tubes being damaged due to moisture. This problem was fully addressed in final production. It was also noted the mount designed to secure the AN/PVS-4 to the M60 machine gun was difficult to install and prone to loss of zero. The sight reticles for both the M16 rifle and M14 rifle required firing a minimum of 20 rounds to seat before the zeroing procedures could begin, leading to an excessive expenditure of ammunition. The sight reticles were subsequently modified, requiring only 2 rounds for seating purposes. Early reticles for the M16/M203 could not be zeroed for each weapon simultaneously. Rubber eye guard separation from the sight was a common problem that was addressed. An example of a noted shortcoming ultimately considered insignificant was an enemies ability to detect the reflection of the AN/PVS-4 objective lens using an active infrared night vision device. The IR device needed to detect the AN/PVS-4 required a straight-on unobstructed view, and AN/PVS-4 detection only occurred using an active device (as opposed to passive). The IR light emitted from the enemy's active device would be clearly observable to a anyone employing an AN/PVS-4, making the enemies position easily known. Reticle burn-in was also addressed. One issue that persists even in production models is the tendency for the projected reticle pattern to become washed out in bright light. This can be somewhat mitigated by adjusting the Reticle Brightness knob to its brightest position and keeping the Tube Brightness knob as low as possible. Variants Although not specifically considered "variants" there are a number of other models that share the AN/PVS-4 lineage and often use interchangeable components, including the image intensifier tube. The addition of a "A" after the model, e.g., AN/PVS-4A, signifies a third-generation image intensifier tube and may be designated either after upgrade or at original manufacture. "A" variants typically use "AA" batteries. AN/TVS-5 The AN/TVS-5 is an observation scope, although it does come with reticles and can be used as a weaponsight as well. With a larger catadioptric lens and higher zoom level, it is rated with a detection range of over 1 Kilometer with a third-generation image intensifier tube. NVS-700 The NVS-700 is the original designation of the Optic Electronics Corp. AN/PVS-4. This model is also manufactured in Iran. NVS-800 The NVS-800 is the original des of the Optic Electronics Corp. AN/TVS-5. This model is also manufactured in Iran. Auto NVS-1 Observation version of