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Questions tagged [light-emitting-diodes]

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0answers
59 views

Superluminescent LED [closed]

How do Superluminescent LED generate amplified spontaneous emission? Moreover," a superluminescent LED combines the spatial coherence of a laser diode and the temporal incoherence of an LED. " Please ...
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2answers
18k views

What's the difference between stimulated and spontaneous emission in lasers and diodes?

I know that an LED emits light by spontaneous emission. To get more coherent and Monochromatic light, we should use a laser. What is the reason that laser light is the most coherent and ...
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2answers
609 views

Why photoluminescene spectrum is not identical to electroluminescence spectrum?

Electroluminescence (EL) and photoluminescence (PL) spectra are not identical: there's a small red-shift of the PL spectrum relative to the EL spectrum. Why is this happening if both EL and PL are ...
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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 ...
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1answer
430 views

Why does this illumination pattern of an LED light going through a small pinhole not emulate the point spread function?

I have an optical setup where a component LED passes through a 50 um pinhole, and creates an illumination pattern on an image sensor approximately 5 cm away that looks like the following: Now, I'm ...
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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 ...
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0answers
138 views

Doesn't this LED created by MIT violate the second law of thermodynamics? [duplicate]

Here is the article I am referring to: http://www.wired.co.uk/news/archive/2012-03/09/230-percent-efficient-leds In their own words (or at least the article writer's), "it appears to draw in heat ...
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0answers
53 views

Can you grow more Biomass in a brickhouse only powered by solar cells than in a greenhouse? Will we ever be able to? [closed]

Right now I read that 1m² of solar cells generates roughly 100Watts of energy. There are really efficient Grow LEDs that use only two spikes in the lightspectrum to make plants grow. I have seen some ...
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2answers
957 views

White LEDs and colour perception

I’m very interested in LED lighting and the different spread of wavelengths that are produced compared to other types (like tungsten). Given that a White LED bulb actually produces 3 different peaks ...
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2answers
1k views

Why doesn't silicon emit light? [closed]

Why doesn't silicon re-emit light when light falls on it? Or if it emits light then why we can't see it?Hence it has valence electrons.
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1answer
537 views

What is the underlying physics that makes Li-Fi “100 times faster” than Wi-Fi? [closed]

Consider this webpage, which compares how the two work. Particularly it says (and I'm paraphrasing) that Lifi transmits using light from LED bulbs and WiFi uses radio waves. Well, both are EMF waves, ...
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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 ...
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1answer
126 views

measuring average directional intensity of LED?

Although I am poor at physics and math I want to compare two light sources of which I haven’t decided the specific brand yet. I was thinking of florescence bulbs versus LED lamps. So something like ...
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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 ...
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2answers
4k views

Why fast eye movement splits a white LED into the RGB components?

I have a "white" LED as the notification light on my phone. When I look at it straight it looks whitish, but when I move my eyes around (or shake my head) I suddenly see glimpses of red and green (and ...
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2answers
1k views

Can I use fluorescent light bulbs to calculate Planck's constant?

We did an experiment today to calculate Planck's constant, $h$. We measured the knee voltages $V$ of different LEDs (red, green, orange, blue, violet) and plotted them against the frequencies $f$ of ...
3
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1answer
1k views

Why did LED lit up while soldering? [closed]

I was soldering an LED when accidentally the soldering wire touched the LED's other Pin while the soldering iron was touching the other and the LED lit up, not bright but bright enough to be seen. I ...
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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 ...
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0answers
865 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
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 ...
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0answers
71 views

What causes this pattern in the reflection of a laser on a screen? [duplicate]

When I shine a red laser behind me at a wall I see a strange reflection occurring on my laptop screen Why does this happen? The pattern on my screen occurs exactly where the laser is hitting the wall. ...
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2answers
380 views

How is the backlight in a LCD monitor dispersed equally?

How is the backlight in a LCD monitor dispersed equally? I mean are there any layers that disperse the light?
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1answer
277 views

Diodes in AC current

If in AC current, the direction of the current is always changing, then if one was to place a diode (for simplicities sake lets say an LED) in an AC current, would it flash to the frequency of the ...
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1answer
48 views

Can high-power LEDs remotely control a whole room? [closed]

Let's say I have 4 appliances in rectangular based room of about 60 square meters with white walls. The appliances are using infrared receivers with a filter of 38 kHz. Would it be possible to apply ...
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4answers
7k views

What determines the forward voltage drop for a diode?

I have always had the idea that the forward voltage drop in a semiconductor diode was related in a simple way to the bandgap energies in the semiconductor. However this is apparently not the case: ...
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1answer
1k views

Where does a LED use energy other than emitting light?

I have a quantum formula describing what kind of photon should be emitted by an LED depending on its voltage. Of course the colour is depending on the material, but every type of LED also needs its ...
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2answers
440 views

Do photons emitted from a LED show bunching?

If photons are emitted from a thermal source, we get photon bunching. For coherent radiation, the detection probability doesn't change after detecting a photon. For single photon sources, we get anti-...
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1answer
1k views

Laser diode spectrum vs LED spectrum, why so much difference in broadness?

I've been reading about how light is produced in both laser diodes (the most common types of lasers) and LED's. The mechanism seems very similar if not exactly the same: one applies a potential ...
2
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2answers
472 views

Why does the light instantly disappear when we switch off the source?

For example i take a box which is completely covered by the most perfect mirrors possible inside and inside that box i have a bulb whose bulb holder is also covered with the most perfect mirrors ...
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1answer
3k views

Diffraction Pattern on LCD Screen

I was up late working with my red LED headlamp on and when I was looking at the black part of my LCD (Apple Retina Macbook Pro) screen I noticed this interesting diffraction pattern. I'm confused ...
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0answers
360 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?
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2answers
4k views

Can a UV LED cause air ionization?

I am thinking of the high power LEDs at around 250nm wavelength and an output power of some 30mW optical, in continuous mode either divergent beam or focused.
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2answers
286 views

Is there something equivalent to a diode for light?

In electronics a diode is a component allowing current passing in only one direction, and blocking the other side. I'm wondering if something similar exists for visible light or other EM waves, like ...
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2answers
246 views

Why do we call a white led with high color temperature “cool”?

one can buy LED bulbs with defined color temperature. why cool white = many kelvins (= high temperature?) why warm white = few kelvins (= low temperature?)
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3answers
365 views

Spatial coherence of SLED light (width of active region?)

As far as I know, super-luminescent diodes (a.k.a. SLEDs) have a very similar structure to laser diodes, except that the front faces of the active region are tilted to prevent the sort of reflections ...
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2answers
31k views

Does staring at a bright LED light damage your eyes?

According to this article it seems that it is the UV part of the spectrum from the Sun that causes damage to the eye. Would it therefore be "safe" to observe directly an equivalent energy density LED ...
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1answer
226 views

IR proportion in white LED spectrum

I have read that any kind of artificial light may emit some energy of IR..my question here that how one can make measurements or computation to find the proportion of IR in the spectrum distribution ...
2
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1answer
118 views

Why there's any voltage on P-N junction in diodes?

If all electrons here have their counterparts in protons, on both, P and N sides, then why there's any voltage (built-in voltage, on the bottom of the image)? Both sides should be neutral, shouldn't ...
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2answers
2k views

How to chose right lens for concentrating IR signal? [closed]

I am looking for the right acrylic lens. Since will be buying at least 1000 pieces I don't want to make any mistake. I want to concentrate the signal from IR LED in a 1cm diameter tube at one point ...
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1answer
173 views

Has the Nobel committee mixed up this years prizes for Physics and Chemistry? [closed]

The title of the question is tongue-in-cheek but the question remains: How does the Nobel committee delineate the fields when awarding work which is of such an inter-disciplinary nature. The chemistry ...
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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 ...
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0answers
372 views

Nobel prize for (dangerous) LED lamps? [closed]

There are many scientific articles about the danger of LED lamps (see below from the LED magazine, from scientific american, from a governmental journal and from Harvard health newsletter): Light and ...
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5answers
7k views

What challenges needed to be overcome to create (blue) LEDs?

In light of today's announcement of the 2014 Nobel laureates, and because of a discussion among colleagues about the physical significance of these devices, let me ask: What is the physical ...
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1answer
164 views

How can monitors display a sharp image, if light travels in all direction?

The question title might be a misnomer, help me improve it. My question is, from my understanding, a light travels in all direction from it's origin point. Also I assume that in the example of a ...
1
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3answers
538 views

How to generate Red light from a Blue-Dominated Spectrum [closed]

I have a white LED light source, the spectrum of which looks as follows --- I want to filter out red light from this white LED source. But as you can see the spectrum dwindles in the red zone. And ...
1
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1answer
70 views

Can magnets destroy notification light on phone? [closed]

I'm posting this question here because it's about magnets and electronics. Please tell me if it's better suited to another site! I've been messing around with some reasonably strong magnets around my ...
2
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1answer
867 views

Why does a LED light up in reverse direction at a DC generator worked by a falling object

I have done the following experiment: A DC generator is worked by an object falling and connected to the DC generator via string. The DC generator is coupled to a LED in reverse direction. The ...
2
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3answers
294 views

In what applications are lasers necessary vs. just convenient?

I’m trying to understand, given the wide array of current laser applications: Which applications actually require laser characteristics, as opposed to just use lasers because they are the most ...
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1answer
118 views

Why do the RGB gamut vertexes not intersect the edges of human vision on the CIExy plot?

As I understand, the CIExy graphic maps "greenness", or rather middle-wavelengthness, to the Y axis and "redness", or rather long-wavelengtsness, to the X axis. The trapping used to reduce the 3d ...
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2answers
21k views

What is the best way to collimate light emitted by a LED?

I'm new to this forum. This is half a question, half a challenge. And it's more engineering then physics but I thought I might get good insights from a physics forum. I would like to cure a UV ...