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If you are operating infrared security cameras you need to be aware that the quality of your images will be affected if there is a light source in proximity to the camera. It can result in a bleaching of the image. You just end up with a white mist on your monitor. Infrared cameras operate best with no light.

Vivotek IP CCTV cameras are high quality.

I would consult your DVR supplier or instruction manual. You could try to reset the factory defaults and set the time again.

Your day time image looks very good quality. The resolution of these CCTV cameras is of a good standard. I think the drop in quality is due to the quality of your IR funtion. If you have the opportunity to add more IR light this would certainly improve things.