When I'm looking at LEDs for sale, it seems to me that some sellers give unrealistic specifications regarding their LEDs' brightness. I was hoping someone could point me in the direction of a resource that would give me the ability to determine when sellers are blatantly lying, so I can avoid buying from them.

What are realistic brightnesses for different sizes (e.g. 3mm, 5mm, 10mm), colours and If currents/powers of LEDs? Would you expect a different brightness between 3mm and 5mm LEDs if they both have the same Vf and If?

 

Go to a reputable manufacturer and read the datasheets..
http://www.kingbrightusa.com/

 

What are realistic brightnesses for different sizes (e.g. 3mm, 5mm, 10mm), colours and If currents/powers of LEDs? Would you expect a different brightness between 3mm and 5mm LEDs if they both have the same Vf and If?

What is your application?

There are several brightness categories for LEDs, but mainly standard and ultrabright with one or more brightness buckets each.

Different sizes will have different perceived brightnesses because the package is designed to disperse light. You also have to consider viewing angle and distance.

If you require a certain brightness in multiple LEDs, you need to drive them with current sources. That minimizes the impact of forward voltage variations. Brightness matching will require using LEDs from the same lot that have perhaps been sorted by brightness by the manufacturer.

 

Cree and Nichia as well as the aforementioned Kingbright are reputable.

It would be nice if manufacturers would give a lumen rating instead of candella, millicandella, wattage, etc.

 

When I'm looking at LEDs for sale, it seems to me that some sellers give unrealistic specifications regarding their LEDs' brightness. I was hoping someone could point me in the direction of a resource that would give me the ability to determine when sellers are blatantly lying, so I can avoid buying from them.

What are realistic brightnesses for different sizes (e.g. 3mm, 5mm, 10mm), colours and If currents/powers of LEDs? Would you expect a different brightness between 3mm and 5mm LEDs if they both have the same Vf and If?

Reference the data sheets instead of the salesperson.
Yes, consistency between manufacturers would be nice.
Brightness just depends on so many factors. Incandescent lights, or LEDs for general room lighting, put out light in all directions. LEDs usually just put out light in one direction and not with the same angle. Some in a 30 degree cone. Some in 120 degree. You have to compare all specs.

 

This web page provides a good overview of the topic.