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When is the light offered to batsmen?
According to the Laws of Cricket, "if at any time the umpires together agree that the conditions of the ground, weather or light are so bad that there is obvious and foreseeable risk to the safety of any player or umpire, so that it would be unreasonable or dangerous for play to take place ...", the light is offered to the batting team.

What if the batsmen don't want to take the light?
The batting team is free to continue, commence, or resume play, even if the light is unsuitable, but the umpires have the discretion to overrule them. If the batsmen don't take the light when it is offered, that does not affect their right to appeal later. If the reading is less at the time of the appeal than what it was when the light was offered, and the same bowler (or a faster one) is on, the batsmen can go off, but if a spinner is on, they will have to continue playing even if the light is bad, since it is not dangerous.

What happens if a batting team choose to continue playing in poor light and later change their minds?
The captain of the batting side can, after having made a decision to play in unsuitable light, appeal against the light to the umpires before the next call of "Time". The umpires are allowed to uphold such an appeal if the light conditions are the same as, or worse than, when they made their previous offer.

How does the light-meter work?
The light-meter is a device with a light sensor at one end and a window at the other that displays the reading. The umpire stands at one end of the pitch and points the light meter down towards the sightscreen to take a reading.