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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?
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.
Most DSLRs have a filter in the sensor stack that prevents infrared light from reaching the sensor. The sensor stack is a set of filters directly over the sensor that protects it from dust and other foreign substances while also allowing only the wavelengths of light for which the designers wish the camera to be sensitive to pass through.
Short of removing the IR filter from the sensor stack the only way to make such a camera sensitive to IR light is to use a device that can detect IR light and translate it to other wavelengths of light that are allowed to pass through the camera's sensor stack. This is how some devices used by humans directly over their eyes to see in the dark work: they translate wavelengths we can't see into wavelengths we can see. These devices, especially the ones that do it well by truly imaging infrared light, can be rather expensive.
Other cheaper options work at near infrared and visible wavelengths and convert the signal to a highly amplified monochromatic image (usually with a red or green tint) because monochrome requires much less intensive signal processing to produce an image. These are often called "Light Intensifiers." Most of the lower cost night vision options use this technology.
The makers of intensifier adapters, such as Astroscope, probably assume that if a buyer is willing to buy a higher end DSLR to shoot in very low light then they also want a very high quality image as a result. So they make a very high quality product that costs more to develop, design, and manufacture.
If a camera doesn't have an IR filter in the sensor stack then the energy from IR light can be detected and the signal from that energy can be displayed with colors that we can see. Some video camera have IR filters, some don't. Most low end "security" type cameras don't cut IR light because they are more concerned with seeing as many details as they possibly can, even in low light, than they are concerned with accurately depicting a scene using the same color with which our human vision would see such a scene. Some higher end video cameras do cut IR light either with an optical filter over the sensor(s) or in the signal processing pipeline.
Some very high end broadcast quality cameras have separate sensors for red, green, blue, and infrared light. In "normal" mode in good light the signal from the IR sensor is ignored. In very dim light the signal from the IR sensor is used to produce the basic structure (details - differences between bright and dark) of the image and the signal from the other sensors are used to add color to the monochromatic signal from the IR sensor. Keep in mind that analog color television broadcast signals used a monochrome signal for the structure of the image and then used much lower resolution color signals to add the color to the scene.
