This type of perimetry is the most commonly used in clinical practice, and in research trials where loss of visual field must be measured.[4] However, the sensitivity of white-on-white perimetry is low, and the variability is relatively high; as many as 25-50 percent of the photoreceptor cells may be lost before changes in visual field acuity are detected.[2] This method is commonly used for early detection of blind spots. The patient sits in front of an (artificial) small concave dome in a small machine with a target in the center. The chin rests on the machine and the eye that is not being tested is covered.