The general principle of such meters is to observe a portion of the visual field against its background and then to bring about a condition such that the observed difference in brightness reaches a threshold value so that it is only just discernible. The instruments differ in their means by which this end is achieved. The threshold may be produced quite simply by interposing a light-absorbing medium, such as an optical wedge, in the field 3509 of view. Other methods include reducing the contrast between the object and its background by superimposing a veiling brightness over the observed field.