Which factor primarily affects the lifespan of a safelight bulb?

The lifespan of a safelight bulb is primarily affected by the frequency of use. Safelight bulbs are designed for use in darkrooms, where they help to illuminate the workspace without exposing photosensitive materials to unwanted light. Each time a bulb is turned on and off, or during prolonged use, it undergoes thermal and electrical stresses that can degrade its components over time. Therefore, the more frequently the bulb is used, the shorter its expected lifespan will be.

While factors such as the color of the bulb, wattage, and surrounding temperature can influence how effective a safelight is in protecting photographic materials, they do not directly correlate with the longevity of the bulb. For instance, certain colors may be more suitable for specific photographic processes, and a higher wattage might provide more brightness, but these factors alone do not determine how long the bulb will last compared to how often it is operated.