# What does a table under "Emission Point" label mean in metal-can laser LED mean?

Screenshot above comes from page 6 of this datasheet That's for OSRAM PLPT5 450KA blue laser diode.

So far I did not have any problems understanding technical drawings, so I suspect this may be a standard notation I've somehow never seen so far, or a custom notation used on laser diode (or other optical elements) datasheets, so it seemed natural to ask here first, some of you probably have already seen this on other datasheets..

The thing I don't understand is the part marked with red line: "⊕ 0.17 A", data apparenly meant for "Emission Point" location. At first I thought it's its radius/diameter, but the same drawing in other places uses typical ø symbol for that, so it's not diameter.

I also do see another "A", I think it's using the same font, but I don't see any correlation between those.

Please solve that mystery for me, I have a feeling that this is something basic, but it eludes me:

1. red line, what does ⊕ symbol mean?
2. red line, what does 0.17 dimension mean, and what is its unit ('A'? ampere?? angstrem??)
3. red line, what does A mean here? (any relations to 'A' from green line?)
4. green line, what is that black triangle denoted by [A] label?
it seems to be some marking/symbol, as it doesn't seem to be drawn in any way on the other two projections

This is Geometric Dimensioning and Tolerancing (aka GD&T). It is a newer and more specific way of dimensioning drawings compared to the traditional way of using dimensions everywhere. It came out of the UK weapons industry during and shortly after WW2, and is quite popular in industries where precise and clearly defined tolerances are important.

I'm not exactly fluent, but if I'm reading them correctly:

• A in green is a datum feature Since the green A is attached to a cylindrical surface, it defines and axis down the centre of the cylinder from which other things can be measured.
• A at the end of the red box means the measurement is taken from that datum.
• The circle with a cross means position, so this control frame is specifying the location of the emission point.
• 0.17 is a tolerance. Units will be the same as the rest of the datasheet. In my experience GD&T is almost always metric (but I'm sure there are exceptions in documents from the USA and/or the last century).

So the emission point is within 0.17mm of the axis defined by the cylindrical surface marked A.

• Thank you very much! Everything fits and makes sense. Commented Dec 4, 2020 at 11:30
• You say that "A is attached to a cylindrical surface", but actually it's attached to a place where material from the cylinder is removed (as shown in the view showing the bottom side). Commented Dec 4, 2020 at 16:39
• @ThePhoton The datum does not seem to be placed correctly per my understanding of GD&T. You are right that the cylinder is not complete there, also it would be normal to place the datum symbol for an axis on a diameter dimension. But I can't see any other features suitable for use as a datum feature near the datum symbol, and using the cylinder makes sense from the shape of the part. Plus, an axis datum combined with the 1.5±0.08 dimension fully locates the emission point - which I can't do if I try to use a different datum feature as A. So I think Osram intended to use the cylinder. Commented Dec 4, 2020 at 17:08