Thermal Image Blocked by Some Plastic Materials...

[Mechatrommer]

i experienced this before while anayzing blocked plumbing pipe using hot water, some plastic/pvc material dont show/register heat info, they just showed up black or blue (means cool) even if they are hot to touch. but some pipe we can see the heat correctly. so in my case i can see pipe left and right is hot but suddenly in the middle pipe tee junction is cool (but its hot to touch), made me scratch head, but since i didnt prepare to take picture and report somewhere, so i just passed, thinking.. yes some materials are opaque.

but today i want to see 3d print operation. my 3d printer is enclosured with clear perspex (plastic) door. with its door closed (see attachment pictures pairs below) nothing can be seen, but when its open, voila! every heat can be analyzed. so my question, is this specific to my thermal camera unit or its lens material (seek thermal compact)? or is this common to all thermal cameras? including military (and ET/predator vision)

maybe Mr Arnie and Samuel L Jackson Danny Glover dont have to poor their body with mud to kill predator, only wear some special plastic material. or can i fool US military bunker if i want to ambush by just wearing custom made clothing out of clear perspex?

[Ultrapurple]

Have a read around the forum and the literature - you'll see that many materials that are transparent to visible light are opaque to LWIR: glass and water are two common examples. It's a fun game seeing what you can and can't see LWIR through. Some materials such as germanium are completely opaque to visible light yet let LWIR through very easily.

Many plastics are LWIR-opaque, to a greater or lesser extent. Some transmit adequately provided the layer is thin - cheap black trash bags and 'cling film' food wrap are two well-known examples. A very few glass-like substances are transparent to LWIR: FLIR and others offer weatherproof glass-like inspection windows for machine cabinets and similar applications that allow one to scan what's going on inside without exposure to whatever hazards lie within (whizzing gears, high voltages, angry leprechauns...)

[Mechatrommer]

some type of metal and materials even reflect LWIR even though they are cold, hence sometime giving false or confusing reading. whats funny is that some materials, even though they are themsekves hot, cannot emit their heat signature, like my pvc plumbing problem earlier istr. i checked the clear perspex, even though it blocks heat from passing, it still can emit its temperature when heated. maybe custom built pvc plastic clothing may block us from military view (is "black body" the right term?)

[Bill W]

The joys of emissivity.....

Bill

[Mechatrommer]

right!... we should keep the black tape around then.. there are lot more "mirrors" than we can actually see...