sunshine

Night Vision, Seeing in the Dark

Seeing in the dark is actually a relative term.  The mouse thinks it’s too dark to see while the owl is able to see perfectly fine at night.  If there’s a full moon we can see things dimly, but if there’s no moon it’s too dark for us and the mouse to see anything. 

There are three ways to improve what we see at night:  We can amplify the small amount of light available with a good low light camera. We can illuminate the area using an IR illuminator or we can detect the heat levels coming off an object using a thermal imaging camera. 

Low Light Sensitivity Cameras
Cameras with good low light sensitivity are just like the owl. They are able to see an image even when it’s too dark to see with our own eyesight.

The owl and all other nocturnal animals have big eyes. So cameras with good night time vision use lenses that accept as much light as possible. Lenses with f number of 1.4 or lower allow a lot of light to get to the camera's sensor.   The lower the f number, the more light can get through the lens, so it’s a good idea to select a lens with the lowest f number available.  

Low-light cameras also have very good image sensors.  Usually the larger the sensor the more light is detected, but the latest CCD and CMOS detectors are extremely good at picking up the light. Low-light cameras are able to switch from color to monochrome to improve light sensitivity, and they do a good job of amplifying the available light.

The owl was able to see even when there was hardly any moonlight. A full moon provides about 1 lux of light so if we want to see as well or better than the owl, we need to have a camera with a low light sensitivity of less than 1 lux.  Good low-light cameras operate at very low light levels.  For example the Axis 232D+ PTZ IP camera switches to monochrome and can detect light as low as 0.005 lux. There are some analog monochrome cameras from Watec with even better low light capability (0.0005 lx).  Cameras that can see at this very low light have to increase the video amplification dramatically. Unfortunately at the limit of their amplification electrical noise is introduced, so it’s likely to get fuzzy images at this very low light level.

IR Sensitive Cameras
Day/night cameras not only can operate at low light levels, they are also sensitive to IR illumination.  IR stands for infrared which means below red (lower frequency).  It is similar to standard light except it is below the visible red light that we can see. The wavelength of this light is between 700 and 1400 nm.

Day/night cameras usually have filters that control IR light entering the camera sensor.  During the day the IR light is blocked because it affects the color. When it’s dark and the camera switches to monochrome mode, the filter is removed to allow the IR light to hit the camera sensor.  The camera should also use a lens that doesn’t block the IR light. For example there are day/night type lenses from Computar and others that are rated for IR applications.
For an IR camera to work you need IR illumination. IR illumination can come from special bulbs or from IR LEDs. Some cameras have the IR illumination built-in, but all day/night cameras can use separate IR illumators.  For example the Vivotek FD6111V has a small IR illuminator that allows the camera to see without any other light source within a distance of 10M (over 30 ft).  IR light is absorbed by the water in the air so it doesn’t have very long range. To get ranges of 500 ft or more you will need a lot of power and a very narrow beam.  One of the more powerful IR illuminators I am aware of is the UF600 from ExtremeCCTV. This model, with a 6 degree beam, has a range of 400M (1300 Ft).

An IR sensitive camera is usually used when you don’t want people to know you are viewing them.  The IR light is not visible so just like the mouse; they don’t know they are being watched.  

Thermal Imaging Cameras
Thermal imaging or thermographic imaging cameras are able to see when there is absolutely no visible light. They go beyond the capability of the owl. An owl or night vision device acquires what little light is available from the moon or stars and amplifies that light to allow it to see at night.  If there is no light available then the night vision device will not work.  Only a thermal imager can see in absolute darkness.

These cameras are sensitive to heat radiation that comes from an object that’s warmer than the surrounding environment.  They can see the heat coming from a person or animal.  Thermal imaging cameras can detect emitted energy through smoke, dust, fog, blowing sand, rain and snow.  Visible light wavelengths bounce or are reflected off the obscurant particles due to the wavelength being short.  The long wavelength of the infrared spectrum can pass through obscurants allowing a thermal imager to detect the emitted energy.

Thermal imaging cameras are sometimes referred to as FLIR (Forward Looking InfraRed) cameras. The name came from the military application of placing a camera on the front of an airplane (forward looking).   

The resolution of thermal imaging cameras is considerably lower than that of optical cameras, mostly 160x120 or 320x240 pixels, up to 640x512 for the most expensive models. These thermographic cameras are much more expensive than their visible-spectrum counterparts, and higher-end models are often export-restricted.  The newest technologies use low cost and uncooled microbolometers FPA sensors.  A microbolometer is a specific type of bolometer used as a detector in a thermal camera. Infrared radiation with wavelengths between 8-13 μm strikes the detector material, heating it, and thus changing its electrical resistance. This resistance change is measured and processed into electrical signals which can be used to create an image. Unlike other types of infrared detecting equipment, microbolometers do not require cooling.
 
Thermal imaging cameras require a special lens that works at this light frequency. Germanium is most widely used for lenses and windows in IR systems operating in the 2 µm - 12 µm range.  Standard lenses block the thermal light while germanium lenses are transparent to this spectrum of light. These lenses are expensive and take a long time to make. 

Like a standard camera, the resolution of the camera is defined by the number of pixels in the sensor; with the more pixels, the better the resolution.  We need a minimum of 4 to 8 pixels per foot to detect a person.  With a narrow angle lens we can detect a person at a distance of 2000 Meters.

There are a number of manufacturers such as: FLIR, HurlyIR and others.  FLIR is probably the largest manufacturer of these types of cameras, while HurlyIR is a small organization with excellent engineering services that create specific camera solutions customized to the exact requirements of the customer. Thermal imaging cameras are much more expensive than standard cameras with prices starting at around $10,000 and can be as high as $60,000 or more.

Summary
Cameras with very good light sensitivity can see things the human eye can’t see; they can even see better than the owl.  By adding IR illumination the camera can see when there’s no visible light.  Thermal imaging or thermographic cameras can see without any light at all, using the thermal energy coming from a heated object. 

In general, if there is some light it’s best to use a camera with good low-light sensitivity.  If there is no light, you can add IR illumination, but the least expensive solution is to add standard lights. IR illumination is added only if you don’t want the person to know that they are being watched.  A thermal imaging camera is usually used by the military because it is best for very long distance detection.

So if you want to learn more about how to see as well as an owl, please contact us at 914-944-3425, at infohome at kintronics.com or by completing the contact form.