Some excellent input TJ. Yes depth perception is definitely a factor, and something to aid in monocular vision. Do also remember, that for the loss of depth perception, there is a gain in field of view. By using umbra, penumbra, and antumbra shadowing techniques, a facsimile of depth perception can be gained for those restricted to monocular vision. I am working up a post on that as well.
For today, I wanted to touch on the difference between IR/FLIR (infrared/forward looking infrared) and thermal imaging (not necessarily one and the same).
Lets review some standard formula:
F=C/ƛ
F frequency
C speed of light
ƛ wavelength
That is how you can figure out what the wavelength of light will be 'while' it's passing through a material. Depending on angular relationships, you will also need to know what angle it will exit. That is where 'snells' law comes into play.
Snells law written for optic refraction
N2/N1 = ƛ1/ƛ2
N1 index of refraction of air = 1.0
N2 index of refraction of glass = 1.6
hyperphysics.phy-astr.gsu.edu/hbase/tables/indrf.htmlhyperphysics.phy-astr.gsu.edu/hbase/geoopt/refr.html#c2Put a pencil or other such object into a straight smooth walled glass of water. Look at it from different angles. You will note that the pencil/object appears to 'shift'. You can also observe this when spear fishing in a creek. The fish is not located 'exactly' where you think it is because the water is acting like a prism 'refracting' the light coming from the fish at an angle.
The index of refraction for glass is the standard for 'crown glass'. Most modern mirrors use that, but remember there are 'flint glasses' and 'barium crown glasses'.
In the normal universe (what you can currently see and touch) Einsteins formula's work.
Those formula go by the wayside when it comes to several points, but for this particular purpose, they work.
Glass, as with most other materials (except metamaterials which breaks into quantum effects etc) has a specific light velocity. As can be calculated by the snells law listed above, light has a difference velocity in different materials (speed at which it travels through the material)
With the basic physics covered, we now need to know how FLIR/IR/thermal imaging works.
Infrared light can be split into three categories:
- Near-infrared (near-IR) - Closest to visible light, near-IR has wavelengths that range from 0.7 to 1.3 microns, or 700 billionths to 1,300 billionths of a meter.
- Mid-infrared (mid-IR) - Mid-IR has wavelengths ranging from 1.3 to 3 microns. Both near-IR and mid-IR are used by a variety of electronic devices, including remote controls.
- Thermal-infrared (thermal-IR) - Occupying the largest part of the infrared spectrum, thermal-IR has wavelengths ranging from 3 microns to over 30 microns.
The difference between thermal-IR and the other two is that
thermal-IR is emitted by an object instead of reflected off it. Infrared light is emitted for the same reasons you see visible light from a fire. That is the largest difference between cheap night vision, and good night vision IR/FLIR. One is picking up the infrared spectrum from the object, while the other is using a source (typically a laser) in much the same way as a flashlight to light the area, but instead of visible light, it's lighting the area with infrared spectrum light.
The same physics as visible light, covers the infrared spectrum. For that matter, it works the same for starlight scopes on the other end of the visible spectrum.
For the purposes of this post, we are going to focus on the emitters (warm bodies).
As noted in the earlier post, subject contrast is important regarding the ability to detect anything. Without a surrounding contrast, this text could not be read. Right click and mouse over the blank area below this paragraph for an example of that. The font color is white, which makes it very difficult to read against the standard forum post background.
See what I mean? Unless you have an exceptionally high resolution monitor, you may be able to read it somewhat, but with the text highlighted, it's much easier to read due to the increased contrast. Now the same text in standard black. " See what I mean? Unless you have an exceptionally high resolution monitor, you may be able to read it somewhat, but with the text highlighted, it's much easier to read due to the increased contrast. ".
Without contrast, you will see nothing.
With that in mind, how does the standard/semi standard thermal imagery work.
The special lens focuses light onto an array of detectors. It is scanned by a laser picking up the differences in phases on the detectors. That in turn creates electrical signals that are interpreted by the machine in much the same way a digital camera works. This information is obtained from several thousand points in the field of view of the detector array and is called a thermograph.
The impulses are sent to a signal-processing unit, a circuit board with a dedicated chip that translates the information from the elements into data for the display.
Effectively, it's a digital camera on the IR wavelength only.
So how do you defeat it?
An internet search will yield multiple references to using glass. In a general sense, it will work, but is not very practical for a mobile shield against thermal imaging. It will also show if you have something hot in contact with it (or cold for that matter).
Remember this?
F=C/ƛ
F frequency
C speed of light
ƛ wavelength
N2/N1 = ƛ1/ƛ2
N1 index of refraction of air = 1.0
N2 index of refraction of glass = 1.6
As the speed of light is reduced in the slower material, the wavelength is shortened proportionately. The frequency is unchanged; it is a characteristic of the source of the light and unaffected by material changes.
Calcs get considerably more complicated when it comes to Impedance of free space usually represented by Z
0I'll spare the Z
0 calc, but demonstrate with the acoustical impedance formula which demonstrates the idea.
Z=PV
Z= impedance
P= Density
V= Velocity in a medium.
The version of that where the refraction of light index comes from is considerably more detailed as it has to account for some quantum effects as well as electromagnetics.
Suffice to say that the reason IR doesn't make it through glass is due to the wavelength of the IR, the specifics of the Z
0, and some quantum effects.
Now, if you had a means of shifting the 'frequency' either up or down out of the detectable range of the IR wavelengths as glass does in a usable form, then you would have something.
www.opticsinfobase.org/oe/abstract.cfm?uri=oe-20-7-8207www.opticsinfobase.org/view_article.cfm?gotourl=http%3A%2F%2Fwww.opticsinfobase.org%2FDirectPDFAccess%2FB9D691D2-CBAF-A960-9E09B1EF17594F68_231514%2Foe-20-7-8207.pdf%3Fda%3D1%26id%3D231514%26seq%3D0%26mobile%3Dno&org=Boiling down the technojargon of that link and study, a layered approach is required for an IR cloak that works constantly. So the myth of 'defeating IR' is not actually a myth no w is it?
You can temporarily block IR by masking it with a thick wool blanket, or space blanket, but that is only short term. As the heat from your body increases the temperature of the blanket over your head, you become visible to IR.
Now with some Upshift phosphors and some woven glass threads intermingled with some woven aluminum threads layered properly. could just maybe make a usable IR cloak......
www.maxmax.com/airinks.htm It should also be remembered that IR is not Xray. Unlike the movies, infrared does not make it through say a block, or large stone, or underneath the ground. As mentioned earlier, it also requires 'contrast', simple reduction of contrast will hide you. Say laying on a stone that is still hot from the day time sun, or ducking under some water. The closer the object is to your temperature, the less contrast the IR camera has. Wool absorbs it for the short term, aluminum can reflect it /mask it short term, glass can phase shift it out of range, and hybrid pigments/upshift phosphors can shift it's wavelength outside of the camera's usable range...
Necessity is the mother of invention, and knowledge is the hammer of invention.