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1.13.3 LED Intensity
LED light output varies with the type of chip, encapsulation, effi-
ciency of individual wafer lots, and other variables. Several LED
manufacturers use terms such as “superbright” and “ultrabright” to
describe LED intensity. Such terminology is entirely subjective since
there is no industry standard for LED brightness.
The amount of light emitted from an LED is quantified by a single-
point on-axis luminous intensity value (Iv). LED intensity is specified
in terms of millicandela (mcd). This on-axis measurement is not
comparable to mean spherical candlepower (MSCP) values used to
quantify the light produced by incandescent lamps.
Luminous intensity is roughly proportional to the amount of cur-
rent (If) supplied to the LED. The greater the current, the higher the
intensity. Of course, design limits exist. Generally, LEDs are designed
to operate at 20 milliamps (mA). However, the operating current
must be reduced relative to the amount of heat in the application. For
example, six-chip LEDs produce more heat than single-chip LEDs
because they incorporate multiple wire bonds and junction points
that are affected more by thermal stress than single-chip LEDs. Simi-
larly, LEDs designed to operate at higher design voltages are subject
to greater heat. LEDs are designed to provide lifelong operation
because of optimal design currents, considering heat dissipation and
other degradation factors.

1.13.4 Visibility
Luminous intensity (Iv) does not represent the total light output from
an LED. Both the luminous intensity and the spatial radiation pattern
(viewing angle) must be taken into account, Figure 1.13. If two LEDs
have the same luminous intensity value, the lamp with the larger
viewing angle will have the higher total light output.
Theta one-half (q½) is the off-axis angle where the LED’s lumi-
nous intensity is half the intensity at direct on-axis view. Two times
q½ is the LED’s full viewing angle; however, light emission is visible
beyond the q½ point. Viewing angles listed in this catalog are identi-
fied by their full viewing angle (2q½°).
The LED viewing angle is a function of the LED chip type and the
epoxy lens that distributes the light. The highest luminous intensity
(mcd rating) does not equate to the highest visibility. The light output
from an LED chip is very directional. A higher light output is achieved
by concentrating the light in a tight beam. Generally, the higher the
mcd rating, the narrower the viewing angle.
The shape of the encapsulation acts as a lens magnifying the light
from the LED chip. Additionally, the tint of the encapsulation affects
the LED’s visibility. If the encapsulation is diffused, the light emitted
by the chip is more dispersed throughout the encapsulation. If the
encapsulation is nondiffused or water clear, the light is more intense