An infrared night vision device is a piece of equipment capable of observing in the infrared wavelengths and is commonly used for observation and monitoring in dark environments. It provides an image by capturing and amplifying infrared radiation in the environment, enabling the user to see objects at night or in low light conditions. The viewing distance and clarity of an infrared night vision device is affected by a number of factors, which are explained in detail below.

1. The observation distance of infrared night vision devices is affected by the technical performance and working mode of the equipment. Infrared night vision devices are usually divided into passive and active types according to the way they emit and receive infrared radiation. Passive infrared night vision devices mainly rely on capturing and amplifying the infrared radiation in the environment to provide images, so its observation distance by the strength of the infrared radiation in the environment has a greater impact. In the case of strong infrared radiation, the observation distance of passive infrared night vision device can reach several hundred metres or even farther. In the case of weak infrared radiation, the observation distance will be reduced accordingly. Active infrared night vision devices emit infrared radiation and receive reflected signals to provide images, so their observation distance is relatively long, and can reach thousands of metres.

2, infrared night vision device observation distance is also affected by the target's reflectivity and size. The higher the reflectivity of the target, the stronger the infrared radiation signal received by the infrared night vision device, the observation distance is relatively far. The size of the target will also affect the observation distance, usually, infrared night vision device can observe a larger target distance is farther.

3, the clarity of infrared night vision devices are also affected by a number of factors. First of all, the clarity of infrared night vision device depends on its resolution. The higher the resolution, the clearer the image. Resolution is the size of the smallest detail that an infrared night vision device can distinguish, usually expressed in pixels. A higher resolution provides more detailed information, making the image clearer.

4, the clarity of infrared night vision devices are also affected by the equipment's image enhancement function and filter. Like the enhancement function can improve clarity by enhancing the brightness of the image under low-light conditions. Filters can reduce haze, dust and other environmental factors on the impact of image clarity, making the image clearer.

In addition, the clarity of infrared night vision devices is also affected by the user's tuning and adjustment of the eyes and equipment. Adjusting the focal length and focusing method of the infrared night vision device can improve the clarity of the observed image. In addition, the user's state of vision can also have an impact on clarity.

To summarise, the observation distance and clarity of infrared night vision devices are constrained by a number of factors. They mainly include the technical performance and working mode of the equipment, the reflectivity and size of the target, the resolution, the performance of the image enhancement function and the filter, and the user's debugging and adjusting ability, and so on. Therefore, when using infrared night vision devices for observation, these factors need to be fully considered and reasonable choices and settings need to be made according to the specific circumstances in order to achieve the best observation results.