Re: what is the difference between night vision 'goggles' and thermal imaging?

Frankly, I didn't know much about thermal imaging but, your question prompted me to surf the web for answers so, thanks for motivating me to learn some things!

Okay, the two types of thermal imaging do have some things in common, but a lot of differences. In both types of thermal imaging, the infrared light is converted to electrical energy , then the electrical signal is modified (amplified in the case of infrared intensifiers, more complex signal processing in the case of FLIR).

Why do we need to convert the infrared light to electrical energy?

Because infrared light is lower in frequency than visible light, it has less energy than visible light:

Infrared Ultraviolet

_________Red______ Orange___Yellow__Green____Blue___Violet

<---Lower Frequency____________________Higher Frequency-->

<---Lower Energy_________________________Higher Energy -->



The problem comes from the first and second law of thermodynamics, which I'll state as:

"energy can neither be created nor destroyed but it can be wasted".

Since ultraviolet light has MORE energy than visible light, you could make an ultraviolet imager by having some material that absorbs ultraviolet light, wastes a little bit of the energy, then emits the remaining energy as visible light. For example, the "brighteners" in some laundry detergents do just that: they absorb ultraviolet light, which is invisible, wastes some energy, then emits light that is visible making the clothes look brighter in normal light, and making them seem to glow in "black light" or ultraviolet light.

Unfortunately, infrared light has LESS energy than visible light, so that trick won't work.

Because objects that are hotter than their surroundings emit infrared light, we refer to the instruments that let us see infrared as & quot;thermal imagers", or as "night vision" devices.

These existing night vision devices fall into two categories:

image intensifier (I2) devices< /P>

Forward looking infrared thermal imagers (FLIRs)

The following information is from the Raytheon website,

http://www.raytheon.com/rtis/docs/thermal/diff.htm

Image Intensifiers:

"I2 devices pass incoming light through a photo cathode, a thin piece of glass coated with a

photosensitive chemical which releases electrons when struck by light particles

called photons. The electrons can then be amplified in a vacuum tube and passed

over a phosphor screen which reverses the function of the photo cathode, turning

electrical energy back into light energy. The phosphor screen usually gives the

scene a green hue. Because I2 devices amplify small amounts of light many

times, they often have difficulties with scenes in which the light level changes or

in which point light sources appear. This gives rise to so-called blooming and

wash out effects.

FLIR:

"FLIRs are true thermal imaging devices that create

pictures based on heat energy emitted by the viewed scene rather than light

reflected off of it. Heat in the infrared region of the electromagnetic spectrum

behaves much like visible light and can be optically focused and collected. FLIRs

employ various materials whose electrical properties change when heated to

perform the transformation from infrared energy to electronic signal levels.

These changing signal levels are translated into video signals in which a different

shade of gray on the TV monitor is assigned for each detectable temperature (i.e.

emitted energy) level. Because the energy being sensed is heat and not light,

FLIRs are totally unaffected by the amount of light in a scene. The ability to

sense differences in temperature can often provide valuable intelligence above

and beyond the ability to see things in the dark.