Very briefly, what is the effect of LED package sizing? i.e. 3mm vs. 5mm vs. 8mm?

[WhiteRabbit]

In particular, I'm wondering what the effect is with respect to straw hat LED's.

e.g. In general, what are the optical differences (if any)? Are bigger packages needed to handle higher power? Or is it just a meaningless marketing thing ("My LED is bigger than yours.")?

My hunch is that an 8mm straw hat is likely to have a more uniform light distribution than a 5mm (all else being equal). True or false? On the other hand, maybe all the extra plastic absorbs more light, so maybe the 8mm's are also likely to produce fewer lumens?

Not all LED's come with spec sheets, and those that do don't always tell you everything you want to know. For those occasions, I thought having some "generally true" rules of thumb would be helpful.

 

[WhiteRabbit]

I notice that neither digikey nor mouser have anything listed as "straw hat" or "strawhat". Is it a BS category? I'll assume it is. Therefore, strike "straw hat" from the query above, unless you know something that digikey and mouser don't.

 

[WhiteRabbit]

In a different thread, I notice JohnR66 is recommending a CREE XP-E as the preferred substitute for a "straw hat" LED because it's "low dome": http://www.candlepowerforums.com/vb...gh-power-LED&p=3095722&viewfull=1#post3095722

 

[WhiteRabbit]

OK, I think I get it now. According to http://www.theledlight.com/LuxeonLEDs.html, "Low dome" produces a narrower beam, and "high dome" a wider beam. Makes sense. So, low dome is the opposite of what I want. What threw me off is that regular 5mm LED's seem to throw a narrow beam, and so I associated that with the steep curvature.

In any case, just to clarify, what I'm looking for is a wide beam, as close to an even distribution over 180 degrees as I can find.

 

[Steve K]

Not all LED's come with spec sheets, and those that do don't always tell you everything you want to know. For those occasions, I thought having some "generally true" rules of thumb would be helpful.

I don't have the answer to your question, but I do have a rule of thumb.... if you are considering buying electronics parts, only buy parts that have a spec sheet or datasheet. If the manufacturer doesn't care enough to provide a datasheet, they probably don't care about producing a good part.

 

[DIWdiver]

5mm LEDs can have narrow beam, wide beam, or diffuse beam. The shape and size of the LED don't necessarily tell you much about the beam, though there are some guidelines.

Bigger packages are required for tightly focusing larger dies.

Milky-looking plastic always results in a diffuse LED.

Flat tops can't be tight beams (I'll bet someone will point out an exception to this one).

I remember back in the day referring to 'top-hat' or 'stove-pipe' LEDs. Those being the ones with a cylindrical portion to the body, and a flange at the back end, similar to the brim of an old 'stove pipe' style of tophat famously worn by Abraham Lincoln and others of his time. It refers to the general shape (excluding the 'lens' portion) of the LED, not to a particular size, power level, or beam characteristic. I'm sure that 'straw hat' fits the same category, and I'd bet anyone who's been around the industry a while would, upon hearing any of those terms, immediately think of that shape LED.

If all your LEDs point the same direction, a uniform 180 degree beam isn't going to happen without optics specifically designed for that purpose. Take a look at the general-purpose LED light bulbs now available. All of them have multiple dies pointing different directions. The big guys like Philips and Cree didn't do that without good reason.

All that said, I don't have the answer either. But if you want to achieve anything even close to 180 degree uniform lighting, you need to start by asking such questions as:

How many different directions can I point LEDs? Closely related, how many LEDs do I need (or can I afford) to use?

Then you can look at the spatial distribution graph in the data sheet and estimate or calculate how overlapping fields will combine.

Don't have a spatial distribution graph in your data sheet? Re-read Steve K's post.

 

[LEDealer]

5mm LEDs can have narrow beam, wide beam, or diffuse beam. The shape and size of the LED don't necessarily tell you much about the beam, though there are some guidelines.

Bigger packages are required for tightly focusing larger dies.

Milky-looking plastic always results in a diffuse LED.

Flat tops can't be tight beams (I'll bet someone will point out an exception to this one).

I remember back in the day referring to 'top-hat' or 'stove-pipe' LEDs. Those being the ones with a cylindrical portion to the body, and a flange at the back end, similar to the brim of an old 'stove pipe' style of tophat famously worn by Abraham Lincoln and others of his time. It refers to the general shape (excluding the 'lens' portion) of the LED, not to a particular size, power level, or beam characteristic. I'm sure that 'straw hat' fits the same category, and I'd bet anyone who's been around the industry a while would, upon hearing any of those terms, immediately think of that shape LED.

If all your LEDs point the same direction, a uniform 180 degree beam isn't going to happen without optics specifically designed for that purpose. Take a look at the general-purpose LED light bulbs now available. All of them have multiple dies pointing different directions. The big guys like Philips and Cree didn't do that without good reason.

All that said, I don't have the answer either. But if you want to achieve anything even close to 180 degree uniform lighting, you need to start by asking such questions as:

How many different directions can I point LEDs? Closely related, how many LEDs do I need (or can I afford) to use?

Then you can look at the spatial distribution graph in the data sheet and estimate or calculate how overlapping fields will combine.

Don't have a spatial distribution graph in your data sheet? Re-read Steve K's post.

Please define flat tops?

Do you mean flat tops like midpower emitters, i.e. dispensed phosphor in a plastic package, or flat tops like a de-domed XP-G2?

 

[DIWdiver]

Basically anything that has a flat surface where the light comes out. For a tight beam you need some kind of optic, and if it's part of the LED, it will be a dome.