Linearity of optoelectronics ?

[vic]

Hi,

What is the linearity of the current/light intensity caracteristic of
usual LEDs, photodiodes and phototransistors ?

I'm building a simple chromatographer to monitor the concentration of a
chemical in a solution, and it's really a simple DIY project, but i'd
like to get not too far off results.

v.

[Nemo]

>What is the linearity of the current/light intensity caracteristic of
>usual LEDs, photodiodes and phototransistors ?

I'm not entirely sure about LED's, probably pretty linear, but keep in
mind that they convert *current* to light. If you drive them with an
analogue voltage waveform, they have a turn-on voltage so there will be
a step in the response. So if you want to control their brightness, do
it with current.

Photodiodes can be linear, depending on how you use them. See this
article for a simple introduction to their foibles:

http://www.national.com/onlineseminar/2004/photodiode/PhotodiodeAmplifers
.pdf#page=2
--
Nemo

[Artemus]

I didn't see the OP so I'm responding to this snippet.

You can greatly linearize the transfer function(s) of opto devices
by employing a feedback loop. See the datasheet for the
Burr-Brown ISO100 for one way of doing this.
Art

"Nemo" <z...@nospam.nospam.nospam.nospam.co.uk> wrote in message

[George Herold]

I like photodiodes. I'm not sure about LED's as sources. How 'bout a
laser diode? What's the wavelength?

George H.

[John Larkin]

Laser diodes are nasty for linearity. They have gross mode jumps as
current, temperature, interferance reflections, or, well, anything
else changes.

Photodiodes are very linear at reasonable currents.

John

[stra...@yahoo.com]

LEDs light output is linear vs current. Some years back I had to build
a headamp for a photo sensor. Rather than attemp a simulation of the
photo diode, I built a light source to test it. We used a high
intensity HP LED because it was fast (10nS turn on / off) and drove
it with a 'diamond transistor' from Burr Brown. We got linearity
better than 1% and response -3dB at 30 MHz.

G²

[Vladimir Vassilevsky]

stra...@yahoo.com wrote:

> On Jul 8, 7:01 pm, George Herold <ggher...@gmail.com> wrote:
>
>>On Jul 8, 4:08 am, vic <n...@bidouille.org> wrote:
>>
>>
>>>What is the linearity of the current/light intensity caracteristic of
>>>usual LEDs, photodiodes and phototransistors ?
>>
>

> LEDs light output is linear vs current.

You should be very carefull about that.
Unless you are getting into the low or high currents, the LED output is
reasonably linear (~1% ballpark). But beware of the strong
nonlinearities at the low or high currents.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

[Robert Baer]

Nemo wrote:
>> What is the linearity of the current/light intensity caracteristic of
>> usual LEDs, photodiodes and phototransistors ?
>
> I'm not entirely sure about LED's, probably pretty linear, but keep in
> mind that they convert *current* to light. If you drive them with an
> analogue voltage waveform, they have a turn-on voltage so there will be
> a step in the response. So if you want to control their brightness, do
> it with current.

* Not a "step", it is a log relationship.

[Tim Williams]

"Robert Baer" <rober...@localnet.com> wrote in message
news:BuudnRSin7y7L8jX...@posted.localnet...

>> If you drive them with an analogue voltage waveform
>

> * Not a "step", it is a log relationship.

*Exponential

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

[George Herold]

On Jul 8, 10:54 pm, John Larkin

> John- Hide quoted text -
>
> - Show quoted text -

Hmm Good point. I should have known better. I was thinking that
laser diodes (run way above threshold) do a better job of turning each
electron into a photon.

Anyone know the efficiency of LED's? How many electrons are wasted
and how many turn into photons?

George H.