1N6266

GaAs INFRARED EMITTING DIODE

INFRARED EMITTING DIODE RADIANT INTENSITY

The design of an Infrared Emitting Diode (IRED)-photode-

tector system normally requires the designer to determine

the minimum amount of infrared irradiance received by the

photodetector, which then allows definition of the photode-

tector current. Prior to the introduction of the 1N6266, the

best method of estimating the photodetector received

infrared was to geometrically proportion the piecewise inte-

gration of the typical beam pattern with the specified mini-

mum total power output of the IRED. However, due to

inconsistencies of the IRED integral lenses and the beam

lobes, this procedure will not provide a valid estimation.

The 1N6266 now provides the designer specifications

which precisely define the infrared beam along the device’s

mechanical axis. The 1N6266 is a premium device select-

ed to give a minimum radiant intensity of 25 mW/steradian

into the 0.01 steradians referenced by the the device’s

mechanical axis and seating plane. Radiant intensity is the

IRED beam power output, within a specified solid angle,

per unit solid angle.

A quick review of geometry indicates that a steradian is a

unit of solid angle, referenced to the center of a sphere,

defined by 4 H times the ratio of the area projected by the

solid angle to the area of the sphere. The solid angle is

equal to the projected area divided by the squared radius.

Steradians = 4 H A/4 H R2 = A/R2 = N

As the projected area has a circular periphery, a geometric

integration will solve to show the relationship of the

Cartesian angle () of the cone, (from the center of the

sphere) to the projected area.

N= 2 H(1 - COS )

2

Radiant intensity provides an easy, accurate tool to calcu-

late the infrared power received by a photodetector locat-

ed on the IRED axis. As the devices are selected for

beam characteristics, the calculated results are valid for

worst case analysis. For many applications a simple

approximation for photodetector irradiance is:

H ≅ Ie/d2, in mw/cm2

where d is the distance from the IRED to the detector in

cm.

IRED power output, and therefore Ie, depends on IRED

current. This variation (Ie/I) is documented in Figure 3,

and completes the approximation: H = Ie/d2 (Ie/I). This

normally gives a conservative value of irradiance. For

more accurate results, the effect of precise angle viewed

by the detector must be considered. This is documented

in figure 8 (Ie/N) giving:

H = Ie/d2 (Ie/N) in mw/cm2

For worst case designs, temperature coefficients and tol-

erances must be considered.

The minimum output current of the detector (IL) can be

determined for a given distance (d) of the detector from

the IRED.

IL = (S)H ≅ (S) Ie/d2

or

IL = (S)H = (S) (Ie/d2) (Ie/N) (Ie/I)

where S is the sensitivity of the detector in terms of out-

put current per unit irradiance from a GaAs source.

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