IIT stands for image intensifier tube which is used to amplify low light level images in a wide light spectrum. It’s a special Image Intensifier Tube (IIT) built in every analog night vision device and it is the heart (most important part) of every night vision device. Quality of image intensifier tube determines in which category or generation night vision device will fall. The higher the quality of IIT technology, the higher the generation. There are 4 IIT Generation (gen) of night vision devices, but due to generation 0 nobody produces (because technology is very old and not effective enough), there are 3 generations on the market.
Before generation 1 there was generation 0 back in the earliest years of 1950. Those night vision devices were based on image conversion rather than intensification and that was the reason they required an IR illuminator (click the info button for IR illuminator wavelength to get to know what IR illuminator means). Generation 0 typically did use an S-1 (photosensitivity of 60 uA/lm) photocathode, which required high voltage to work and get an optimal image. Companies do not produce them anymore, due to this being an old technology that provides a bad image and high geometric distortion. The system of generation 0 also usually failed when exposed to bright light sources, due to not having sophisticated electronics that provides Bright-Source Protection (BSP) or bright light cut off.
First generation night vision (GEN 1 NV) devices were invented in the 1960s. Some of them had 3 photocathodes in the system, because of that, they were heavy and bulky. The prices of gen 1 are now low, but for this, you get an image that is clear at the center but distorted or blurry around the edges. Gen 1 devices use an S-20 photocathode (photosensitivity of 180-200 uA/lm). They also need (like gen 0) high voltage to work properly but provide higher photosensitivity. These were the first truly passive image intensifiers. Gen 1 is also known for geometric distortion and fails when exposed to bright light sources (like gen 0 it doesn't have BSP). The performance is poor at low light levels, but, you can get a brighter image when adding additional IR illuminator, which also means, you will be visible to other night vision users. With a good IR illuminator detection range is around 100 m. Gen 1 can easily be damaged, if they are used in daylight, so the user has to take care not to switch the device ON without the lens cover. A sudden bright light source can also damage GEN 1 night vision device. Consider this, even with the lens cover on it is still not recommended to have the device switched ON for a longer time. The gain (This is the number of times a night vision device amplifies light input) is typically from 500 – 2000 times. Gen 1 NV devices normally (when pressing the OFF button) don’t turn OFF immediately, but will provide an image for a longer time. These NV devices are most commonly used by civilians and are suitable for the most application. The price/performance is relatively good.
Second Generation Night vision (Gen 2) devices were developed in the 1970s. Inside the system, there is a sophisticated microchannel plate and electronics. Because of this, image quality is much better compared to gen 0 and 1. And the size of night vision devices is smaller. All of the newer gen 2 devices have built-in BSP or bright light cut off. This provides an automatic adjustment in different light situations and protects the photocathode from damage. Gen 2 typically uses an S-25 extended red photocathode (photosensitivity 240+ uA/lm). They provide a good performance in low light situations and provide low image distortion. With an additional IR illuminator, you can get even better image quality. Generally, Gen 2 NV is more sensitive than the Gen 1, which means, you see further and the image is brighter in dark conditions. Even the clarity is improved against Gen 1. Normally Gen 2 can be used without IR illuminator but in very dark areas it is a good accessory to have. The system gain for Gen 2 devices is from 10.000 – 20.000 times. The newer Gen 2+ have a gain value of even 25.000 – 45.000 (photocathode Photonis XR-5).
Gen 3 has two big differences against gen 2. The main improvements are high-quality gallium arsenide (GaAs – it’s a compound of the elements gallium and arsenic) photocathode and ion-barrier film on the microchannel plate. GaAs photocathode enables detection of objects at greater distances under much darker conditions then Gen 2 or lower generation night vision devices. Ion-barrier filmed microchannel plate provides an increase of the operational lifetime of photocathode up to 10000 hours and more. Gen 3 devices use high-quality photocathode with photosensitivity of over 800 uA/lm. For Gen 3 devices you don’t need an additional IR illuminator. If we put everything together, shortly written, Gen 3 devices provide excellent performance in low light situations without perceptible distortion and with a long photocathode lifetime. Sure they also have BSP and all other features with lots of improvements from Gen 2. The gain is from 40.000 – 65.000 times.
These are only the main IIT Generations. There are also generations like gen 1+, gen 2+ and different quality photocathodes inside generations. Again, many times heard, the higher the quality the higher the price.
Source: AR15 (Gen 2 vs Gen 3)