Viewing Angle
Viewing angle
LEDs have a fixed light output pattern and are available with many different light output patterns. The LED light output pattern is selected in order to match the application requirement. Further adjustment to light pattern is possible with secondary lens systems.
The Light output pattern of an LED is mainly determined by two factors: -
1. The design of the reflective cup in which the LED die is mounted
2. The Epoxy enclosure
The diagram below illustrates that the Light being emitted by the LED die is reflected upwards by the cup and then is further shaped by the epoxy encapsulation acting as a lens: -
The most common LED has a circular light output where the spread of light is specified by the term: - 2θ½ (measured in degrees). LED manufacturers will often publish a Beam Pattern diagram giving a graphical image of the out put of the LED.
Leaded Round LEDs are available with beam angles (2θ½) ranging from ~8° to 135°. Standard SMT chip LEDs, which have much less space to create a lens and therefore cannot create tight output beam, usually have 2θ½ values in the range of 100° to 135°. Optical designers will often use Light Pipes with SMT LEDs to carry the light from an SMT LED to desired area. There also are designs of SMT LEDs that have similar Lens structure to through-hole devices that can create tighter beam patterns.
As an example an LED specified with a 2θ½ beam angle of 30° will have Beam Pattern: -
The solid curved line represents the light output (Iv) of the LED, for this LED the maximum Iv (100%) is at 0° i.e. it is brightest looking straight at the LED. The intensity falls off until at 15°(θ) from the centre line, Iv drops by 50% i.e. the light is half as intense. Hence: -
2θ = 2x15°
½ = When the light output (Iv) is 50% of the maximum.
Representation of the Light Output of a Round LED.
Although Round LEDs are by far the most common there an almost infinite number of shape producing different light output patterns. Another common LED package is the ‘Oval’ that gives an elliptical pattern. The light output pattern this type has two values for 2θ½, in the case below 70°/35° where 70° represents the horizontal value and 35° the vertical value (or vice versa depending on how the LED is mounted)
This will give an elliptical light output pattern: -
Light Output pattern of Oval LED
