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diff --git a/posts/led-characterization/index.html b/posts/led-characterization/index.html new file mode 100644 index 0000000..c5f210e --- /dev/null +++ b/posts/led-characterization/index.html @@ -0,0 +1,470 @@ +<!DOCTYPE html> +<html lang="en-us"> + <head> + <meta charset="utf-8"> + <meta name="viewport" content="width=device-width, initial-scale=1"> + <title>LED Characterization | blog.jaseg.de</title> + <link rel="stylesheet" href="/css/style.css" /> + <link rel="stylesheet" href="/css/fonts.css" /> + + <header> + + + <link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/atom-one-light.min.css"> + <script src="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/highlight.min.js"></script> + <script>hljs.initHighlightingOnLoad();</script> + <nav> + <ul> + + + <li class="pull-left "> + <a href="https://blog.jaseg.de/">/home/blog.jaseg.de</a> + </li> + + + + + </ul> + </nav> +</header> + + </head> + + <body> + <br/> + +<div class="article-meta"> +<h1><span class="title">LED Characterization</span></h1> + +<h2 class="date">2018/05/02</h2> +<p class="terms"> + + + + + +</p> +</div> + + + +<main> +<div class="document"> + + +<div class="section" id="preface"> +<h2>Preface</h2> +<p>Recently, I have been working on a <a class="reference external" href="https://blog.jaseg.de/posts/wifi-led-driver/">small driver</a> for ambient lighting using 12V LED strips like you can get +inexpensively from China. I wanted to be able to just throw one of these somewhere, stick down some LED tape, hook it up +to a small transformer and be able to control it through Wifi. When I was writing the firmware, I noticed that when +fading between different colors, the colors look <em>all wrong</em>! This observation led me down a rabbit hole of color +perception and LED peculiarities.</p> +<p>The idea of the LED driver was that it can be used either with up to eight single-color LED tapes or, much more +interesting, with up to two RGB or RGBW (red-green-blue-white) LED tapes. For ambient lighting high color resolution was +really important so you could dim it down a lot without flickering. I ended up using the same driver stage I used in the +<a class="reference external" href="https://blog.jaseg.de/posts/multichannel-led-driver/">multichannel LED driver</a> project for its great color resolution and low hardware requirements.</p> +<figure> + <img src="images/rgb_cube.svg" alt="An illustration of the RGB color cube."> + <figcaption>An illustration of the RGB color cube. + <a href="https://commons.wikimedia.org/wiki/File:RGB_color_cube.svg">Picture</a> by + <a href="https://commons.wikimedia.org/wiki/User:Maklaan">Maklaan from Wikimedia Commons</a>, + <a href="https://creativecommons.org/licenses/by-sa/3.0/">CC-BY-SA 3.0</a> + </figcaption> +</figure><p>To make setting colors over Wifi more intuitive I implemented support for HSV colors. RGB is fine for communication +between computers, but I think HSV is easier to work with when manually inputting colors from the command line. RGB is +close to how most monitors, cameras and the human visual apparatus work on a very low level but doesn't match +higher-level human color perception very well. When we describe a color we tend to think in terms of "hue" or +"brightness", and computing a measure of those from RGB values is not easy.</p> +</div> +<div class="section" id="colors-and-color-spaces"> +<h2>Colors and Color Spaces</h2> +<p><a class="reference external" href="https://en.wikipedia.org/wiki/Color_space">Color spaces</a> are a mathematical abstraction of the concept of color. When we say "RGB", most of the time we actually +mean <a class="reference external" href="https://en.wikipedia.org/wiki/SRGB">sRGB</a>, a standardized notion of how to map three numbers labelled "red", "green" and "blue" onto a perceived +color. <a class="reference external" href="https://en.wikipedia.org/wiki/HSL_and_HSV">HSV</a> is an early attempt to more closely align these numbers with our perception. After HSV, a number of other +<em>perceptual</em> color spaces such as <a class="reference external" href="https://en.wikipedia.org/wiki/CIE_1931_color_space">XYZ (CIE 1931)</a> and <a class="reference external" href="https://en.wikipedia.org/wiki/Lab_color_space">CIE Lab/LCh</a> were born, further improving this alignment. In +this mathematical model, mapping a color from one color space into another color space is just a coordinate +transformation.</p> +<figure> + <img src="images/hsv_cylinder.png" alt="An illustration of the HSV color space as a cylinder."> + <figcaption>An illustration of the HSV color space as a cylinder. + <a href="https://commons.wikimedia.org/wiki/File:HSV_color_solid_cylinder.png">Picture</a> by + <a href="https://commons.wikimedia.org/wiki/User:SharkD">SharkD from Wikimedia Commons</a>, + <a href="https://creativecommons.org/licenses/by-sa/3.0/">CC-BY-SA 3.0</a> + </figcaption> +</figure><p>CIE 1931 XYZ is much larger than any other color space, which is why it is a good basis to express other color spaces +in. In XYZ there are many coordinates that are outside of what the human eye can perceive. Below is an illustration of +the sRGB space within XYZ. The wireframe cube is (0,0,0) to (1,1,1) in XYZ. The colorful object in the middle is what +of sRGB fits inside XYZ, and the lines extending out from it indicate the space that can be expressed in sRGB but not in +XYZ. The fat white curve is a projection of the <em>monochromatic spectral locus</em>, that is the curve of points you get in +XYZ for pure visible wavelengths.</p> +<p>As you can see, sRGB is <em>much</em> smaller than XYZ or even the part within the monochromatic locus that we can perceive. In +particular in the blues and greens we loose <em>a lot</em> of colors to sRGB.</p> +<figure> + <video controls loop> + <source src="video/sRGB.mkv" type="video/h264"> + <source src="video/sRGB.webm" type="video/webm"> + Your browser does not support the HTML5 video tag. + </video> + <figcaption>Illustration of the measured sRGB color space within XYZ. The thick, white line is the spectral + locus. + + <a href="video/sRGB.mkv">mkv/h264 download</a> / + <a href="video/sRGB.webm">webm download</a> + </figcaption> +</figure><p>The wrong colors I got when fading between colors were caused by this coordinate transformation being askew. Thinking +over the problem, there are several sources for imperfections:</p> +<ul class="simple"> +<li>The LED driver may not be entirely linear. For most modulations such as PWM the brightness will be linear starting +from a certain value, but there is probably an offset caused by imperfect edges of the LED current. This offset can be +compensated with software calibration. I built a calibration setup for driver linearity in the <a class="reference external" href="https://blog.jaseg.de/posts/multichannel-led-driver/">multichannel LED +driver</a> project. Below are pictures of ringing on the edges of an LED driver's waveform.</li> +<li>The red, green and blue channels of the LEDs used on the LED tape are not matched. This skews the RGB color space. +In practice, the blue channel of my RGB tape to me <em>looks</em> much brighter than the red channel.</li> +<li>The precise colors of the red, green and blue channels of the LEDs are unknown. Though the red channel <em>looks</em> red, it +may be of a slightly different hue compared to the reference red used in <a class="reference external" href="https://en.wikipedia.org/wiki/SRGB">sRGB</a> which would also skew the RGB color +space.</li> +</ul> +<figure> + <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 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 in front of the MOSFET gate to slow the transition dampened the ringing + somewhat, but ultimately it cannot be eliminated entirely.</figcaption> + </figure> +</figure><p>These last two errors are tricky to compensate. What I needed for that was basically a model of the <em>perceived</em> colors +of the LED tape's color channels. A way of doing his is to record the spectra of all color channels and then evaluate +their respective XYZ coordinates. If all three channels are measured in one go with the same setup the relative +magnitudes of the channels in XYZ will be accurate.</p> +<p>To map any color to the LEDs, the color's XYZ coordinates simply have to be mapped onto the linear coordinate system +produced by these three points within XYZ. LEDs are mostly linear in their luminous flux vs. current characteristic so +this model will be adequate. The spectral integrals mapping the channels' measured responses to XYZ need only be +calculated once and their results can be used as scaling factors thereafter.</p> +</div> +<div class="section" id="measuring-the-spectrum"> +<h2>Measuring the spectrum</h2> +<p>In order to compensate for the cheap LED tape's non-ideal performance I had to measure the LED's red, green and blue +channels' spectra. The obvious thing would be to go out and buy a <a class="reference external" href="https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_spectroscopy">spectrograph</a>, or ask someone to borrow theirs. The +former is kind of expensive, and I did not want to wait two weeks for the thing to arrive. The latter I could probably +not do every time I got new LED tape. Thus the only choice was to build my own.</p> +<p>Luckily, building your own spectrometer is really easy. The first thing you need is something that splits incident light +into its constituent wavelengths. In professional devices this is called the <em>`monochromator`_</em>, since it allows extraction +of small color bands from the spectrum. The second thing is some sort of optics that project the incident light onto a +screen behind the monochromator. In professional devices lenses or curved mirrors are used. In a simple homebrew job a +pinhole as you would use in a <a class="reference external" href="https://en.wikipedia.org/wiki/Pinhole_camera">camera obscura</a> does a remarkably nice job.</p> +<p>For the monochromator component several things could be used. A prism would work, but I did not have any. The +alternative is a <a class="reference external" href="https://en.wikipedia.org/wiki/Diffraction_grating">diffraction grating</a>. Professional gratings are quite specialized pieces of equipment and thus +rather expensive. Luckily, there is a common household item that works almost as well: A regular CD or DVD. The +microscopic grooves that are used to record data in a CD or DVD work the same as the grooves in a professional +diffraction grating.</p> +</div> +<div class="section" id="household-spectra"> +<h2>Household spectra</h2> +<p>From this starting point, a few seconds on my favorite search engine yielded an <a class="reference external" href="http://www.candac.ca/candacweb/sites/default/files/BuildaSpectroscope.pdf">article by two researchers from the +National Science Museum in Tokyo</a> providing a nice blueprint for a simple cardboard-and-DVD construction for use in +classrooms. I replicated their device using a DVD and it worked beautifully. Daylight and several types of small LEDs I +had around did show the expected spectra. Small red, yellow, green, and blue LEDs showed narrow spectra, daylight one +continuous broad one, and white LEDs a continuous broad one with a distinct bright spot in the blue part. The +single-color LED spectra are quite narrow since they are determined by the LED's semiconductor's band gap, which is +specific to the semiconductor used and is quite precise. White LEDs are in fact a blue LED chip covered with a so-called +<em>phosphor</em>. This phosphor is not elementary phosphorus but an anorganic compound that absorbs the LED chip's blue light +and re-emits a broader spectrum of more yellow-ish wavelengths instead. The final LED spectrum is a superposition of +both spectra, with some of the original blue light leaking through the phosphor mixing with the broadband yellow +spectrum of the phosphor.</p> +<figure> + <figure class="side-by-side"> + <img src="images/spectrograph_step1_parts.jpg"> + <figcaption>The ingredients. The cup of coffee and Madoka Magica DVD set are essential to the eventual + function of the appartus.</figcaption> + </figure><figure class="side-by-side"> + <img src="images/spectrograph_step2.jpg"> + <figcaption>Step 1: Cut to size and mark down all holes as described in <a + href="http://www.candac.ca/candacweb/sites/default/files/BuildaSpectroscope.pdf">the manual</a></figcaption> + </figure> + <figure class="side-by-side"> + <img src="images/spectrograph_step3.jpg"> + <figcaption>Step 2: Cut out all holes</figcaption> + </figure><figure class="side-by-side"> + <img src="images/spectrograph_step4_complete.jpg"> + <figcaption>The finished result with the back side showing. The viewing window is on the bottom of the other + side.</figcaption> + </figure> +</figure><p>Now that I had a spectrograph, I needed a somewhat predictable way of measuring the spectrum it gave me.</p> +</div> +<div class="section" id="measuring-a-spectrum"> +<h2>Measuring a spectrum</h2> +<p>Pointing a camera at the spectrograph would be the obvious thing to do. This produces pretty images but has one critical +flaw: I wanted to acquire quantitative measurements of brightness across the spectrum. Since I don't have a precise +technical datasheet specifying the spectral response of any of my cameras I can't compare the absolute brightness of +different colors on their pictures. Some other sensor was needed.</p> +<figure> + <img src="images/daylight_spectrum_dvd.jpg"> + <figcaption>The daylight spectrum as seen using a DVD as a grating. + <a href="https://commons.wikimedia.org/wiki/File:SpectresSolaires-DVD.jpg">Picture</a> by + <a href="https://commons.wikimedia.org/wiki/User:Xofc">Xofc from Wikimedia Commons</a>, + <a href="https://creativecommons.org/licenses/by-sa/4.0/">CC-BY-SA 4.0</a> + </figcaption> +</figure><div class="section" id="measuring-light-intensity"> +<h3>Measuring light intensity</h3> +<p>Looking around my lab, I found a bag of <a class="reference external" href="https://dammedia.osram.info/media/resource/hires/osram-dam-2495903/SFH%202701.pdf">SFH2701</a> visible-light photodiodes. Their +datasheet includes their spectral response so I can compensate for that, allowing precise-ish absolute intensity +measurements. Just like LEDs, photodiodes are extremely linear across several orders of magnitude. The datasheet of the +classic <a class="reference external" href="http://www.vishay.com/docs/81521/bpw34.pdf">BPW34</a> photodiode shows that this photodiode's light current is exactly proportional to illuminance over at +least three orders of magnitude. The <a class="reference external" href="https://dammedia.osram.info/media/resource/hires/osram-dam-2495903/SFH%202701.pdf">SFH2701</a> datasheet does not include a similar graph but its performance will be +similar. The <a class="reference external" href="https://dammedia.osram.info/media/resource/hires/osram-dam-2495903/SFH%202701.pdf">SFH2701</a> photodiodes I had at hand were perfect for the job compared to the vintage <a class="reference external" href="http://www.vishay.com/docs/81521/bpw34.pdf">BPW34</a> since their +active sensing area is really small (0.6mm by 0.6mm) compared to the BPW34 (a whopping 3mm by 3mm). If I were to use a +<a class="reference external" href="http://www.vishay.com/docs/81521/bpw34.pdf">BPW34</a> I would have to insert some small apterture in front of it so it does not catch too broad a part of the +spectrum at once. The <a class="reference external" href="https://dammedia.osram.info/media/resource/hires/osram-dam-2495903/SFH%202701.pdf">SFH2701</a> is small enough that if I just point it at the projected spectrum directly I will +already get only a small part of the spectrum inside its 0.6mm active area.</p> +<p>To convert the photodiode's tiny photocurrent into a measurable voltage I built another copy of the <a class="reference external" href="https://en.wikipedia.org/wiki/Transimpedance_amplifier">transimpedance +amplifier</a> circuit I already used in the <a class="reference external" href="https://blog.jaseg.de/posts/multichannel-led-driver/">multichannel LED driver</a>. A <a class="reference external" href="https://en.wikipedia.org/wiki/Transimpedance_amplifier">transimpedance amplifier</a> is an +amplifiert that produces a large voltage from a small current. The weird name comes from the fact that it works kind of +like an amplified resistor (which can be generalized as an <em>impedance</em> electrically). Apply a current to a resistor and +you get a voltage. A transimpedance amplifiert does the same with the difference that its input always stays at 0V, +making it look like an ideal current sink to the connected current source.</p> +<p>Transimpedance amplifiers are common in optoelectronics to convert small photocurrents to voltages. In this instance I +built a very simple circuit with a dampened transimpedance amplifier stage followed by a simple RC filter for noise +rejection and a regular non-inverting amplifier using another op-amp from the same chip to further boost the filtered +transimpedance amplifier output. I put all the passives setting amplifier response (the gain-setting resistors and the +filter resistor and capacitors) on a small removable adapter so I could easily change them if necessary. I put a small +trimpot on the virtual ground both amplifers use as a reference so I could trim that if necessary.</p> +<figure> + <img src="images/preamp_schematic.jpg" alt="A drawing of the photodiode preamplifier's schematic"> + <figcaption>The photodiode preamplifier schematic. Schematic drawn with an unlicensed copy of + DaveCAD.</figcaption> +</figure><p>Following are pictures of the preamplifier board. The connectors on the top-left side are two copies of the analog +signal for the ADC and a small panel meter. The SMA connector is used as the photodiode input since coax cables are +generally low-leakage and have built-in shielding. The circuit is powered via the micro-USB connector and the analog +ground bias voltage can be adjusted using the trimpot.</p> +<p>For easy replacement, all passives setting gain and frequency response are on a small, pluggable carrier PCB made from a +SMD-to-DIP adapter.</p> +<p>Flying-wire construction is just fine for this low-frequency circuit. In a high-speed photodiode preamp, the +transimpedance amplifier circuit would be highly sensitive to stray capacitance, but we're not aiming at high speed +here.</p> +<figure> + <figure class="side-by-side"> + <img src="images/preamp_front.jpg"> + <figcaption>The front side of the preamplifier board.</figcaption> + </figure><figure class="side-by-side"> + <img src="images/preamp_back.jpg"> + <figcaption>The wiring of the photodiode preamp.</figcaption> + </figure> +</figure><p>Given a way to measure intensity what remains missing is a way to scan a single photodiode across the spectrum.</p> +</div> +<div class="section" id="scanning-the-projection"> +<h3>Scanning the projection</h3> +<p>A cheap linear stage can be found in any old CD or DVD drive. These drives use a small linear stage based on a +stepper-driven screw to move the laser unit radially. Removing the laser unit and connecting a leftover stepper driver +module I was left with a small linear stage with about 45 steps per cm without microstepping enabled. The driver I used +was an <a class="reference external" href="https://www.pololu.com/file/0J450/A4988.pdf">A4988</a> module that required at least 8V motor drive voltage. I used a small micro USB-input boost converter +module to generate a stable 10V supply for the motor driver, with the USB's 5V rail used as a logic supply for the motor +driver.</p> +<p>The <a class="reference external" href="https://dammedia.osram.info/media/resource/hires/osram-dam-2495903/SFH%202701.pdf">SFH2701</a> can easily be mounted to the linear stage using a small SMD breakout board glued in place with thin wires +connecting it to the transimpedance amplifier. The DVD drive linear stage is not very strong so it is important that +this wire does not put too much strain on it.</p> +<p>Above the photodiode, I mounted a small piece of paper on the linear stage to be used as a projection screen to align +the linear stage in front of the spectrometer viewing window. A line on the screen paper points to the photodiode die in +parallel to the linear stage allowing precise alignment.</p> +<p>The whole unit with photodiode preamplifier, linear stage, photodiode and stepper motor driver finally looks like this:</p> +<figure> + <img src="images/electronics_whole.jpg" alt="The complete electronics setup of the spectrograph. In the back + there is the DVD drive stepper stage. In front of it, mounted on a piece of wood are a small USB-to-12V + switching-regulator module to power the stepper motor in the top left, below on the bottom left is the + photodiode preamp and on the right is a breadboard with the stepper driver module and lots of jumper wires + interconnecting everything. On the right of the breadboard, a buspirate is attached to interface everything to a + computer. On the bottom edge of the piece of wood, two LED panel meters are mounted for readout of the preamp + output and the stepper supply voltages."> + <figcaption>The complete electronics setup. The buspirate on the right interfaces to a computer and controls the + stepper driver and ADC'es the preamp output. The two panel meters show the preamp output and stepper voltage for + setup.</figcaption> +</figure><p>The projection of the spectrum can be adjusted by moving the light source relative to the entry slot and by moving +around the grating DVD.</p> +</div> +<div class="section" id="the-capture-process"> +<h3>The capture process</h3> +<p>To capture a spectrum, first the light source has to be mounted near the spectrograph's entry slot. The LED tape I +tested I just taped face-down directly into it. Next, the grating DVD has to be adjusted to make sure the spectrum +covers a sensible part of the photodiode's path. Mostly, this boils down to adjusting the photodiode distance and height +to match the vertical extent and wiggling the grating DVD to adjust the projection's horizontal position.</p> +<p>After the optics are set-up, the photodiode preamplifier has to be adjusted. In my experiments, most LED tape at 5GΩ +required a high-ish amplification. The goal in this step is to maximize the peak response of the preamp to be just +shy of its VCC rail to make best use of its dynamic range. To adjust the pre-amp, I took several very coarsely-spaced +measurements to give me an estimate of the peak while I did not yet know its precise location.</p> +<p>Since stray daylight totally swamped out the weak projection of the LED's spectrum I shielded the entire setup with a +small box made of black cardboard and two black t-shirts on top. This shielding proved adequate for all my measurements +but I had to be careful not to accidentially move the DVD that was stuck into the spectrograph with the shielding +t-shirts.</p> +<p>For capturing a single spectrum I wrote a small python script that will automatically move the stepper in adjustable +intervals and take two measurements at each point, one with the LED tape off that can be used for offset calibration and +one with the LED tape on. All measurements are stored in a sqlite database that can then be accesssed from other +scripts.</p> +<p>I built a small script that shows the progress of the current run and an jupyter notebook for data analysis. The jupyter +notebook is capable of live-updating a graph with the in-progress spectrum's data. This was quite useful as a sanity +check for when I made some mistake easy to spot in the resulting data.</p> +<p>After one color channel is captured, the LED tape has to be manually set to the next color and the next measurement can +begin.</p> +<figure> + <img src="images/raw_plot_cheap_rgb.svg" alt="A plot with three wide peaks, two large peaks on both sides and + one smaller one in the middle. The middle one overlaps the two on the sides. The large ones are about 2.5V in + amplitude. Overall, the plot is about 300 stepper steps wide with each peak being around 130 steps wide."> + <figcaption>A plot of the raw preamp output voltage versus stepper position. From left to right, the three peaks + are blue, green and red. Step 0 corresponds to the bottommost stepper position and the shortest wavelength. + </figcaption> +</figure></div> +<div class="section" id="data-analysis"> +<h3>Data analysis</h3> +<p>Data analysis consists of three major steps: Offset- and stray light removal, wavelength and amplitude calibration and +color space mapping.</p> +<div class="section" id="offset-removal"> +<h4>Offset removal</h4> +<p>The first task is to remove the offset caused by dark current as well as stray light of the LED's bright primary +reflection on the DVD. The LED is very bright and only a small part of its light gets reflected by the grating towards +the photodiode screen. The remaining part of the light is reflected onto the table in front of the DVD spectrograph. +Though I covered all of this with black cardboard, some of that light ultimately gets reflected onto the photodiode. +This causes a large offset, in particular in the blue part of the spectrum since in this part the photodiode is closest +to the spectrograph's opening.</p> +<p>The composite offset can be approximated with a second-order polynomial that is fitted to all the data outside of the +main peak's area. Since at this point the wavelength of each data point is still unknown this is done with a rough first +estimate of the three colors' peaks' locations and widths.</p> +</div> +<div class="section" id="wavelength-and-amplitude-calibration"> +<h4>Wavelength- and amplitude calibration</h4> +<p>The photodiode's response is strongly wavelength-dependent. In particular in the blue band, the photodiode's sensitivity +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.</p> +<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><p>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 +using this coarse measurement. Then all three channel peaks are measured in the resulting data and a fine wavelength +estimate is produced by a least-squares fit of a linear function. This fine estimate is then used for a second +sensitivity correction of all original measurements and the scale is changed from stepper motor step count to +wavelength in nanometers.</p> +<figure> + <img src="images/processed_plot_cheap_rgb.svg" alt="A plot with three wide peaks, all three of different + heights. The leftmost peak is highest at 6nA, the middle peak lowest at 1.6nA and the rightmost peak in between + at 4nA. The middle one overlaps the two on the sides. Overall, the plot spans about 300nm on its x axis with + each peak being around 100nm wide."> + <figcaption>A plot of the processed measurements. From left to right, the three peaks are blue, green and red. + </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 --> +</div> +<div class="section" id="color-space-mapping"> +<h4>Color space mapping</h4> +<p>Finally, to achieve the objective of measuring the LED tape's channels' precise color coordinates the measured spetra +have to be matched against the color spaces' <em>color matching functions</em>. The color matching functions describe how +strong the color space's idealized <em>standard observer</em> would react to light at a particular wavelength. Going from a +measured spectrum to color coordinates XYZ works by integrating over the product of the measurement and each color +coordinate's color matching function.</p> +<p>The result are three color coordinates X, Y and Z for each channel R, G and B yielding nine coordinates in total. When +written as a matrix conversion between XYZ color space and LED-RGB color space is as simple as multiplying that matrix +(or its inverse) and a vector from one of the color spaces.</p> +<p>In XYZ space, the set of colors that can be produced with this LED tape is described by the <a class="reference external" href="https://en.wikipedia.org/wiki/Parallelepiped">parallelepiped</a> spanned by +the three channel's XYZ vectors. In the following figures, you can see a three-dimensional model of the RGB LED's color +space (colorful) as well as sRGB (white) for comparison plotted within CIE 1931 XYZ. There is no natural map to scale +both so for this illustration the LED color space has been scaled to fit. These figures were made with blender and a few +lines of python. The blender project file including all settings and the python script to generate the color space +models can be found in the <a class="reference external" href="https://github.com/jaseg/led_drv">project repo</a>.</p> +<figure> + <video controls loop> + <source src="video/led_within_srgb_scale=1.0.mkv" type="video/h264"> + <source src="video/led_within_srgb_scale=1.0.webm" type="video/webm"> + Your browser does not support the HTML5 video tag. + </video> + <figcaption>Illustration of the measured LED color space scaled to fit within XYZ with sRGB (light gray) for + comparison. The thick, white line is the spectral locus. + + <a href="video/led_within_srgb_scale=1.0.mkv">mkv/h264 download</a> / + <a href="video/led_within_srgb_scale=1.0.webm">webm download</a> + </figcaption> +</figure><p>As you can see, the result is pretty disappointing. The LED's color space parallepiped is very narrow, which is because +the blue channel is much brighter than the other two channels. An easy fix for this is to scale-up the RGB space and +drop any values outside XYZ. The scaling factor is a trade-off between color space coverage and brightness. You can +produce the most colors when you clip all channels to brightness of the weakest channel (green in this case), but that +will make the result very dim. Scaling brightness like that stretches the RGB parallelepiped along its major axis. Up to +a point the number of possible colors (the gamut) increases at expense of maximum brightness. When the parallelepiped is +stretched far enought for all three channel vectors to be outside the 1,1,1 XYZ-cube, maximum brightness continues to +decrease but the gamut stays constant. I don't know a simple scientific way to solve this problem, so I just played +around with a couple of factors and settled on 2.5 as a reasonable compromise. Below is an illustration.</p> +<figure> + <video controls loop> + <source src="video/led_within_srgb_fancy_camera_path_scale=2.5.mkv" type="video/h264"> + <source src="video/led_within_srgb_fancy_camera_path_scale=2.5.webm" type="video/webm"> + Your browser does not support the HTML5 video tag. + </video> + <figcaption>Illustration of the measured LED color space at scale factor 2.5 within XYZ with sRGB (light gray) + for comparison. The thick, white line is the spectral locus. + + <a href="video/led_within_srgb_fancy_camera_path_scale=2.5.mkv">mkv/h264 download</a> / + <a href="video/led_within_srgb_fancy_camera_path_scale=2.5.webm">webm download</a> + </figcaption> +</figure></div> +</div> +</div> +<div class="section" id="firmware-implementation"> +<h2>Firmware implementation</h2> +<p>In the end, the above measurements yield two matrices: One for mapping XYZ to RGB, and one for mapping RGB to XYZ. Of +the several versions of CIE XYZ I chose the CIE 1931 XYZ color space as a basis for the firmware because it is most +popular. Mapping a color coordinate in one color space to the other is as simple as performing nine floating-point +multiplications and six additions. Mapping Lab or Lch to RGB is done by first mapping Lab/Lch to XYZ, then XYZ to RGB. +Lab to XYZ is somewhat complex since it requires a floating-point power for gamma correction, but any self-respecting +libc will have one of those so this is still no problem. Lch also requires floating-point sine and cosine functions, but +these should still be no problem on most hardware.</p> +<p>My implementation of these conversions in the ESP8266 firmware of my <a class="reference external" href="https://blog.jaseg.de/posts/wifi-led-driver/">Wifi LED driver</a> can be found <a class="reference external" href="https://github.com/jaseg/esp_led_drv/blob/master/user/led_controller.c">on Github</a>. You +can view the Jupyter notebook most of the analysis above <a class="reference external" href="http://nbviewer.jupyter.org/github/jaseg/led_drv/blob/master/doc/Spectrum%20Measurement.ipynb">here</a>.</p> +</div> +</div> +</main> + + <footer> + +<script> +(function() { + function center_el(tagName) { + var tags = document.getElementsByTagName(tagName), i, tag; + for (i = 0; i < tags.length; i++) { + tag = tags[i]; + var parent = tag.parentElement; + + if (parent.childNodes.length === 1) { + + if (parent.nodeName === 'A') { + parent = parent.parentElement; + if (parent.childNodes.length != 1) continue; + } + if (parent.nodeName === 'P') parent.style.textAlign = 'center'; + } + } + } + var tagNames = ['img', 'embed', 'object']; + for (var i = 0; i < tagNames.length; i++) { + center_el(tagNames[i]); + } +})(); +</script> + + + <div id="license-info"> + ©2020 by Jan Götte. 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