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authorjaseg <git@jaseg.net>2018-05-19 14:43:28 +0200
committerjaseg <git@jaseg.net>2018-05-19 14:43:47 +0200
commit7bc656ca2a026d91a845dd4d8bfeb812021cdf4d (patch)
tree4f1896fe65afc9079e766602184cf0126d29913a /content/posts/led-characterization
parent24ad9ee7a0a8095d56c330a39321f40dee020dcb (diff)
downloadblog-7bc656ca2a026d91a845dd4d8bfeb812021cdf4d.tar.gz
blog-7bc656ca2a026d91a845dd4d8bfeb812021cdf4d.tar.bz2
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Add multichannel LED driver post
Diffstat (limited to 'content/posts/led-characterization')
-rw-r--r--content/posts/led-characterization/images/photodiode_sensitivity.svg874
-rw-r--r--content/posts/led-characterization/index.rst21
2 files changed, 889 insertions, 6 deletions
diff --git a/content/posts/led-characterization/images/photodiode_sensitivity.svg b/content/posts/led-characterization/images/photodiode_sensitivity.svg
new file mode 100644
index 0000000..e845444
--- /dev/null
+++ b/content/posts/led-characterization/images/photodiode_sensitivity.svg
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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