Abstract: An active matrix display includes a display element for producing a visual output when the display element is driven with a constant current, and drive circuitry for controllably driving a substantially constant current through the display element. The drive circuitry includes a two-transistor inverter having an inverter input and a common node output, and the common node output of the inverter is connected, directly or indirectly, to supply or control the current passing through the corresponding display element.

Abstract: Tilt (?(r, ?)) is measured in a measuring location (P(r, ?) optical disc (2). A pivotable objective lens (34) is brought to a first focus measuring location such as to focus a light beam (32) in a first anchor point (P1(r??r1, ?))having the same angular coordinate ? as said measuring location (P(r, ?)) and having a small radial distance ?r1 from said measuring location. The objective lens is brought to a second focus measuring location such as to focus the light beam in a second anchor point (P2(r+?r2, ?)) having the same angular coordinate ? as said measuring location and having a small radial distance ?r2 from said measuring location, wherein said first and second anchor pints are located on opposite sides of said measuring location. Tilt in said measuring location is calculated from the coordinates of said two focus measuring locations of said objective lens.