I am working on a project in which I need to find a PWM duty cycle to dim an LED to the same brightness level as an LED being dimmed with CCR. Initially, I was considering using a camera for this: I would take a picture of each LED, and compare the intensity with photo-editing software. I am not sure how accurate this would be, though. I was wondering if there were any devices out there (light sensors, etc.) that could help me achieve this.
Electrical – How to build a light sensor for PWM dimmed light (for comparison against CC dimmed light)
brightnessdimmingledlight-sensorpwm
Related Solutions
Wikipedia has some good examples about dimming. For as simple solution as possible (by electrical terms) I would use a transformer, full wave DC bridge and a thyristor. Again, wikipedia has a great picture of that
Note that thyristor dimmers usually make some radio frequency noise but as I see it, this is one of the simplest AC dimmers.
You will find more info about different dimmers on http://en.m.wikipedia.org/wiki/Light_dimmer
And although you would like to keep out from MCUs, at least think about them. If the thyristor-stuff goes over your head, try to use MCUs. You will find them to be very useful for your future projects.
It is permissible within certain limits. The best place to look is probably the associated IEC standards (IEC 60285 Laser Safety and IEC 62471 Lamp Safety), which are generally internationally recognized as best practice. Unfortunately I can't post excerpts of them here since they're copyrighted.
Choosing which standard to apply depends on how the LED is used. OSRAM has a very comprehensive appnote describing how these standards apply to infrared LEDs and how to calculate permissible exposure.
Your particular case focuses on pulsed light. In general, PWM'd light is weighted against its averaged value, so long as the individual pulses do not exceed an irradiance limit (given by a chart in the standard of pulse length vs. irradiance). This is all outlined in the OSRAM appnote, although since you're in the visible range you'll have to refer back to the source standards to see what the particular limits are for your wavelengths.
Edit: Found another appnote which may be useful to you - OSRAM has an appnote on 62471 as a whole, not just IR.
The best place of course to look is the standard itself, but it costs about $250. If this is a product you're designing, that's probably worth it, but if this is just a hobby project I would scavenge information based on appnotes.
Best Answer
I'd suggest that the Intersil ISL29125 might be suitable for your application. Available on a breakout board from either Digikey or Sparkfun for under $8.
These are I2C devices with very high resolution and integration times that are adjustable to reject 50/60 Hz and above modulation effects, so they should reject your PWM frequency very well.
You could calibrate 2 individual sensors or use just a single one in a strict comparison mode.
There are multiple solid state light sensors available, but this is the only one I've seen with such good calibration control.
Depending on how professional a solution you are developing there are Arduino libraries for this device from Sparkfun, though they lack much in the way of calibration capability.