[Laser] Can some explain Steradian please

[Andrew T. Flowers, K0SM]

Paul,

My understanding, and (correct me if I'm wrong), is that mW/SR is 
analogous to a EIRP figure for an RF antenna and transmitter.  
Basically, it tells you what the equivalent isotropic radiator's power 
would have to be to be as bright as the LED you have.

So, if you were to put your 200mw/SR LED next to a 200mw output LED that 
radiated equally in all directions, they would be "equally bright" at a 
given distance.  I have to imagine even a little 5mW laser must be in 
the kW/SR range.

If you goal is to be pointing at a point target, the higher power/SR 
device is going to be "brighter" at the target.  

Andy



Paul Cianciolo wrote:
> Hello Zack,
>
> I have read that Wikipedia article....
>
> I guess it boils down to this.
>
> So answer this question please, Would the brighter 940 nm LED be the one
> with the higher mW/Sr rating?
>
> PauLC
> W1VLF
> http://www.rescueelectronics.com/
>
>
> -----Original Message-----
> From: laser-bounces at mailman.qth.net
> [mailto:laser-bounces at mailman.qth.net]On Behalf Of Zack Widup
> Sent: Thursday, July 31, 2008 5:59 PM
> To: Paulc at snet.net; Free Space LASER Communications
> Subject: Re: [Laser] Can some explain Steradian please
>
>
>
> This is about as simple as you can get:
>
> http://en.wikipedia.org/wiki/Steradian
>
> 73, Zack W9SZ
>
> On Thu, 31 Jul 2008, Paul Cianciolo wrote:
>
>   
>> Hello Folks,
>>
>> I wonder if there is some one out there in Laserland or LEDland, that can
>> explain the term steradian in layman's terms.
>>
>> As the term applies to LED's and intensity.
>>
>> For example if we start with a "reference LED"
>>
>> And drive it with say 2 volts and 100 ma that equals 200 milliwatts of DC
>> input power
>> What that equates to in terms of light output at a given wavelength I
>>     
> don't
>   
>> know.
>>
>> But lets pick a wavelength 940 nm.
>>
>> Please forgive me if I use some terms in appropriately.
>>
>>
>> So we have an LED with a set DC input level, and a set radiation pattern,
>> Lambertian lets say.
>>
>> SO........
>>
>> There is a math formula that defines what a Steradian is and I assume that
>> our reference LED illuminates with a given level.
>>
>> LEDs are made with all sorts of 1/2 power beam angles. for 160 deg to 3
>> degs.  So if we put a lens in front of our reference LED and try to
>> collimate the beam into a narrower 1/2 power angle, does that increase the
>> mW/Sr?
>>
>> Is it as simple as looking for the LED with the highest mW/Sr for the
>> "brightest" infrared LED?
>>
>> PauLC
>> W1VLF
>> http://www.rescueelectronics.com/
>>
>>     
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