Questions tagged [light-emitting-diodes]

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65
votes
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 ...
21
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4answers
3k views

LED conversion efficiency exceed 100%

I have read this article, Which says that conversion efficiency of the LED have exceeded 100%. The results are published in Physical Review Letters In their experiments, the researchers reduced the ...
18
votes
2answers
5k 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 ...
17
votes
4answers
4k views

Why can't single LEDs produce white light directly?

Why does production of white light using a LED require combining a short wavelength LED such as blue or UV, and a yellow phosphor coating? Why can't a single LED produce pure white light?
15
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6answers
2k views

Can someone explain the science behind MIT's 230% efficient LEDs?

I was reading Gizmodo the other day and I didn't quite understand the Physics behind this. Could anybody shed some light on how this effect actually works?
13
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3answers
5k views

LED Thermal Modeling (How to solve heat equation with constant heat source)

I have a mechanical design with LEDs that generate heat. I want to estimate the temperature at the LED junction vs. time, but especially at steady state. Knowing the LED voltage drop and current, I ...
12
votes
3answers
373 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 ...
11
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8answers
20k views

Why is a 1mW laser dangerous?

In our Physics lab we have a 1 milliwatt (0.001W) helium neon laser. Despite the low power, we were cautioned not to even look at reflections of the beam as it could cause permanent eye damage - why ...
11
votes
1answer
393 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 ~...
10
votes
2answers
891 views

Light reflecting off rain from bike light

The other day when I was cycling back from work in the rain I noticed an effect from the light on my bike. A single raindrop, as it passed the light, appeared as multiple dots or dashed that followed ...
10
votes
1answer
912 views

What's the origin of the four rays that come out of the reflection of candlelight in our TV?

The reflection of a candlelight in our TV shows four emerging rays. The rays vary spatially and periodically and have a repeating pattern of the colors of the rainbow. The further the flame is from ...
9
votes
2answers
913 views

Could anyone help explain this current voltage graph for an LED in liquid nitrogen?

I've been doing my coursework investigating LEDs at various temperatures and I've come across an interesting phenomenon which nobody I've asked has been able to explain thoroughly - wheras at room ...
8
votes
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 ...
7
votes
1answer
175 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 ...
6
votes
1answer
5k views

How to determine divergence of a LED source from a single biconvex lens

I'm trying to determine the divergence angle of light from a single lens that is completely illuminated by a high power LED. Most optics textbooks only deal with imaging optics and I'm having a hard ...
6
votes
2answers
22k 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 ...
6
votes
2answers
1k 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 ...
5
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4answers
9k views

Can light emitting diodes be used to generate UV wavelengths?

I don't know much about light-emitting diodes, but I imaging if you had a panel of RGB diodes you could produce any wavelength of color within the visible light spectrum. However, if I also wanted to ...
5
votes
1answer
717 views

LED heat radiation

Consider two bodies in space exchanging heat solely by radiation. Let's call them a star and a planet. If the star contains a heat source and the planet doesn't, it can be shown that temperature of ...
5
votes
1answer
4k 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 ...
5
votes
2answers
297 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 ...
5
votes
2answers
109 views

Opposing Laser Beams Turns One Laser Off

When I send two separate laser beams directly into each other one of the beams goes completely out. These are inexpensive 5mv red diode lasers. What’s happening inside the laser to make it “turn off”? ...
4
votes
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.
4
votes
1answer
120 views

Placement of an LED in a circuit

When an LED is connected in a circuit it needs a resistor so that excess current does not pass through it, but when a resistor is placed after the LED, it works the same as when the resistor is placed ...
4
votes
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: ...
4
votes
1answer
47 views

Colour Temperature and the thermal temperature [duplicate]

I understand that Colour Temperature correlates with the object's thermal temperature, such as a red-hot piece of iron being ~360c and the incandescent bulb filament is about 2500c. Why are sources ...
4
votes
1answer
356 views

How to make a green LED as visually bright as a 0 magnitude star?

I'm trying to estimate the distance and power I'd need for a green LED to appear visually roughy as bright as a relatively bright star - say a visual magnitude of zero. Here is what I have so far. Be ...
4
votes
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 ...
4
votes
1answer
111 views

If the energy of photons at the bottom of the visible spectrum (~720 nm) is 1.72 eV, why does a red LED light up at only 1.48 V?

I have a red LED (623 nm peak wavelength) which I am able to light up at 1.48 V. I thought the switch-on voltage should be determined by $hc/(e\lambda)$, but that would give 1.99 V for 623 nm. Even ...
3
votes
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 ...
3
votes
3answers
201 views

Difference in spectrum of green laser and green LED

In an experiment I conducted I used a spectrometer to find the spectrum of green laser and green led. this is what I found: LED spectrum: Laser spectrum: why is the spectral width of the LED is ...
3
votes
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 ...
3
votes
1answer
4k views

How does infrared light behave through water?

I have a project involving the detection of the presence of water in a cylindrical transparent plastic container (diameter=25cm) And I cannot put any device inside, so I was thinking of using and IR ...
3
votes
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
votes
2answers
194 views

Can zinc nitride be used as light emitting material?

I cannot manage to find any journal papers about the applicability of zinc nitride as active layer of an light emitting diode (LED). But certain papers got mention that zinc nitride with a direct ...
3
votes
2answers
978 views

What is the mechanism behind the halo around LED lights?

Was at a musical event last night, outdoors in what might be called a "beer garden", and I noticed that many of the lights in the area were surrounded by a sort of "aura". The more distant lights (...
3
votes
1answer
2k views

Is a 3 LED flashlight brighter than a single LED one? (same LEDs, same power through each LED)

Say I have one LED in a flashlight, it produces some light. Now, if I have 3 of those LEDs in the same flashlight, each LED receiving the same amount of energy than the single LED was receiving in ...
3
votes
0answers
247 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
374 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 ...
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
votes
6answers
554 views

Why does the LED turn off if the high current is going through the short?

None of the other questions related to short circuits seems to answer my question. I understand how short circuits work theoretically, that if there is a path with no resistance in a circuit then all ...
2
votes
3answers
429 views

How to smooth the spectrum of a light source?

Could somebody please tell me if there's a reasonably cheap substance or device can I use to smooth the spectrum of a light source? For example, if the spectrum has spikes as in the blue graph below, ...
2
votes
2answers
476 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 ...
2
votes
2answers
247 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?)
2
votes
3answers
297 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 ...
2
votes
2answers
188 views

Why can blue LEDs be used for generating white light, but red LEDs cannot

LEDs consist of pn-junctions, so why can blue LEDs be used for generating white light, but red LEDs cannot
2
votes
2answers
21k 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 ...
2
votes
1answer
125 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 ...
2
votes
1answer
134 views

Finding the Injection Efficiency and IV Characteristics of an LED

I have taken IV measurements for several LEDs and now need to calculate their injection efficiencies. I understand that the injection efficiency: $\eta_{pn} = I_{D}/I$ Where $I$ is the total ...
2
votes
1answer
148 views

Circular polarizer rotation

If I use a circular polarizer to a uniformly illuminated white LED light and rotate it I observe a change in intensity accross the wavelengths. Especially towards the blue side. Its almost to the ...