Bjerrum tangent screen for testing central visual fields

Historical Significance: This apparatus, known as a tangent screen or a Bjerrum screen, is for locating and measuring blind areas in the central 30 degrees of the visual field. The visual field is explored by the clinician moving white or coloured targets of various sizes, most commonly one 3 to 5 mm in diameter, at the end of a black wand to locate areas of the visual field where the target cannot be seen by the patient. See Cat no. 1302 for an example of the targets. It was first devised Hermann Aubert and Richard Forster about 1855 but was improved and popularised by Danish ophthalmologist Jannik Bjerrum in the last decade of the 19th century. It was a procedure commonly used by optometrists in the 1950s and 1960s and became superseded in the 1970s by easier to use static perimeters, especially when that method of perimetry became automated and computer controlled. (See Catalogue nos. 1333, 1645, 1664 for examples)