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Questions tagged [light-emitting-diodes]

49 questions with no upvoted or accepted answers
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11
votes
1answer
351 views

Ruby fluorescence activation spectra

You might have seen the absorption spectrum of ruby: it has distinct absorption peaks near 400nm and 550nm. Looking at its absorption graph, one could expect some fluorescence when excited from a ~...
3
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0answers
223 views

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 ...
3
votes
0answers
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 ...
2
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0answers
22 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 ...
2
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0answers
298 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 ...
2
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0answers
99 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
539 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 ...
2
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0answers
99 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
310 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 ...
2
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2answers
117 views

Why is my measured LED activation voltage lower than theoretical?

I have done an experiment to measure Planck's constant. I measured the activation voltage of the LED for each wavelength used. I calculated theoretical activation voltage results using the energy of ...
1
vote
1answer
21 views

Diode Diffusion Capacitance

I'm confused on the topic of diffusion capacitance in a diode. In the book, Solid State Devices by Streetman and Banerjee, I see an equation for the diffusion capacitance which gives an exponential ...
1
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0answers
11 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 ...
1
vote
1answer
42 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 ...
1
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0answers
15 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 ...
1
vote
1answer
45 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 ...
1
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0answers
19 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 ...
1
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0answers
59 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 ...
1
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0answers
59 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 ...
1
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0answers
37 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 ...
1
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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?
1
vote
1answer
202 views

How can a photo diode turn its current on and off in nano seconds?

I know that in a photo diode current is carried by minority charge carriers whose concentration depends upon the incident light's intensity. I wonder if incident light has once created electron hole ...
1
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0answers
105 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 ...
1
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0answers
34 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 ...
1
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0answers
31 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 ...
1
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0answers
864 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 ...
1
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0answers
62 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 ...
0
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0answers
79 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 ...
0
votes
0answers
29 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 ...
0
votes
1answer
25 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 ...
0
votes
2answers
14 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 ...
0
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0answers
21 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 ...
0
votes
1answer
52 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 ...
0
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0answers
24 views

(Color Temperature) How to calculate combined color temperature (kelvin) of two different sources

I have been looking for a while but I can't seem to find any information on this. I would like to figure out the formula to calculate the CCT of two calibrated sources of light relative to their ...
0
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4answers
74 views

With the same power output, which color of light give the most intensity?

Imagine we have a power source produce constants power in Watt. If I placed blue LED, then red LED, which color gives more intensity and wht color gives the most?
0
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2answers
63 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 ...
0
votes
1answer
380 views

How to find out the wavelength for the blue led diode?

I need your help as I am stuck and I dont know where to start. I have got four blue leds diode that come with dark blue led lights that I really want to get the same led colours for my 3 fans, but I ...
0
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0answers
24 views

SiO2 layer in diodes

Can somebody please explain the purpose of this SiO2-layer? Thank you!
0
votes
1answer
43 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 ...
0
votes
1answer
64 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 ...
0
votes
2answers
32 views

Why The Phase of Electic field of a wave is defined like this?

$$ E_x = E_0 \cos(\omega t - k z + \phi_0) $$ $$ \phi = \omega t - k z + \phi_0 $$ Why do we need $kz$ in this equation?
0
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1answer
64 views

Ohm's law; LED Circuit; measuring current using a multimeter (ammeter)

I have a $9V$ power supply, connected to a $2.37k \Omega$ resistor followed by a LED to the ground. I want to use Ohm's law to predict my current. So $\frac{9V}{2.37k \Omega} = 3.79 mA$. When I ...
0
votes
2answers
416 views

Measuring output optical power of an LED

How do I measure the output optical power of an LED? I have a detector that measures output power of a laser incident on a small opening. The problem with using the LED with the same detector is that ...
0
votes
1answer
126 views

LED tubes (fluorescent replacements) refresh rate

I replaced fluorescent lights in our kitchen with LEDs. I can see an occasional flicker with the LEDs. Is the flicker caused by the way the bulb(s) was made or the AC input to the bulb? I haven't been ...
0
votes
1answer
154 views

Matching LED Brightness Using PWM

I am currently working on a project that involves the use of LEDs. For this project, I have to match the brightness of an LED (receiving constant current) using another LED with PWM (the LEDs are ...
0
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0answers
343 views

What is “unseen blue light” from LED lighting?

In the Washington Post story Some cities are taking another look at LED lighting after AMA warning I noticed the phrase "unseen blue light" in the first full paragraph: The American Medical ...
0
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0answers
56 views

Interpreting LED output distribution specification

I have specifications for an LED which gives the following graph of spacial output distribution. I am not sure how to interpret this. As I see it, it could be one of two ways. To make explanations ...
0
votes
1answer
92 views

Direct bandgap semiconductor reaction to heat. Produce light?

In an indirect bandgap semiconductor if you apply heat the number of free electrons increases, conductivity increases, and the bandgap decreases. In a direct bandgap semiconductor is there any ...
0
votes
1answer
32 views

How to configure two infrared light to produce two different intensity

I have two infrared illuminators that have the same properties (same wavelength, same number of leds, same angles, same max voltage and amperage). suppose that these illuminators are attached at the ...
0
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0answers
359 views

How far away are we from a UV LED operating at around 100nm?

So far, we are down to around 250nm for UV LEDs. If I could get one at around 100nm that would ionize air I would find it very useful. Any ideas on whether this wavelength is feasible for a LED?