If infrared light is not visible to the human eye, why do most tv remote controls and security cameras appear to have a visible red colored LED lit when the infrared light is being emitted?
Is that visible red light present as a convenience (introduced by grace of the component designer?) or as a by-product of emitting actual infrared light?
Is it possible to have infrared light emitted without this visible red color?
Answer
The simple answer is that they are using near IR. LED manufacturers have a good handle on how to make them so they are affordable.
Their center frequencies may be invisible to the M-1 eyeball (i.e. human eye), but unless they put a filter in front of the LEDs (which cause them to produce less illumination) there will be some of it that you can see.
The effect is minor. Basically, to see it you must look directly at the emitter. You're not going to see it in reflections or scene illumination.
Far-IR is completely invisible. But a whole lot more expensive because the manufacturing process is different.
Near-IR emitters are mass-produced. Far-IR not so much.
IR lasers are another story. They emit on a single frequency, so there is no gaussian curve describing their output in the frequency domain. They are so invisible that they can be dangerous. Working around lab CO2 lasers, for instance, requires removal of all jewelry and controlling the beam. They will not trigger a blink response so you can sustain a lot of damage in a short time and not know it right away.
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