How is led half angle measured

[mercurial]

Hi,
Was wondering if the Led half angle is measured from the tip of the led or from the actual light origin inside the led package.
I couldn't find any definitive references regarding this.

For example this document

https://www.silabs.com/documents/public/application-notes/AN521.pdf

shows 2 different figures (attached)

Its not really clear from what point is the half angle measured.

[CosteC]

Generally from optical source. Almost every led has some lenses and mirror inside.
However in practice it does not matter if real LED is shifted 1 mm from optical source with optics. Partially also due to not-so-great repeatability of encapsulation process.

[mercurial]

Generally from optical source. Almost every led has some lenses and mirror inside.
However in practice it does not matter if real LED is shifted 1 mm from optical source with optics. Partially also due to not-so-great repeatability of encapsulation process.

Hi thanks for the info.
So if I want to draw rays from the Led extending outward it would be from the led inside the package correct? However we do not know the behaviour of the internal lens, we only know that the datasheet specifies an half angle of some degrees say 10 deg. But I'm not sure from where do I extend the rays? using that 10 deg value.

[ArdWar]

It's often specified from the optical center, i.e. the center of the semiconductor surface area.

In practice soldering will be much less repeatable than encapsulation in my experience. A bit of shift in optical center is much less impactful than tilting from uneven soldering.

[CosteC]

Drawing rays is also bit tricky.
Half angle is angle at which intensity drops to 50% (common criteria). So a little more than half angle there is 49% of full brightness. Most LEDs do not have sharp characteristics.

I would assume cone of light starts at structure. It is probably not exactly true due to lens but also not far from truth.
@ArdWar pointed also inaccuracies of soldering, which add too.

[DavidKo]

The LEDs are not point light source. This angle is only approximation from big distance where you can replace it with the point light source. For optical simulations the LED suppliers have measured light sources which can be imported to optical simulation programs. 

[CosteC]

The LEDs are not point light source. This angle is only approximation from big distance where you can replace it with the point light source. For optical simulations the LED suppliers have measured light sources which can be imported to optical simulation programs.

This is nicely visible in many LED torches with ZOOM option - you usually can project die image on wall.
Anyway, normal, soldered LED is not optically precise instrument.