AVR Freaks

Post a datasheet for the photodiode for starters.

When you say flash light from an iPod are you referring to the LED flashlight function? If yes that's white light and would expect the photodiode to be much slower responding.

Jim

I think we need to see your circuit. I find it hard to imagine the diode itself would have such a delayed response.
 

Why do you blame your IR diode?

Are you sure you are not measuring the response time of your  LED?

It might be far fetched, but a lot of different technologies are used in LED's.

White leds use a white phosphor excited by a uv led.

Some.  Some use a blue LED to excite a yellowish phosphor.  There are >>many<< LED/phosphor combinations to achieve specific colour temperatures or CRI.

The rate of change of the diode output depends on the current generated by the diode. It is after all, the simple I = C dV/dt. The current, I, depends on two factors: (1) the wavelength/color distribution of the light falling on the diode surface and (2) the intensity AT the diode surface. Intensity includes source brightness, distance from source to detector, and orientation of the detector sensitive surface with respect to the light source.

 

Once you have ALL of these factors known and under control, the reason(s) for differences in response speed should be obvious.

 

Jim

 

Optical triggering of thyristors  has been used with the real high power semiconductor stuf. Think power station here, not electric car. 100's of kV where whole stacks of high voltage thyristors are put in series to reach high enough breakdown voltages. The advantage of optical triggering with long glass fibres is obvious here.

It could be that this is old technology, maybe IGBT's are taking over in that area and I don't know if those can be optically triggered.

 

Some saying goes: "When you hear hooves, think horses, not unicorns".

When response times change drastically by changing the led, I suspect the led (or led incompatibility with the photo diode).

Or something even simpler, it might be an alignment error, where most of the led's light misses the photo diode.

No unicorns in #17.

 

Edit / Addition:

Different human's brains work in different ways. Different wordings can help a lot if the "other" explanation fits the target's brain better.

I had to re-read and think about #17 to see the similarities...

Perception of brightness by eye is notoriously difficult, especially if it is not a side-by-side comparison. First off, the eye, like many sensory organs of the human body, is logarithmic.

 

Second, memory of brightness is actually quite poor. This comes into play if you look at an illuminated surface, then after a period of time, look at a different illuminated surface and try to judge which one is/was brighter. It is much easier if you can look at the two surfaces, simultaneously.

 

Yet a third problem arises with flashes. When the flash is short compared to the visual time constant of the eye, your perception of the brightness may not match the actual brightness.

 

It is almost impossible to compare brightness for two different colors, because that is strongly affected by the frequency response(s) of the sensor (the eye, with its rods and cones that are sensitive to different wavelengths). Here, you get into the very significant philosophical issue of what "brightness" is. Is it simply the number of photons per second arriving at the eye (photon flux)? Or, is it the perception of brightness which is photon flux modified by the color response of the eye? Even light sensors such as photo-diodes, are subject to the latter constraint. 

 

Personally, I think that the OP is on a wild goose chase, with no clearly defined end-point.

 

Jim

Gosh, I never envisioned that IR diodes have a limited frequency response

Well said Jim, you have my vote.