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I have a HP LP2475w display with a cold cathode fluorescent light (CCFL) backlight. It is approximately 8 years old. However, it has only 9041 backlight hours, meaning it has been used for only 12.9% of the time and the rest of the time it has spent idle.

When compared with newer HP Z24i monitors, the LP2475w at 85% brightness will eventually after turning on and waiting be equivalent to Z24i at 90% brightness. However, Z24i at 100% brightness is only 300 cd/m2, whereas LP2475w at 100% brightness should be 400 cd/m2. Thus, I estimate the LP2475w has lost 20% of its backlight intensity. Yet it has only 9041 backlight hours.

Is it possible to lose CCFL intensity just by being idle due to e.g. diffusion or does the intensity of CCFLs degrade only when actually being used?

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Probably the ~10k hours of actual use. According to this article in Appliance Design a typical CCFL at room temperature will be down more than 20% after 10K hours at room temperature. Of course designs will vary in their lifetime, but your 25% drop does not appear to be far out of line.

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Interestingly, it appears that even a short time operating at very low temperatures can greatly reduce the lifetime for CCFL lamps.

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