From 7bc656ca2a026d91a845dd4d8bfeb812021cdf4d Mon Sep 17 00:00:00 2001 From: jaseg Date: Sat, 19 May 2018 14:43:28 +0200 Subject: Add multichannel LED driver post --- .../images/photodiode_sensitivity.svg | 874 +++++++++++++++++++++ content/posts/led-characterization/index.rst | 21 +- 2 files changed, 889 insertions(+), 6 deletions(-) create mode 100644 content/posts/led-characterization/images/photodiode_sensitivity.svg (limited to 'content/posts/led-characterization') 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 @@ -0,0 +1,874 @@ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 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:
Strong ringing on the LED voltage waveform edge at about 100% overshoot during about 70% of the cycle time. -
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.
+
The LED strip being at the end of a couple meters of wire caused extremely bad ringing at high + driving frequencies.
Weak ringing on the LED voltage waveform edge at about 30% overshoot during about 20% of the cycle time. -
Adding a resistor dampened the ringing somewhat, but ultimately it cannot be eliminated - entirely.
+
Adding a resistor in front of the MOSFET gate to slow the transition dampened the ringing + somewhat, but ultimately it cannot be eliminated entirely.
@@ -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 + +
+ 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. +
A plot of the photodiode's relative sensitivity in the visible spectrum. The sensitivity is + normalized against its peak at 820nm. +
+
+ 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. - .. 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 -- cgit