Light distribution from multi LED lamp?

[treez]

If you are doing a 300W LED bank comprising say ten COB LEDs. You will want a certain light area distribution, and would use lenses to get this.
The thing is, do you make one lens giving the required light distribution and use that same lens on all ten LEDs…or…is it better to make several different lenses , each pointing in a different direction, each concentrating the light to a more limited area, and then get the overall required light distribution from all ten of the different ones acting together?

 

[Audioguru]

All the LED bulbs in my home have no lens. The ceiling LED light in my computer room has 72 little chip-on-board LEDs with no lens. It lights the entire room properly.

 

[c_mitra]

You will want a certain light area distribution, and would use lenses to get this.

If you want any specific distribution of light, you need to use individual lenses. It may not be possible to use a single lens to focus on a desired distribution. For example, to get an uniform distribution over a certain area, you need to choose the focal length and adjust them individually.

 

[treez]

Thanks. Please just to clarify….
So for example, say I was doing a 300W LED lamp, and I need a specific light area distribution…

A)….I can do it with one large 300W COB LED with one lens.
Or
B)… I can do it with ten 30W COB LEDs and ten different lenses…each lens shining light in a slightly different direction than all the others, in order to create the total light distribution.
Or
C)....I can do it with ten 30W COB LEDs and ten identical lenses

….so we are saying that B is best?

Presumably A and C are the same?....or do the ten LEDs mean interference patterns are produced, and even light cancellation?

 

[Audioguru]

I have never seen huge high power COB LEDs. Have you? Their heatsink will need to be the size of a bus, or need a leaf blower cooling them.

 

[FvM]

It seems to me that you are discussing "solutions" without specifying an exact problem.

or do the ten LEDs mean interference patterns are produced, and even light cancellation?

Nothing of this kind. Similar effects are observed with coherent light sources, e.g. lasers. LED light power simply adds up.

 

[treez]

Solution...any outdoor lighting situation...eg a car park light etc etc.
So then maybe just one lens type will do?
Eg ten 30W LED COBs with ten identical lenses...(ie, C in the above) .this is OK?

 

[KlausST]

Hi,

So then maybe just one lens type will do?

Yea, exactly: maybe.

With optical sytems you have:
* a light source with it's light emitting area dimensions
* a lense with it's optical specifications
* the distance between both
* the area you want to illuminate, with it's dimensions and the light distribution expectance
* the distance to the lense
* light color distribution
And so on.....

--> You need to define your requirements first.

so we are saying that B is best?

Asking for the "best" without defining it is useless.
The bst may be:
* lowest overall cost
* lowest development cost
* lowest piece price at volume xx
* smallest size
* only standard parts
* best light distribution / equal brightness
* smallest spot point
* sharp edges in brightness
* material
* mechanical stability
* and a lot more..

Klaus

 

[treez]

Thanks...its these three that are of interest

best light distribution / equal brightness
* smallest spot point
* sharp edges in brightness

How do A , B and C (from post #4 above) fayre with these three points?

To have ten different lenses will be more expensive, so can we say that it is pointless because there are no optical advantages of having ten different lenses?

Though Intuition tells me that ten different lenses may be worse optically, because there will inevitably be overlap in their light spreads, and so wasted light?

I also suspect that it depends on the angle of light spread required…eg, a very wide light spread would be impossible to achieve with just one lens, and so several different lenses would be needed…each lens taking its own more limited portion of the light spread.
Am I wrong?

 

[c_mitra]

Their heatsink will need to be the size of a bus, or need a leaf blower cooling them.

300W is not really much; I presume that 300W reflects the input power.

Just as a rough approximation, say about 50% of this input is converted into light. The rest will be converted into dissipation, i.e., heat.

Even cheap LEds will have efficiencies around 30% so that the dissipation will be around 210W. For white LEDs there will be some dissipation in the phosphors too.

At 60C, you may need about 2000-3000 cm2 effective area to dissipate this much heat. If you use a small fan, this will be much more reduced.

Not really the size of a small bus.

 

[KlausST]

Hi,

You have to say how it relates, because we don't know.
And this lack of information makes it impossible to abswer the question "what is best".

Multiple LEDs will cause multiple sources of light.
Single lense or multiple lenses ... it now depends on focus adjustment whether you get one big blur spot or multiple sharp spots. Maybe you want the one, the other or soemething inbetween...
Multiple single lenses are more expensive, but give you more flexibility...

With non monochromatic light you additionally get chromatic dispersion problems.

I need a specific light area distribution…

What does specific mean? Specific in which manner?


Klaus

 

[c_mitra]

To have ten different lenses will be more expensive, so can we say that it is pointless because there are no optical advantages of having ten different lenses?

The situation will be different if you want a small spot or a wide area of illumination - these are different requirements.

For practical purposes, you can consider each COB as a point source; A lens can be used to cast an image of the COB on the final area in a spot lighting experiment.

But 10 of these need 10 different lenses with a slightly different orientation. This is true if you want a spot lighting or a wide area to be lighting.

Simpler way for a wide area illumination is to mount the COBs on a hyperboloid shaped heat sink.

 

[treez]

Supposing i have a "light engine" which is ten COB LEDs on a 15cm by 15cm heatsink surface.....i require them to light a wide area.
Each led has its own power supply, and i can dim the light by turning off one or more leds......if they each have the same lens...then this dimming method is workable.
However, if they all have different lenses , pointing their maximum ray in a different direction, then i cant do this kind of dimming.

Therefore I wish to find out if ten similar lenses will be OK? (i hope so) ...or would it sometimes be inferior to having ten different lenses?

If it is ever inferior, what is the case where it is inferior?...presumably wide area light distribution cases?

 

[c_mitra]

i require them to light a wide area.

Why use a lens at all?

15cm X15cm heatsink for 10 COB LED with 300W power input may be difficult to focus on a wide area - best may be to skip the lens altogether. Unless the light source is quite high altitude.

In fact, I should say that you need to use 10 heatsinks with the LED mounted to shine light in a slightly different direction.

You use a lens to focus diverging light rays; a reflector does the same job better with higher efficiency. Light from a bare LED is usually divergent.

 

[treez]

Thanks, the led light is divergent but gets less in strength as you get more angled from the zenith.......so we need lenses to put out decent light strength to the peripheral areas.

 

[volker@muehlhaus]

So for example, say I was doing a 300W LED lamp, and I need a specific light area distribution…

A)….I can do it with one large 300W COB LED with one lens.
Or
B)… I can do it with ten 30W COB LEDs and ten different lenses…each lens shining light in a slightly different direction than all the others, in order to create the total light distribution.
Or
C)....I can do it with ten 30W COB LEDs and ten identical lenses

….so we are saying that B is best?

The difficulty with using 10 small COB to illuminate 10 different areas is this: depending on color tolerance of the individual COB, you will notice different color on your wall/target. Color difference for SDCM=6 is easily visible and obvious. So you better point all individual COB to the same direction.

 

[c_mitra]

Thanks, the led light is divergent but gets less in strength as you get more angled from the zenith.......so we need lenses to put out decent light strength to the peripheral areas.

I understand.

You are planning to use the LEDs at a low height. In this case, the peripheral rays can reach the ground and you want to divert some of the central rays to the edges. But most of the common lenses catch only the central rays and just ignore the peripheral rays.

I do not have a ready answer but a diffuser (some are made using corner reflectors) can work better than a lens in such cases.