In addition to what hansmex correctly writes, you may want to consider:
- The easiest way of adding lights, is to use the included light-sources in the library and place them where the lamp is.
- Lights have a default power of 50%.
- Lights will only show in photo-rendering (highest qualities).
- Lights are not very visible if they compete with sunlight, try a time later in the evening.
- The "auto ceiling lights" checkbox may interfere with the effect of accurately placed light-sources.
- Pay attention to the colour of the light-source you choose.

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