night vision - a general overview

see also:

• bushwalking in Australia
• infrared photography
• outdoor thermal imaging devices - search and rescue, hunting, hiking
• near IR night vision devices for seeing wildlife

• it takes ~ 30 to 60min for your eyes to adapt to the dark
• this process must restart if there is significant exposure to lights, especially bright lights
  • minimise this by using faint red lights but if you can see that it's red on the paper you are looking at, it's too bright
• if you go out for long on a sunny day, expect to lose about three-quarters of a magnitude in your dark adaptation stellar magnitude threshold the succeeding night—after such extended exposure to high-intensity scenes (beach, snow-skiing on sunny days), it takes more than 24 hours to become fully dark-adapted! The usual half-hour or hour won't do. Thus wear “glacier glasses” when outside during daytime if you want the best night adapted vision.

• near IR night vision devices
• far IR thermal imaging devices
• optical monoculars or binoculars

• near IR night vision devices detect reflected near IR light in wavelengths shorter than 1µm (eg. 850 or 940nm IR lamps) or heat sources 250-500degC which give heat off in this wavelengths
  • these generally give better subject definition and eye shine than thermal imagers but 850nm lamps will scare most prey and even 940nm lamps will be visible while the lamps have a shorter range of use
  • essentially most of these are just digital camera sensors without an IR blocking filter and optimised for IR wavelengths
  • see near IR night vision devices for seeing wildlife

• thermal imaging devices do NOT use IR lamps for imaging but detect the longer long wave mid-IR wavelengths (usually 7-14µm) that are given off by objects and animals with the wavelength dependent upon the object's temperature and use a microbolometer type sensor
  • detection range will depend upon sensor technology, lens aperture and base magnification
  • subject detection also requires thermal contrast between subject and background hence will not detect cold blooded animals and will not be as effective when ambient temperatures match warm blooded animal temperature
  • ability to see in heavy fog or rain will require a NETD of < 40mK
  • see outdoor thermal imaging devices - search and rescue, hunting, hiking

• optical devices which amplify ambient light
  • normal optical binoculars or monoculars amplify light by having a larger light collection “funnel” than your naked eyes with light gathering increasing in proportion to the area of the objective (ie. by the square of the radius with the fully dilated naked eye having a diameter of 4-9mm and in adults averaging around 6mm)
    • a 42mm objective gives 49x light gathering power of the average adult naked eye (21mm/3mm = 7x the radius of the dilated pupil)
    • a 50mm objective gives 64x light gathering power of the average adult naked eye (25mm/3mm = 8x the radius) thus a 10×50 binoculars are limited to an apparent stellar magnitude of +9.5 to +11 depending on sky conditions and observer experience (dark adapted naked eyes are limited to stellar magnitude of around +6.5 with each integer step of magnitude being 2.5x dimmer than the previous)
    • a 70mm objective gives 121x light gathering power of the average adult naked eye (35mm/3mm = 11x the radius) or twice that of the 50mm objective
  • in addition, the larger diameter allows one to resolve more detail as resolving power is directly proportional to diameter assuming similar optic quality
• twilight factor
  • theoretical ability to see detail in low light, the higher the value the better (although transmission and optical quality will also affect performance, indeed modern high quality coatings and optics may be more important than the twilight factor in determining detail in low light)
  • = sqroot (magnification x objective diameter mm)
  • needs to be > 17 to have utility in low light
• relative brightness index
  • used to compare the brightness of binoculars or spotting scopes of similar magnification. The larger the relative brightness number, the brighter the image.
  • = square of exit pupil in mm
  • NB. note again that an exit pupil larger than the person's actual pupil will not result in further brightness
  • one can multiply this by 1.5 when comparing multicoated vs uncoated vintage binoculars

• they have an advantage over the above digital devices in that they do not disrupt your eye's night vision and do not require batteries (apart from those that have built-in image stabilisers or laser rangefinders)
• these are obviously also useful for seeing much fainter astronomy subjects than are visible to the naked eye although serious astro users will go for heavy, expensive models with 70-150mm optics
  • those more powerful than 15×70 require support of some type such as a tripod and even those with magnifications more than 10x may need support or image stabilisation
• they do need some ambient light and thus work best on nights with a moon but will have trouble seeing into the shadows or through fog whereas the above will not have these issues
• unlike the other devices they are not as limited by distance although there is still an optical limitation due to atmospheric effects and magnitude/resolution factors
• can be used behind a glass window
  • near IR devices with a built-in IR lamp will need to be pressed against the window otherwise the IR light will just reflect off the window as if you are taking a photo with flash on
  • far IR thermal devices do not generally see through windows as most windows will block far IR light longer than 3 micron wavelengths (humans give off IR at 9-10 microns) and thus the devices will just see reflected IR light

• in day light, a good-quality 8x32mm binocular delivers all the light a person can use, producing just as bright an image as a 50mm binocular, but where the larger objective delivers is in low light.
• for a given magnification, there is little advantage of a larger size objective if the exit pupil is larger than your night eyes - for young adults this is 7mm, for older adults it is 4-6mm (exit pupil = objective size / magnification), in addition, a large exit pupil also collects more light from the background sky, effectively decreasing contrast, making the detection of faint objects in the sky more difficult except perhaps in remote locations with negligible light pollution.
• mobile hunters will want a waterproof, sturdy, relatively light (under 1kg) and compact low light binocular with an exit pupil to match their eyes and a magnification to match their needs - perhaps a 8×42, 8×56, 10×50 or 12×56
• some may have angular height mil markings (mil reticle)
  • these allow optical range finding via the formula distance in metres = object height in metres x 1000 / mil
  • more accurate than a LRF over water - hence mainly in marine, tactical or military binoculars
  • work over longer distances than a LRF
  • do not need batteries
  • can estimate the size of an object if the distance is known (eg. by use of a LRF)
  • BUT are slower to use and not as accurate as a LRF
• some may have a built in laser range finder (LRF) which is more suited to hunting than a optical rangefinder but also adds weight and need for batteries
• avoid zoom binos, good zoom eyepieces are expensive, and most will reduce optical quality
• the best prisms are made from BAK-4 glass, while others use BK-7 glass
• roof prism designs are smaller and lighter but have less stereo effect and require higher quality and phase control - cheap versions will not be good!
• check the shape of the exit pupil - if they are not circular, the prisms may be under-sized or lower quality
• ensure they are fully multi-coated - physically check each end to ensure reflections are dark
• if you need to wear glasses for astigmatism make sure you can see the full field of view with glasses on
• check the two optical barrels are aligned properly (collimated) - if they are out of alignment they will make you feel sea-sick, or if really bad you will get double vision
• test it on stars:
• can you bring a star to perfect focus?
• if, as you turn the focus, little rays start growing out of the star in all directions before the rest of the star comes down to focus, you're looking at spherical aberration - this could be your eyes which can't be corrected by glasses but can be reduced by using a smaller exit pupil
• once a star is focused in the centre, move it to the edge to check the field of focus is flat and not curved
• see also: https://skyandtelescope.org/astronomy-equipment/binoculars-for-astronomy/