Lighting Design Glossary

Right Hand Rule

(Terms of mathematics/geometry)

1.
A rule-of-thumb method to determine the sequence of axae in a right handed (sic!) cartesian cordinate system. If the thumb, index finger and middle finger of the right hand are held so that they form three right angles, then the thumb indicates the X-axis, the index finger the Y-axis and the middle finger the Z-Axis.

Right handed cartesian coordinate systems are the standard coordinate systems for most applications and are used in most current CAD systems internally. Left handed cartesian coordinate systems follow the same rule, just using the left hand. They are most often used in surveillance, since they correlate well with flat projections of the longitude and latitude grids of orthogonal projections of the earth surface.

2.
A "rule of thumb" method to determine the normal vector of polygons in computer graphics applications. The rule is derived from the mathematical rule to determine the forward direction of helixes (screws). When the observer looks at the polygon from one side, and the points of the polygon are defined in counter clockwise sequence, then the normal vector points towards the observer. This can be symbolised by a "thumbs up" closed right hand fist. The four remaining fingers point into a counter clockwise direction (the sequence of the points), while the thumb shows the direction of the normal vector.

It is to note here, that while the definition of right- or lefthandedness of screws and helixes is a mathematical rule, the orientation of polygons in computer graphics is only a convention, though one that is followed fairly consistently by most programs