Thermal Night Vision Cameras

[LakeSuperior]

Originally Posted by Tkeeth

clouds show up as warmer becuase they are reflecting the heat in the atmoshpere. as clouds get thicker, you cannot see through them. lay back on a partly cloudy night and watch a jet dissapear. even better, look down at the top of your fish tank and see if you can spot a fish.

FWIW! Clouds do not reflect IR radiation but rather emit IR radiation at the lapse temperature (air temperature at elevation). Clouds are for all practical purposes blackbodies.

[Paramotorgreg]

I have a night vision monocular. It can either amplify low light or project IR for vision in zero light. It is useful when anchored for identifying objects ashore at night, but with any significant level of magnification if the boat is moving much e.g. over waves, the image is so unstable it is difficult to focus and virtually useless.

[Tkeeth]

Originally Posted by LakeSuperior

FWIW! Clouds do not reflect IR radiation but rather emit IR radiation at the lapse temperature (air temperature at elevation). Clouds are for all practical purposes blackbodies.

Clouds also have a major role in reflecting some of the Sun's short wavelength (visible light) radiation back into space. The proportion of incident radiation reflected by a substance is called its albedo. The albedo of low thick clouds such as stratocumulus is about 90 percent. The albedo of high thin clouds such as cirrus may be as low as 10 percent. The albedo could vary with the wavelength of the radiation, but for clouds it does not as evidenced by the fact that they are white under white light. At sunrise and sunset the incident light is red, orange or yellow and the clouds reflect this light without modification. The albedo of clouds for infrared radiation is likely the same for visible light. There are two sides, top and bottom, to clouds that may be involved in the reflection of radiation.

Clouds, Cloudiness, Surface Temperature, the Greenhouse Effect and Global Climate Change

If the cloud is optically thick, it radiates like a blackbody so that the observed emission is equivalent to the actual temperature at the top (if the upper portion of the cloud is tenuous, then the radiating level will be within the cloud). If the cloud is optically thin, then the emission will appear to be larger than that for the cloud top temperature because additional radiation is transmitted from the warmer atmosphere and surface below. The ISCCP parameter has been corrected for this effect using the measured optical thickness to determine the transmission (daytime only).
https://isccp.giss.nasa.gov/cloudtypes.html

[Dockhead]

I have a third generation night vision device, forward looking sonar, AND radar on my boat, so I can comment from experience on the relative usefulness of all these devices for avoiding running into things in the dark.

Forward looking sonar sees something like only 8 times the depth -- and not the water depth, but the depth of the object you are seeing. So it is great for picking around slowly in a shallow anchorage, but utterly useless for avoiding hitting things. It would see something like a floating container only maybe 2 seconds before impact at sea speed.

Night vision only sees warm things. And are you going to sit with your eyes glued to it all night?

A good radar set, on the other hand, especially one with good target discrimination like the CW sets (3G, 4G, etc.), will see almost everything down almost to crab pot size, assuming of course that the sea is calm enough in relation to the size of the object you want to see. It can distinguish well enough between clutter and real objects that guard zone alarms are really useful, so you don't have to sit with your eyes glued to it.

Radar doesn't see objects underwater, but nothing available to recreational sailors does. The only possibly effective means of avoiding hitting a whale at night is -- prayer and good sailor's karma.

So on a scale of 1 to 100, the relative usefulness of these devices for avoiding running into stuff at night is, I would say:

1. Radar: 99
2. Night vision: 3
3. Forward looking sonar: 0

That's based on experience, and not arm chair speculation.

P.S. Some of you may know that I consider the so-called "Mark I Eyeball" to be greatly overrated, and will know that I always get irritated at the argument that you don't need technology if you're a real man and get back to basics with your good old Mark I Eyeballs. HOWEVER, I must say, that for avoiding running into stuff, the Mark I Eyeball is actually a very useful tool, even on a pretty dark night. It's not actually all that common for it to be so dark, that you can't see anything with well adapted eyes. Really only moonless nights in heavy overcast. Even starlight is enough to see quite a lot.

[LakeSuperior]

Originally Posted by Dockhead

Night vision only sees warm things. And are you going to sit with your eyes glued to it all night?

Lots of confusion out here... terminology must be clarified.

Night vision devices amplify photons that are reflected from an object into the input aperture of the device. Consequently, night vision optics can see everything that reflects wavelengths that are just near the red end of the visible spectrum. Therefore, night vision devices do not see only warm things they see all reflective things. The temperature is not relevant for this discussion.

FLIRs or thermal imaging systems are sensitive to long wave length thermal emissions and reflections from an object (3-5 or 8-12 um wavelengths). The combinations of reflections and emissions that depend on temperature (reflection/emission combinations change with the angles of the surfaces) create apparent temperature differences. These differences are what are seen in a thermal imaging system as different shades of gray.

[Dockhead]

Originally Posted by LakeSuperior

Lots of confusion out here... terminology must be clarified.

Night vision devices amplify photons that are reflected from an object into the input aperture of the device. Consequently, night vision optics can see everything that reflects wavelengths that are just near the red end of the visible spectrum. Therefore, night vision devices do not see only warm things they see all reflective things. The temperature is not relevant for this discussion.

FLIRs or thermal imaging systems are sensitive to long wave length thermal emissions and reflections from an object (3-5 or 8-12 um wavelengths). The combinations of reflections and emissions that depend on temperature (reflection/emission combinations change with the angles of the surfaces) create apparent temperature differences. These differences are what are seen in a thermal imaging system as different shades of gray.

Thanks for the correction.

[hellosailor]

And as TK didn't quite mention, the meteorologists are always going on about how one night will be particularly cold, because there is no cloud cover, and that allows the heat from the earth to go out into space, rather than being reflected back down. Seems like a lot of people with alleged degrees in the subject, think clouds reflect IR.

Dockhead-
The MarkI Eyeball can be an incredible piece of equipment. But the variation among units can be incredible too. Some folks can read newspaper classified ads by moonlight. Others can't see a pedestrian on the roadside--and that's even before cataracts set in. Or floaters (aka "asteroids") or retinal problems set in, as they frequently do with age.

Good sharp young eyes? Pretty useful. But also, very much not standard issue.

[Tkeeth]

This is fun.....

Abstract
We have derived a simple approximation for the emissivity and flux emissivity of water clouds inside the atmospheric window between 8 and 14 m. In our approximation the emissivity in the 8–11.5 m band is a function only of the cloud's liquid water content and cloud thickness. When compared with the exact radiative transfer calculations the broad-band flux emissivities (in the 8–11.5 m region) differ by less than 10%. At wavelengths > 11.5 m the emissivity is a function of the droplet size distribution as well. By considering a typical droplet size distribution for stratus, altostratus and cumulus clouds, we have shown that the effect of the size distribution on the broad-band flux emissivity in the 8–14 m band is about 35%. Our approximation should be useful for treatment of cloud infrared properties in climate models.

http://journals.ametsoc.org/doi/abs/...O%3E2.0.CO%3B2

So....if the emssivity is approx 35%, the Reflectivity is closer to 65%...

http://www.optotherm.com/emiss-physics.htm

[Tkeeth]

Originally Posted by Dockhead

I have a third generation night vision device, forward looking sonar, AND radar on my boat, so I can comment from experience on the relative usefulness of all these devices for avoiding running into things in the dark.

Forward looking sonar sees something like only 8 times the depth -- and not the water depth, but the depth of the object you are seeing. So it is great for picking around slowly in a shallow anchorage, but utterly useless for avoiding hitting things. It would see something like a floating container only maybe 2 seconds before impact at sea speed.

Night vision only sees warm things. And are you going to sit with your eyes glued to it all night?

A good radar set, on the other hand, especially one with good target discrimination like the CW sets (3G, 4G, etc.), will see almost everything down almost to crab pot size, assuming of course that the sea is calm enough in relation to the size of the object you want to see. It can distinguish well enough between clutter and real objects that guard zone alarms are really useful, so you don't have to sit with your eyes glued to it.

Radar doesn't see objects underwater, but nothing available to recreational sailors does. The only possibly effective means of avoiding hitting a whale at night is -- prayer and good sailor's karma.

So on a scale of 1 to 100, the relative usefulness of these devices for avoiding running into stuff at night is, I would say:

1. Radar: 99
2. Night vision: 3
3. Forward looking sonar: 0

That's based on experience, and not arm chair speculation.

P.S. Some of you may know that I consider the so-called "Mark I Eyeball" to be greatly overrated, and will know that I always get irritated at the argument that you don't need technology if you're a real man and get back to basics with your good old Mark I Eyeballs. HOWEVER, I must say, that for avoiding running into stuff, the Mark I Eyeball is actually a very useful tool, even on a pretty dark night. It's not actually all that common for it to be so dark, that you can't see anything with well adapted eyes. Really only moonless nights in heavy overcast. Even starlight is enough to see quite a lot.

and I appreciate the real world response that directly relates to the OP's question. In some things you do see a bigger picture.

[LakeSuperior]

Originally Posted by Tkeeth

This is fun.....

Abstract
We have derived a simple approximation for the emissivity and flux emissivity of water clouds inside the atmospheric window between 8 and 14 m. In our approximation the emissivity in the 8–11.5 m band is a function only of the cloud's liquid water content and cloud thickness. When compared with the exact radiative transfer calculations the broad-band flux emissivities (in the 8–11.5 m region) differ by less than 10%. At wavelengths > 11.5 m the emissivity is a function of the droplet size distribution as well. By considering a typical droplet size distribution for stratus, altostratus and cumulus clouds, we have shown that the effect of the size distribution on the broad-band flux emissivity in the 8–14 m band is about 35%. Our approximation should be useful for treatment of cloud infrared properties in climate models.

http://journals.ametsoc.org/doi/abs/...O%3E2.0.CO%3B2

So....if the emssivity is approx 35%, the Reflectivity is closer to 65%...

Physics of Emissivity

I had already Googled the same paper and if you read it you will find that broad band, 8 - 14 um, emissivity for a cloud that is thicker than about 500 meters is 1, a blackbody. Droplet size does not make a big difference. See figure 2.

[Tkeeth]

[QUOTE=LakeSuperior;2430537]I had already Googled the same paper and if you read it you will find that broad band, 8 - 14 um, emissivity for a cloud that is thicker than about 500 meters is 1, a blackbody. Droplet size does not make a big difference. See figure 2.[/QUOT

But your argument was they do not reflect, so is it "never" or "only if" and how does this help identify a log in the water?

[LakeSuperior]

[QUOTE=Tkeeth;2430542]

Originally Posted by LakeSuperior

I had already Googled the same paper and if you read it you will find that broad band, 8 - 14 um, emissivity for a cloud that is thicker than about 500 meters is 1, a blackbody. Droplet size does not make a big difference. See figure 2.[/QUOT

But your argument was they do not reflect, so is it "never" or "only if" and how does this help identify a log in the water?

I was providing you, a proclaimed IR expert, with a correction regarding your misconception about cloud emissivity.

As has already been pointed out it would be very difficult for an IR imaging device to "see" a wet log in the water.

[Tkeeth]

Thanks for helping me out.

[LakeSuperior]

Originally Posted by hellosailor

And as TK didn't quite mention, the meteorologists are always going on about how one night will be particularly cold, because there is no cloud cover, and that allows the heat from the earth to go out into space, rather than being reflected back down. Seems like a lot of people with alleged degrees in the subject, think clouds reflect IR.

In the IR business a "cold sky reflection" is often referred to. This is shorthand for saying that a cold sky has a small emission. So there is little energy to be reflected from a reflective surface. Consequently, a reflective surface looks cold to an IR imager because there is no energy to reflect and it is a poor emitter. I could see where someone outside the business could misinterpret the use of this terminology.

[W3GAC]

One of the posts asked about recreational forward looking sonar. I've had what is now probably an out of production scanning sonar on our 46' cruising ketch. While some aspects I'm sure has been improved... there are some limiting physics that likely have not. The main issue we quickly learned is the time it takes to scan using sound. For a scanning sonar to work it has to send a narrow focused 'chirp' AND WAIT for its echo, then adjust the beam angle over a little (or up or down, depending on mode) and keep repeating until the desired overall scan pix has been painted on the screen... then the whole process starts again. Turns out those pings and waiting for the echo return actually takes more time than one might think... and the longer the distance setting... the longer it takes to paint a scanning sonar. Then there is a limitation of how close to horizontal in front it can beam a chirp. Because a sound chirp bean is not as sharp as we'd like and anything closer than 10 degrees results in too much back scatter from the discontinuity of the water's surface and air. That minimum angle gets worse the rougher the sea state.
So if you are blind to floating objects above a 10 degree line down from where you have mounted the scanning sonar... what's the minimum distance you will detect a 10' high container, a log? Well, the reality is you will have hit it before you see it! Certainly you will not have time to stop/ change course before hitting it. Back to the scanning/ painting time... if you had set it on 200' out... it would take about a minute to scan... and at 6 kts... how many feet would you travel in a minute? Again, you would have already hit it before seeing it... unless you are just creeping ahead. So why do I still use it? In the up- down mode it does warn up the bottom coming up as you approach a rocky coastline. Very valuable (along with radar, GPS/ chartplotter, depth). But it really shines when making an approach into a narrow entrance with shifting sand bars or not we'll detail entrance with coral heads. If you set it to scan left and right you'll easy see the low point of the trough or clear path between coral heads. But again the scanning back and forth is also s-l-o-w! So we have to just creep into such tight rocky/ coral head narrow entrances. Fun to see your anchor chain/ anchor at anchor. That's also how I began to discover/ understand iur scanning sonar's limitations... the chain image disappeared 10' from the surface!