The Peephole Viewer uses a 2.5" LCD panel to display who is at your door without alerting them that you're home if you choose not to answer. The image is easy to see for both people of any heights and people with poor eyesight, and is similar to that on the display of a digital camera. The viewer compensates for low-light images and eliminates "fish eye" distortion. It turns off automatically after 10 seconds to conserve battery life. It uses a 2.0 mega pixel CMOS sensor and provides 720p HD quality image displays. When the batteries are low, the red Low Battery Light flashes. After the red light flashes, the batteries can still produce images, and are good about 200 times. Quick, easy installation! The viewer requires 2 AA batteries (good for approx. 2000 times without the use of an SD card). You can use an SD card of any memory size to store photos taken at the time that you press the POWER button to see who is at the door. The viewer can be used with most existing peepholes in doors 3.5cm~5.5cm (1.38"~2.17") thick. It replaces existing peepholes in the doors of the aforesaid sizes.