The brightness of a light bulb is primarily determined by its wattage, especially with incandescent bulbs, which convert only about 20% of their energy into light and produce significant heat. Using a bulb with a wattage higher than the fixture's rating can pose a risk of overheating, potentially damaging the fixture or causing a fire hazard. For older lamps, it is advisable to avoid bulbs rated at 100 watts or higher. In contrast, modern fluorescent and LED bulbs are more efficient, producing more light with less heat, making them safer for use in various fixtures. Therefore, it is crucial to match the bulb's wattage with the fixture's specifications to ensure safety.