"I'm shopping for a new computer monitor to use both for my PC and as an HDTV screen with HDMI. I'm trying to decide between two that appear to be the same except that one says 250 cd/m2 and the other one says 400 cd/m2. The monitor with 400 cd/m2 is more expensive, but I don't see any other difference. What is cd/m2 and why does it make a computer monitor cost more?"
cd/m2 is the measure of brightness for a monitor in terms of how well the screen will show up in brightly lit conditions. cd/m2 stands for candelas per square meter and is roughly a measure of how many candles of light are emitted from the screen being measured. Think of a candle as being approximately one candela. The thing to keep in mind about cd/m2 as it relates to your computer monitor is where you plan to use it.
If you will always be using a monitor in a room with controlled lighting, the cd/m2 might be less of an issue than if you have an office with a bright window that may drown out the screen. If you've ever had a digital camera with a screen you couldn't read when you were outside, you know exactly what I'm talking about. The digital camera screen in that case had a cd/m2 far too low for outdoor lighting conditions.
The cost difference of the two monitors you are comparing is likely due, at least in part, to the overall brightness of the two monitors. The one at 400 cd/m2 will be far more versatile in many room situations and probably uses higher quality components. If it were me making the purchase decision and the only difference is truly the cd/m2 brightness, I'd go for the more expensive monitor, because even in most controlled lighting situations the picture will appear better. At a bare minimum, look for a monitor with a 300 cd/m2. 250 cd/m2 isn't very bright and you may find yourself dimming the room lights, or suffering buyer's remorse with the cheaper monitor.