A detection threshold is the minimum stimulus strength that can be perceived. Even with such a humble definition, the measurement of detection thresholds has become a golden, powerful tool for psychophysicists, but it is elusive to pinpoint. Some examples of visual detection tasks discussed in this entry are:

• Detection of a tumor. In order to diagnose cancerous tumors, radiologists must learn to detect their presence in a complex X-ray.
• Detection of a feature. The ability to detect a gap in the letter C can indicate whether new eyeglasses are necessary, or whether, even with new eyeglasses, one is fit to drive a car.
• Detection of a needle in a haystack. Many real-life detection tasks are similar to finding Waldo in a crowd or a needle in a haystack. ...