Questions tagged [light-emitting-diodes]

60 questions with no upvoted or accepted answers
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How to interpret the UPS spectrum of quantum dots?

I have been using the ultraviolet photoemission spectroscopy (UPS) to look at the energy levels of quantum dots. My quantum dot has a core/shell structure, where the shell is supposed to have a larger ...
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1k views

Increase in Wavelength of an LED immersed in liquid nitrogen

"Hi, I've been working on an experiment where I dip LEDs into liquid nitrogen and note the color change. All online sources I've seen predict that the wavelength of the light emitted by the LED will ...
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23 views

Calculating required amount of lumen for camera sensor satuation

I am stuck on this problem at work - I hope some of you can help me further. I work with underwater vision and am trying to find way or methodology to get a somewhat estimate of the required light ...
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0answers
364 views

Why is a white LED color spectrum more continuous than CFL spectra?

As far as I understand, white LEDs and CFLs both use phosphor mixtures to convert part of their primary emission spectrum to longer wavelengths. What puzzles me is the difference in the resulting ...
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0answers
106 views

Does the intensity of LED lights decrease after turning on?

LED lights come on immediately the power is supplied. This is different to the older compact fluorescent-tube lights. I am always impressed by the brightness of the LED lights when they come on. Later ...
2
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0answers
652 views

Is it possible to use a tapered fiber to collimate a single-mode light beam?

I would like to collimate a light beam stemming from a single-mode LED source. As the source is pretty big (a few millimeters), collimation with a single lens gives a bad result, i.e. a quite ...
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101 views

Interpretation of the diode constant in a LED

For a real diode the current as function of the potential difference between its terminals (for big enough voltage) is given by: $$I=A e^{\frac{eV-E}{\eta k_B T}} \tag{1}$$ So in the case of LEDs ...
2
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0answers
358 views

Formulas for Haitz's law?

The Nobel Prize in Physics 2014 is for LED. There is a related Haitz's law: ...every decade, the cost per lumen (unit of useful light emitted) falls by a factor of 10, and the amount of light ...
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18 views

Lab Construction of LED

If given a semiconductor with known bandgap, how might someone construct an experimental LED within a lab? It seems common to prepare solar cells using FTO glass plates with a paste of semiconductor ...
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15 views

Is the current in reverse biased PN junction due to drift or diffusion?

Wikipedia says :https://en.m.wikipedia.org/wiki/Saturation_current The saturation current (or scale current), more accurately the reverse saturation current, is that part of the reverse current in a ...
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0answers
13 views

How to calculate LED light output

I'm trying to assemble an array of LEDs to be configured as a solar simulator. I need to compare this array's output to the solar spectrum as a function of W/m^2/nm. To do this I need to go through ...
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0answers
30 views

Shockley diode equation - temperature influence

when looking at Shockleys ideal diode equation I wonder what "meaning" the temperature has in there? If I assume $$I(T) = I_0 \left(\exp \left( \frac{U}{k_BT/q}\right)-1 \right)$$ and vary ...
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1answer
14 views

How can I find the external quantum efficiency of an LED?

I have a setup where I can measure the optical power, in watts, the current, the voltage and the spectrum of an LED. Is it possible from this to calculate the external quantum efficiency? I realise I ...
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18 views

Model radiated power from an LED

For a range of LEDs I'd like to model their performance and predict the flux density across an area using manufacturers datasheets. Eventually I'd like to take various power measurements and calculate ...
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35 views

Semiconductor structure confinement modes

Today, I would like to simulate the field intensity in an emitting structure such as a laser diode. With this field, I will be able to calculate the confinement factor of my device. I put an ...
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0answers
39 views

Calculate irradiation from a LED lamp

I am working with cell culture and I want to irradiate a 96-well plate with a single color led lamp (let's say the peak is at 450nm). It's just a test, so a rough estimation would do. I've been ...
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0answers
14 views

Calculating mix ratio of primaries for given chromacity

I'm trying to calculate the intensities of a Red, Green and Blue, such that they mix into a (white) color. I'm given the wavelengths of the three LED's and their luminous intensity for a given drive ...
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1answer
102 views

Calculate Retinal irradiance (W/cm² to log photons/cm²/s)

I have been trying to calculate what retinal irradiance value I get with a 1 blue LED system. Since the manufacturer didn´t give the spectral distribution information, I will approximate LED as a ...
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18 views

Optimal paint for UV fluorescence?

If we assume that a light source with 365 nm is the optimal source for how strongly we perceive UV light when it hits a UV fluorescent surface, what would be the optimal colour to paint such a surface ...
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1answer
46 views

What is the simplest way to get the light of 50 visible LEDs into a multimode glass fiber?

LEDs are cheap. How can I get all their light into a fiber? Even if it is only 1% from each LED, it would be useful. Any proposal is welcome. I read about fiber combiners, but they are not helpful. I ...
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34 views

What is the fastest way to photodissociate/bleach melanin - with lasers or LEDs - without heat?

Melanin is the chemical that makes our skin dark. Melanin absorbs in very well in the UV, but when one does so, the solution/molecule gets very hot. Is there a way to photodissociate melanin without ...
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0answers
64 views

What causes observed color differences in white LED's for near and far fields?

While running towards traffic early morning before sunrise I recently noticed that the new automobile LED (white) lights when viewed at a distance appear blueish, but when the light is viewed much ...
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92 views

Intuitive explanation: how does a LED work?

I'm looking for an intuitive explanation of how a LED works. I have a bachelor level understanding of math (I did take some physics courses as well, but nothing too advanced). I'll explain what I know ...
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40 views

Object that is visible only at certain light intensity

I have a lux meter, by which we can measure light intensity. We have one box, in which we inject the light from a hole. What we want is, if light intensity for e.g. Lux value is between 400 to 450 ...
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0answers
60 views

What restricts the size of LED?

Diameter of crystal can be several cm, why isn't it possible to make LED with larger emitting area? I've also read that emitting area of single OLED can be pretty large, but why?
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110 views

LED uniform spread of light

I am using an LED in my project. I need to provide uniform intensity or uniform spread of light using this LED in a particular area(Small region). Please tell some techniques for this. I have read ...
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0answers
38 views

why are triplet excitons more tightly bound than singlet?

In organic light emitting diodes, the Jablonski diagrams always show he T1 states to be lower in energy to the S1 states. They are also declared to be more tightly bound as a result. I don't ...
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0answers
37 views

What size aluminum foil capacitor would power an LED?

Inspired by this video where a $9 \mu F$ capacitor is made from aluminum foil, if I had students put this together in a classroom, one question I would have is what would it take to have this power an ...
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0answers
974 views

Variable wavelength LED

Does there exist a type of LED (or other type of light source) that can change it's wavelength depending on the current/voltage it receives on a second wire? If not, would something like this even be ...
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0answers
64 views

Where does the reverse current come from in a organic semiconducting diode?

Unlike traditional inorganic diodes, organic semiconducting materials hold few thermally exited free carriers under room temperature, so the reverse current should be small if pinholes of the films is ...
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9 views

Collimating 3 LED light sources into one point

I am creating a bespoke piece of lighting which requires modifying an existing piece of equipment Currently the equipment is lit up by a single SMD white led chip, however I am going to be replacing ...
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1answer
72 views

Should I be concerned about UV light from my IKEA LED lamp?

I have an LED lamp from IKEA very similar to "JANSJÖ" (see https://www.ikea.com/ca/en/p/jansjoe-led-usb-lamp-black-70291232/). One day, I noticed that white paper illuminated by the lamp ...
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1answer
9 views

What does the turn on voltage for LEDs depends on except for the bandgap of the material?

I am trying to find out why my LEDs show a different turn on voltage. The material emitting is the same (therefore same bandgap). The stack I have is ITO/Polyvynilcarbazole (hole injection layer)/...
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19 views

How to design a collimated (no hot-spot) RGB light source?

I'm an electrical engineer tinkering with a tiny display that has an LED back light. It looks like a little square and it quickly cycles through red, green, blue light to produce the final display. I ...
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0answers
15 views

Relationship between Optical Power output and luminous intensity

I have an LED with the following specification which I'm using for oxygen saturation measurements. Here the optical output power is given as >1.2mW for RED LED and >0.5mW for IR LED. How to ...
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1answer
18 views

Is an LED's voltage response to current non-linear with temperature?

I have been measuring the voltage response to current of an LED. It's a point source LED with an oval emission area of 40x150um (link to datasheet https://www.daido.co.jp/en/products/point_source_led/...
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0answers
16 views

How to calculate lux value?

I tried calculating lux value for my LED by using the formula the (luminous flux / area), i obtained viewing angle and luminous flux value from LED datasheet, then with the angle i calculated the area ...
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16 views

How can I calculate the UV LED dose having few parameters such as wavelength and output power of LED?

i have UVC led with 270nm wavelength and output power of 70mW. The total length of the passage for the de-contamination is 40cm and the total time is 60 seconds. Is it possible to calculate the UV ...
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1answer
24 views

Reverse biased semiconductor diode: Name of the current due to which the depletion layer is getting widened?

I know, that in a reversed biased connection the reverse saturation current flowing is negligible but before the depletion layer is widened to its maximum capacity. It seems the current flowing ...
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37 views

How much R and B spectrum changes over time for LED?

I am trying to see if there is any data on the R and B spectrum change over time for a LED. For example after 5000 hrs of operation, there are few light which gets dull, which is mostly that the ...
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0answers
173 views

Why is silicon absorbing visible light?

Here it is shown that silicon absorbs visible light: but the band gap energy for direct transitions is 3.6 eV which does not match to this absorption coefficient. Instead, the band gap energy for an ...
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0answers
31 views

How many photons will a scintillation crystal emit?

The question is already in the title: How many photons will an excited scintillator emit? It is probably mainly given by the energy of the impinging particle and the bandgap energy. But also probably ...
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1answer
162 views

Change the color of a LED Display (Make it appear different)

I want to know if it is possible to make a red L.E.D. display appear a different color, at least theoretically, but ultimately for a D.I.Y project. Based on answers to this question (How do color ...
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2answers
25 views

Optics - Polarization's Effect in a Blazed Reflective Diffraction Grating

I'm doing an undergraduate research project, part of which requires me to build an extended cavity diode laser (ECDL). The new diode mounts take receive the diode in a 90 degree rotation of the old ...
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0answers
26 views

Is activation voltage the sum of voltage required to collapse depletion layer and band gap energy?

Is the energy released when an electron moves from the valence to conductance band states the activation voltage or is activation voltage the energy required for electrons to overcome the depletion ...
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1answer
57 views

How do electrons exit LEDs? Aren't they in the valence energy state?

If a conducting electron reduces to the valence band in an LED, where does it get the energy to go back to the conductance band upon leaving the diode so current can flow? I'm confused as to how ...
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2answers
106 views

Is this possible to focus common light (not laser) to a small point in the wall and control it with mirrors?

I want to make something like a laser projector for an experiment but with common light. My question is: is this possible to focus a small point of common light to the wall and control it with ...
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0answers
26 views

SiO2 layer in diodes

Can somebody please explain the purpose of this SiO2-layer? Thank you!
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1answer
56 views

Why does the light from high-power LED source look bigger in the image plane?

Maybe my question is not clear enough so I will post a picture I found for simplicity: https://upload.wikimedia.org/wikipedia/commons/8/8d/Ronja_beam_Prostejov.jpg In the document it is explained ...
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1answer
65 views

Why is LED lifetime limited? Why do they degenerate?

As I know LED is based on electrons and holes recombining in a piece of semiconductor. Emitted heat is very low. So why aren't LED's almost eternal? Why do they degenerate with time? My guess is that ...