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Diffstat (limited to 'content/posts/led-characterization/index.rst')
| -rw-r--r-- | content/posts/led-characterization/index.rst | 21 |
1 files changed, 15 insertions, 6 deletions
diff --git a/content/posts/led-characterization/index.rst b/content/posts/led-characterization/index.rst index 0f6e32b..ebcc16c 100644 --- a/content/posts/led-characterization/index.rst +++ b/content/posts/led-characterization/index.rst @@ -103,14 +103,13 @@ over the problem, there are several sources for imperfections: <figure class="side-by-side"> <img src="images/driver_ringing_strong.jpg" alt="Strong ringing on the LED voltage waveform edge at about 100% overshoot during about 70% of the cycle time."> - <figcaption>The shift register logic output of the multichannel LED driver directly driving a small mosfet's - gate through an inch or so of PCB trace caused extremely bad ringing at high driving - frequencies.</figcaption> + <figcaption>The LED strip being at the end of a couple meters of wire caused extremely bad ringing at high + driving frequencies.</figcaption> </figure><figure class="side-by-side"> <img src="images/driver_ringing_weak.jpg" alt="Weak ringing on the LED voltage waveform edge at about 30% overshoot during about 20% of the cycle time."> - <figcaption>Adding a resistor dampened the ringing somewhat, but ultimately it cannot be eliminated - entirely.</figcaption> + <figcaption>Adding a resistor in front of the MOSFET gate to slow the transition dampened the ringing + somewhat, but ultimately it cannot be eliminated entirely.</figcaption> </figure> </figure> @@ -371,6 +370,17 @@ The photodiode's response is strongly wavelength-dependent. In particular in the gets very poor down to about 20% at the edge to ultraviolet. This effect is strong enough to move the apparent location of the blue peak towards red. +.. raw:: html + + <figure> + <img src="images/photodiode_sensitivity.svg" alt="A plot of photodiode sensitivity against wavelength relative + to peak sensitivity at 820nm. The sensitivity rises from 20% at 380nm approximately linearly to 80% at 620nm, + then the rise rolls off."> + <figcaption>A plot of the photodiode's relative sensitivity in the visible spectrum. The sensitivity is + normalized against its peak at 820nm. + </figcaption> + </figure> + The problem is that in order to remove this non-linearity, we would already have to know the wavelength of the measured light. Since I don't, I settled for a two-step process. First, a coarse wavelength calibration is done relative to the red peak and the short-wavelength edge of the blue peak. The photodiode measurements are then sensitivity-corrected @@ -390,7 +400,6 @@ wavelength in nanometers. </figcaption> </figure> - .. FIXME re-do these measurements, avoiding clipping .. FIXME re-do calibration using CCFL .. FIXME calibration for brightness imbalance due to wedge-shaped projection of spectrum |