The germicidal effect of ultraviolet (UV) lamps relies heavily on the direct exposure of UV light to microorganisms. UV light can be divided into several different wavelength bands, of which the UVC band (wavelengths between 200-280 nm) is commonly used for disinfection and sterilization because it effectively destroys the DNA or RNA of microorganisms, thus preventing them from replicating.

To ensure effective sterilization by UV lamps, the following conditions need to be met:

1. Direct exposure: Microorganisms must be exposed to UV radiation. If an object blocks the light, then the area behind the blocking object will not be sterilized. 

2. Sufficient dose: The germicidal effect depends on the intensity of the UV light and the duration of exposure. It must be ensured that the microorganisms receive a dose of UV light sufficient to inactivate them.

3. Appropriate wavelength: Different wavelengths of ultraviolet light have different effects on microorganisms, with the UVC band being the most effective. 

4. Cleanliness: The cleanliness of the surface being irradiated also affects the germicidal effect. Dirt, dust or other contaminants may absorb some of the UV light and reduce its efficiency. Therefore, when sterilizing with UVC lamps, it is important to ensure that the light reaches the target area directly and adequately.