High-key lighting

This was originally done partly for technological reasons, since early film and television did not deal well with high contrast ratios, but now is used to suggest an upbeat mood.

High-key lighting is usually quite homogeneous and free from dark shadows.

The advantage to high-key lighting is that it doesn't require adjustment for each scene which allows the production to complete the shooting in hours instead of days.

Part of the reason for this is the advent of new lighting fixtures that are easier to use and quicker to set up.

Another reason is the growing sophistication of the audience for TV programs and the need to differentiate.