Night vision seems to be a "easy to add" -function when it comes to video recorders, even the cheapest versions seems to sometime offer this option. But when it comes to dslr, these adapters costs roughly ≈ +7000$, why is this?
Is there a way to achieve this effect in other ways that I'm not aware of?
Answer
Are you looking for an "effect" or are you asking about the actual night vision equipment? Assuming you're asking about the equipment, you're confusing IR capable imagers (video cameras, webcams, etc.) and night vision "light amplifiers".
IR imagers take advantage of the fact that most/all cmos/ccd sensors are sensitive into the near infrared. Most cameras have a filter to prevent IR from getting to the imaging chip. Those that don't can use IR LED's to illuminate a scene.
Night vision equipment (as seen in the example photo) amplifies the ambient lighting environment (moonlight, starlight, etc.) to a useable level. These are sensitive to IR as well, and can be used with IR flashlights, beacons, etc.
You can read about the process of light amplification here or here but briefly, the process is as follows (for newer systems).
Photons enter the lens in front of the device and are focused on a transparent but conductive screen that has a high voltage charge. These photons knock loose electrons that enter something called a Microchannel Plate (MCP). They bounce down narrow, tightly packed channels and every time they hit the wall of the channel the electrons knock loose more electrons- amplifying the signal. At the far end of the MCP can be a phosphor screen (giving the green color of the example image) or another MCP for further amplification. Ultimately there is either an eyepiece for direct view of the phosphor, or a lens system for attachment to a camera.
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