# Question on LED light dispersion. How to achieve maximum dispersion ??



## charanchakravarthi (Dec 10, 2013)

Hello Everyone,

Firstly I am not sure if this is the right place to ask the question. However, I have taken my chances. I have implemented an access reader setup which has a series of 8 LEDs in a loop to show access grant and access rejected conditions. Currently after the plastics have been identified and implemented, I can see that the LEDs are very close to the plastic casing and this causes "Bright Spots on the plastic where the LED is directly under it and Dim spots on the places where there are no LEDs. Hence I am not able to achieve a circular loop illumination, instead I get bright and dark spots along the circular ring, making it look awkward or more of a hexagon. I have tried different options of make the plastic above the LED as "Matt" finish by grinding / etching the plastic and that does not spread the light too. I can bring back the LED assembly to about 4mm and achieve uniform illumination of the ring, but that is not a option as read performance of the reader is affected.

It would be great, if the light experts like you guys could spare some thoughts of improving the same.


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## RoGuE_StreaK (Dec 10, 2013)

What type of LEDs? Standard 5mm? Undomed SMD? Piranha?
Probably the quickest and easiest is to change your source LED to a more suitable one, but we need details / photos. As an example, swapping standard 5mm LEDs for "straw hat" ones will give larger output angles in essentially the same body.


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## charanchakravarthi (Dec 10, 2013)

RoGuE_StreaK said:


> What type of LEDs? Standard 5mm? Undomed SMD? Piranha?
> Probably the quickest and easiest is to change your source LED to a more suitable one, but we need details / photos. As an example, swapping standard 5mm LEDs for "straw hat" ones will give larger output angles in essentially the same body.



Hello Rogue_Streak,

Thank You for your quick response. I am using a wide angle SMD Multicolour (RGB) LED. CLV1A-FKB from Cree. Viewing angle 120 Deg 1mm Height ,SMD. I have also attached a picture for your reference. I can see that there are LEDs with 160 degree light dispersion, but I am not sure if I can find one in the same form factor. From you image, you can clearly see the bright and dim spots on the plastic causing it to look spotted ring than a solid ring. 

Link to Image : http://www.screencast.com/users/Cha...lt/media/f13e3326-7652-468c-9fbf-39e0345aeb9e







Thank You

--CC


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## jason 77 (Dec 11, 2013)

Couldn't you just double the number of LEDs? As in "fill in" the dark spaces with more LEDs?


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## dc38 (Dec 11, 2013)

charanchakravarthi said:


> Hello Rogue_Streak,
> 
> Thank You for your quick response. I am using a wide angle SMD Multicolour (RGB) LED. CLV1A-FKB from Cree. Viewing angle 120 Deg 1mm Height ,SMD. I have also attached a picture for your reference. I can see that there are LEDs with 160 degree light dispersion, but I am not sure if I can find one in the same form factor. From you image, you can clearly see the bright and dim spots on the plastic causing it to look spotted ring than a solid ring.
> 
> ...



instead of having them fire directly through the ring material, maybe have them facing in towards the center of the ring? That way, the light should be more dispersed within the enclosure before coming out of the ring.

also, what if you tried buffing the domes of the LEDs?


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## RoGuE_StreaK (Dec 11, 2013)

Seems they don't have domes.

Are you using PWM to control? As an experiment, how does it look if you dial down the brightness?

I'm thinking you might need to apply an optic to decrease the output in the middle and redirect it out the sides more, possibly a "bat wing" optic? I don't know if they make them for these (seem to be roughly a "5050"?)
Maybe you could try casting some clear acrylic, experiment with sanding it back in various manners, see how it effects the output, try again, then if you get something "OK", take an RTV silicone mould and cast duplicates? It won't be perfect, pretty, or overly efficient, but it might work "well enough"


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## Ken_McE (Dec 11, 2013)

charanchakravarthi said:


> I can bring back the LED assembly to about 4mm and achieve uniform illumination of the ring, but that is not a option as read performance of the reader is affected.



Drop back to 4mm for evenness and use twice as many LEDs to maintain brightness.


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## charanchakravarthi (Dec 12, 2013)

jason 77 said:


> Couldn't you just double the number of LEDs? As in "fill in" the dark spaces with more LEDs?



Hi Jason_77,

Yes, that is an option and its long gone. But that would involve for cost on the overall reader. Also, I am in the final stages of finishing the product and cannot afford to go back to changing the PCB design. If things to pan put, well I would live with the spots rather than changing the design. 

Thanks 

--CC


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## charanchakravarthi (Dec 12, 2013)

dc38 said:


> instead of having them fire directly through the ring material, maybe have them facing in towards the center of the ring? That way, the light should be more dispersed within the enclosure before coming out of the ring.
> 
> also, what if you tried buffing the domes of the LEDs?



Hi Dc38,

Moving the LEDs inside was something I thought too. But I need to completely change the PCB design because there are components on the other side of the PCB that will interfere with it. In regards to buffing the LEDs, I did try coating the LED with white marker, pasting a plaster on it to diffuse light more evenly but that did not work as the bright and dim spots reduce proportionally and arrive at the same original problem but with lesser intensity. 

I have tried using diffusion paper to cover the LED assembly and that works to an extent of "milkishly" smoothing the dark and bright spots. But the spots are not completely removed. 

Would you advice on any type of plastic to achieve an even dispersion of light ? I use a poly carbonate plastic. PC RTP300 to be specific. 

Thank You

--CC


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## charanchakravarthi (Dec 12, 2013)

RoGuE_StreaK said:


> Seems they don't have domes.
> 
> Are you using PWM to control? As an experiment, how does it look if you dial down the brightness?
> 
> ...



Hi Rougue_Streak,

Yes, I tried bringing down the brightness by reducing the current to the LEDs but that again, brought down the light intensity overall and again with the same dark and bright spots. With longer streaks of dark spots to be frank. Casting it with plastic causes a similar outcome as the latter one. Sanding the plastic caused no effect, but I am not sure of the types of sanding. I simply grinded the plastic facing the LEDs and that did not yield better results than the current state. 

I also tried pouring some glue / silicon on the plastic facing the LEDs and that dint help either. 

Let me know your thoughts,

--CC


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## John Pombrio (Jan 21, 2014)

Might be too late, but here are my thoughts. Turn the negative into a positive by deliberately separating the LEDs use black electricians tape to mask off the dimmer spots and say that the segmented LED lighting is a feature! Otherwise, it is damn hard to evenly light a plastic surface. Amazon Kindlewhite does it with microscopically grooved etches in the surface. I guess you could dim the whole thing by using a heavily frosted surface but you would lose a lot of light. 
Your picture looks fine and I doubt anyone would notice the difference in brightness but YOU, heh. show it to some folks and get their opinion.


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## greenlight (Jan 21, 2014)

I have a PIC countertop hotplate with a glass surface. The underside where the LEDs light up is highly textured. It makes the readout look unusual, but still readable. You might consider an etched pattern in the plastic to draw attention to the LED's.


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