this is a YG1006 NPN Photo transistor. Popularly known as IR Receiver.
I have a question , why the top is black? I know a perfect black is a good reflector ,but what it does matter for infrared receiver?

 

Black actually is a good absorber (that's why it's black).
Here it blocks visible light but lets through the IR.

 

Hi,
The black finish is an ambient white light filter, passes only IR.
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If you could see in IR, it would look clear.

 

If you could see in IR, it would look clear.

Ummm...no.

 

Ummm...no.

If your eyes operated in the band the detector does, the plastic would appear clear (or neutral gray but not opaque). That is the purpose of the filter.

It is opaque to visible light but transparent to the IR band it targets.

 

Black absorbs all wavelengths of light so no light would reflect from or pass though the receiver to reach your eye.

 

Probably less about color and more about the plastic characteristics for being bandpass filter for IR.

Side tidbit (good to know) - if you are doing UV-B work, std glass and plastics will block UV-B.

 

Side tip:

Older digital cameras respond to IR light.
Simply view a working IR LED on the camera's screen.

Newer smart phones are less sensitive to IR.

 

Black absorbs all wavelengths of light so no light would reflect from or pass though the receiver to reach your eye.

The opacity of that plastic is not from a pigment, it is from the bandpass characteristics. The plastic is opaque to the visible light spectrum. That is its purpose. It passed IR and blocks visible light, so it looks "black".

An IR illuminator behind it, and an IR camera in front, would make it look transparent.

If this was not the case, the detector embedded in it could not operate.

 

Side tip:

Older digital cameras respond to IR light.
Simply view a working IR LED on the camera's screen.

Newer smart phones are less sensitive to IR.

They have better filtering, but the sensors themselves are still responsive to the IR. There are services that will take DSLRs and remove the IR cut filter to turn the camera into an IR camera.

 

@Yaakov Thanks for clearing that up, I don't know what I was thinking...of course it's only a filter and not opaque.

 

@Yaakov Thanks for clearing that up, I don't know what I was thinking...of course it's only a filter and not opaque.

No problem. To be honest you had me questioning my sanity because you are not flaky and you were so certain.

 

If you look at that sensor with an IR viewer scope, as I have done, you will see right thru it, just as I have done! It is not really black it just looks that way because it blocks visible light.
Seeing and thinking that you can believe what you see is often an error. The human eye is easy to fool.

 

The human eye is easy to fool.

It's obviously limited by its visual bandwidth.
Some species of animals and insects can see further into the ultraviolet and infrared than we can.

 

If you look at that sensor with an IR viewer scope, as I have done, you will see right thru it, just as I have done! It is not really black it just looks that way because it blocks visible light.
Seeing and thinking that you can believe what you see is often an error. The human eye is easy to fool.

With the right tool, you can see right through aluminum.

But "black". So where does all the visible light energy go if it's not being reflected?
Is the IR receiver (the plastic) bandpass or lowpass filter?

 

It's obviously limited by its visual bandwidth.
Some species of animals and insects can see further into the ultraviolet and infrared than we can.

I watched a very interesting documentary where they took cameras of different bandwidths into the jungle and watched how certain species interacted with plants. The attractive "colours" of flowers towards insects was especially cool. It's literally a different world for these creatures and the evolutionary mechanisms at work are incredible to witness which was previously impossible to see before technology. The sensory organs of some species will blow your mind.

https://en.m.wikipedia.org/wiki/Evolution_of_the_eye

Check out this article, humans are fairly primitive compared to what some species can detect. And it doesn't stop at sight, birds can detect magnetic fields and sharks / snakes can "smell" in the parts per million range. Incredible.

 

If you have ever used an IR scope then you might consider that if IR was also visible to us everything that we see would be so very much different and our visual sight would be rather overloaded. Like when you can hear 50 people talking all at once, and not understand very much of it.

 

But "black". So where does all the visible light energy go if it's not being reflected?

it's being absorbed like a black body.
In this case it's a black body for visible light, but transparent to IR.
That's rather the opposite of window glass which is transparent to visible light, but absorbs/blocks IR.

 

So the black absorbs light energy (visible for the most part), so all that energy goes where, heat?