Lots of people feel blue when sunlight is scarce. But users of clinical-strength light boxes swear by light therapy as something that can help lift mood and make them feel more awake and energized. Not just any “happy lamp” will do, however. Experts say they must have at least 10,000 lux (a measure of light intensity), the lamp must be close enough to your face, and must be used for 20 to 30 minutes at a time. If you’ve decided it’s time to get more light into your life, here’s what to look for in a lamp.