The basic principle behind most photography is that you need light! Light emitted from a light source (artificial or natural) is reflected by an object and then enters the camera through the lens. An image of the object is captured when the light rays hit the image sensor (or photographic film).
Figure 1: Light is reflected by an object and enters the camera through the lens.
The essence of this principle is that without light, there will be no image, and poor light will result in a poor image. Anything reducing the light between the object and the sensor will impair image quality. Examples are windows that partially block light, smoked dome covers or lenses with poor optics and small apertures.
If the scene you wish to view is lacking in light, you may need to add some. Auxiliary lamps illuminating the object can often increase image quality considerably.
Also, consider the fact that a camera mounted and tested during daytime can give entirely different results at night, or as the seasons shift. Make sure you understand the entire range of light in your scenario, and set up your camera accordingly.