Utente:NathanSwiss/Sandbox
NSI (Night Sensor light Intensifiers) Technology
Night Vision Types
Night vision devices come in multiple basic technologies. Digital night vision, NSI (Night Sensor light Intensifiers) technology, and thermal-imaging are the most common. Both NSI technology and digital night vision amplify whatever available light there is, while thermal-imaging (or infrared) devices look at heat. These three technologies give very different results, are different in design, cost, and image appearance. We will look at all of these technologies but the majority of the night vision devices available on the market are NSI technology and we will focus most of our discussion there. The “Night Vision” terminology we use in this article refers to NSI (image-enhancement) technology. The night vision industry has grown dramatically in the last ten years. Originally developed and funded by the military, the technology is becoming much more popular and affordable for consumers. This is still an emerging technology. The more advanced designs can only be sold within the US and are restricted for export.
Definition (NSI technology)
NSI (Night Sensor light Intensifiers) technology is what most people think of when they talk about night vision. Since this technology uses available light, it does not work in complete darkness where there is no light to amplify. However, most of these devices also come with a built-in infrared (IR) illuminator. The IR illuminator emits a beam of near-infrared light, invisible to our eyes, that the night vision device can use to illuminate the scene.
Night vision devices come in three accepted generations of design (Generation 1, 2, and 3). These can be thought of as levels of design sophistication, although the basic components and technology (defined below) remain mostly constant. The different design generations are also significantly different in image quality, cost and capability. These three and the proposed “Generation 4” devices will be explained in greater detail in the Generations section of this article.
One of the major misconceptions about night vision devices is that they are like a binocular that collects light and focuses the image onto the eyes. Night vision devices are closer to a camcorder in that available light is converted to electrons that are then amplified and projected onto a screen that converts them back into a visible light image that you look at through an eyepiece. Examining this a bit closer, there are several major components that all NSI night vision devices use.
NSI (Night Sensor light Intensifiers) Technology Design
- 1) Objective Lens
- 2) Photons
- 3) Photocathode
- 4) Microchannel Plate
- 5) Electrons
- 6) Power Supply
- 7) Phosphorus Screen
- 8) Eyepiece
Objective lens
In front is a conventional objective lens, which captures ambient light and some near-infrared light. This lens focuses the incoming light onto a photocathode at the front end of an image intensifier tube. The objective lens is adjustable and is used to focus at different distances similar to the focus knob on a pair of binoculars.
Image-intensifier tube
The photocathode at the front end of the image-intensifier tube converts the photons of light to electrons (electrical energy). These electrons pass down through the tube and are accelerated using 5,000 volt bursts between oppositely charged electrodes. The electrons are now effectively “amplified”. The problem with this approach is that the acceleration of the electrons distorts a linear path and the subsequent image. Generation 1 devices have significant distortions around the edge because of this.
Starting with Generation 2, a microchannel plate (MCP) was added into the tube to solve the distortion problem. The MCP is a glass disk with millions of microscopic holes (microchannels), made using fiber-optic technology. The electrons in the microchannels are both accelerated and multiplied in number. As electrons pass through the microchannels, they cause thousands of other similar electrons to be released as they collide with the microchannel walls. Therefore, each incoming electron sends thousands of similar electrons out the other end. This was a major improvement in night vision.
In summary, the image-intensifier tube has performed two of the essential steps toward light amplification: first it converts photons of light into electrons of energy and second, it amplifies the flow of electrons. The addition of a microchannel plate in Generation 2 image-intensifiers improved the orderly output of electrons and significantly multiplied their number.
Power supply
The power supply powers the unit and display. It also sends pulses of electricity to the electrodes that accelerate the electrons in the image intensifier tube. The power supply may have a quiet buzz in Generation 1 units, while the device is on. This is normal and unless it affects the usage of the device should not cause concern.
Phosphorus screen
The electrons coming out of the image-intensifier tube are hurled against a green phosphorus screen. This converts the electrons back to photons of light and the final light-intensified image is created. Green phosphorus is used because the human eye can distinguish more shades of green than any other color. This is why night vision is associated with the eerie green display color.
Eyepiece
Last of the basic standard components is the eyepiece through which one looks to see the image that has been projected onto the phosphorus screen. These are adjustable so you can focus in sharp on the screen. Once focused for your eye this should not have to be readjusted (unless switching between users). This adjustment might be thought of like a diopter adjustment on a pair of binoculars that you adjust for your eyes.
Infrared (IR) Illuminator
In front is a conventional objective lens, which captures ambient light and some near-infrared light. This lens focuses the incoming light onto a photocathode at the front end of an image intensifier tube. The objective lens is adjustable and is used to focus at different distances similar to the focus knob on a pair of binoculars.
NSI PLUS
The NSI PLUS is the enhanced version of Nsi. With this technology the role of the WDR stands for Wide Dynamic Range (WDR). There could be situations when the CCTV could be subject to a wide range of light levels. For example: a CCTV mounted inside a building looking at the building entrance - daylight outside and ambient lighting inside. To compensate for this "wide range of lighting levels", WDR is an awesome feature.
WDR is measured in dB. The decibel (dB) is a logarithmic unit used to express the ratio of two values - in this case the intensity of the ratio of the radiance of the brightest and least-brightest object in the scene. Since WDR is a logarithmic ratio, 1000:1 would be equivalent to a dB value of 60 dB. A higher WDR range (for example 140 dB), means that the CCTV can capture a greater scale of brightness. Some manufacturers implement DWDR. DWDR or Digital wide dynamic range is a software-based technique that enhances the image quality by adjusting the gamma value to brighten dark areas. True WDR is a sensor-based technology. A True WDR CCTV can produce images with an extremely wide dynamic range. The WDR image sensor can capture several images with short and long exposures, then combining them into a single frame. For all types of WDR, proper calibration of WDR levels is essential to achieve the optimum image quality.
BLC or Backlight compensation is different from WDR. BLC increases the level of exposure for the entire image, thereby making the object of interest clear but can cause overexposure in brightly lit areas. See example image below: When comparing technical specifications from different CCTV manufacturers, it is important to understand that the dB unit is just an approximation of the capabilities of the CCTV. For the user, there is no way to verify the exact WDR levels. The best way to check the WDR levels is to verify it under required conditions.
Conclusions • Night vision devices come in three designs: NSI technology, digital night vision and thermal-imaging. NSI technology designs are the most common and popular type of night vision for consumers. • NSI technology night vision works by amplifying available light and can not function in total darkness without the aid of an IR-illuminator. • The basic components of all NSI technology night vision devices consist of an objective lens, image-intensifier tube, phosphorus screen, eyepiece and power supply.