Page 67 - Sensors and Control Systems in Manufacturing

FIGURE 1.13 LED The Role of Sensors in the 21st Century 29
4
I v (0 )
actual radiation
pattern. I (0) = 1 I (0°)
v
/ 2 v
1
θ / 2

4
(0 )
I v
1
2θ / 2
4
I (0) = 1 I (0 )
/ 2 v
v

but has a narrower viewing angle. Nondiffused and water clear LEDs
have identical viewing angles; the only difference is, water clear
encapsulations do not have a tint to indicate color when the LED is
not illuminated.
Overall visibility can be enhanced by increasing the number of
LED chips in the encapsulation, increasing the number of individual
LEDs, and utilizing secondary optics to distribute light. To illustrate,
consider similar red GaAlAs LED chip technologies in four different
configurations.
In each case, the amount of visible light depends on how the
LED is being viewed. The single chip may be appropriate for direct
viewing in competition with high ambient light. The six-chip LED
may be better suited to backlight a switch or small legend, while
the cluster or lensed LED may be best to illuminate a pilot light or
larger lens.

1.13.5 Operating Life
Because LEDs are solid-state devices, they are not subject to cata-
strophic failure when operated within design parameters. DDP LEDs
are designed to operate upwards of 50,000 hours at 25°C ambient
temperature. Operating life is characterized by the degradation of
LED intensity over time. When the LED degrades to half of its