|The LED, the solid-state light source of the future, differs in many
aspects from every other classical light source.
For now, let us consider only one aspect: LEDs are surface emitters, and because of the planar substrates on which they are currently deposited, the die generally exhibits a half-space emission
Therefore, all LED applications can generally
be divided into two groups: direct and
indirect (see Fig. 1). The terms direct and
indirect describe the relation between the
LED and its secondary optic components,
or the light target (the main optical axis).
Also, the term direct (as for direct light/flux)
is also used to emphasize the fact that LED
applications use the flux from the primary
optic lens of the emitter to create the light
Direct: A direct optical configuration is
characterized by the alignment of the optical
axis of the emitter(s) with the main optical
axis of the light. This need not necessarily
be coaxial. It can also be considered an issue
of the direct visibility of the emitter (from
a point of view within the desired angular
emission range of the light). A descriptive
example is a projector application using
aspherical lenses for image creation. Such
arrangements would be addressed as direct
type, but they strongly restrict the visibility
angles of the emitter, which is observed only
through secondary optics.
Indirect: In contrast to the direct configuration,
an indirect optical configuration
will always prevent the emitter from being
observed directly. Instead, the flow of light
will be controlled completely by secondary
optics, such as free-form
reflectors. In the archetypical
representation of an indirect configuration,
the optical axis of the emitter will be antiparallel
to the optical axis of the light (the reflector); for example, a perfectly
reverse orientation. Any semi-direct solutions are also possible.
This article was published in the Sept/Oct 2009 issue of LEDs Magazine.
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