Questions tagged [light-emitting-diodes]

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1answer
47 views

Depletion layer of solar cell? [closed]

What happens to depletion layer of solar cells when light is falling on it? Please reply as soon as possible. Thank you
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2answers
379 views

Difference between diode and LED

What is the difference between a diode and a led? I know a led is a diode and I know how they are used and so on but I'm not really sure why a regular is not also light emitting. Therefore, what is ...
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1answer
85 views

Breadboard LED circuit

Please, just a quick question, am I right that the reason why LED is not lighting is: LED is connected parallel to the wire The wire has much lower resistance, so nearly all current is flowing ...
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3answers
314 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
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1answer
43 views

Missing link re: how does kelvin relate to temperature when talking about color temperature?

I am an electrician at a commercial electrical company. There is an in-house training program that posits the datum that when talking about the designations of color temperature for lights (for ...
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3answers
290 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 ...
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2answers
52 views

Blue LED light source

is it possible to change the color of a blue LED light source? Can the blue be filtered out to make white or some other color?
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0answers
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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4answers
369 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?
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2answers
96 views

Need of LED Modulation [closed]

I have an experiment of calculating the velocity of light by using phase difference produced (link of the experiment is at the end). The experiment has a control unit which has an LED installed to ...
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2answers
93 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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1answer
50 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 ...
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1answer
108 views

How to evaluate LED illuminance based on luminous flux and directivity / radiation pattern?

I am building an LED-light and am trying to figure out how many LEDs I need to be on it. I know how much illuminance I need as well as the distance to the subject and the luminous flux of one of the ...
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4answers
775 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 ...
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1answer
502 views

Diffusing a laser?

I haven't been able to find much information about this, maybe I'm searching the wrong thing but, how would you diffuse a laser beam from a thing collimated beam, into a "cone" of light? More ...
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2answers
269 views

Alignment of Michelson interferometer using IR LED

I am trying to align the Michelson interferometer using 780 nm LED. But I am not getting any interference pattern in the CCD. Initially I align the interferometer using Laser to get the equal path ...
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2answers
1k views

Focusing white LED light into a spot as small as possible

An aperture (about 1mm) is placed 2cm in fount of the LED, collimated by a f=50mm lens, then focused by a f=100mm lens into a 3x3mm spot. The focal distance of the second lens cannot be decreased. ...
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1answer
140 views

reverse / dark / leakage current

what is the difference between reverse, dark and leakage current of a (photo-)diode? From my current understanding it all seems to be the same?
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SiO2 layer in diodes

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

Can a controlled lighting environment be a substitute for bandpass light filters?

Could illuminating a subject with different LEDs be used instead of bandpass filters? I am experimenting with capturing surface reflectance using a monochrome camera. The traditional approach to ...
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1answer
82 views

What wavelength of light has the greatest rate of reflection off of ice?

We are using a camera that is nearly flush against frosted ice (not perfectly clear, but pretty close). We are currently using white LED's, but very little of the light gets reflected back into the ...
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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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0answers
87 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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1answer
639 views

Why do parallel LEDs glow brighter when one is replaced with an incandescent bulb?

I have a chandelier with 10 LED lights in parallel. I replaced one with an incandescent lamp and now the 9 LEDs glow much brighter. Why does happen?
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1answer
964 views

Converting illumination to irradiance

I need get intensity of an LED light source, but I cannot convert lux to $W/M^2$ How does one convert illumination to irradiance? And what is the irradiance formula?
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2answers
3k views

How is it I can see the infrared Face ID on my iPhone X? [closed]

I’ve tried using Face ID in the dark on my new iPhone X and I can very clearly see the infrared flood Illuminator light up as well as the infrared dot projector. Is it possible for humans to see ...
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1answer
54 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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2answers
108 views

Bike light: what consumes more current, continuously on or blinking?

I've been riding my bike many nights wondering the following question: What consumes my LED bike-light battery: the continuously on setup or the blinking setup? It might seem trivial, since the ...
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6answers
750 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 ...
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0answers
356 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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1answer
441 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 ~...
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1answer
32 views

Noise produced by foam illuminated by a pwm led

I have a led headlight which seems to use pwm modulation to ajust brightness (led lenser xeo 19R). With an intermediate light power (i.e. not 0% or 100%), the light produce a really small noise, which ...
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2answers
1k 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 (...
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1answer
725 views

Focusing LED using a Fresnel lens at different distances

Simple setup: A Red LED in front of a small fresnel loupe/lens (credit card size). Using a screw I can adjust the distance between the LED and the lens. The LED is centered with regard to the lens. ...
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0answers
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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1answer
96 views

Why does the spectrum of blue LEDs appear to have two peaks?

Due to having poor distance vision and thus strong corrective lenses, I'm "blessed" with the ability to see the spectrum of a light source whenever I look at it from an angle. One thing I've ...
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2answers
231 views

Does the lumen value of a light bulb take into account the inverse square law and volume of a room?

For example, I am trying to calculate the number of 800lm light bulbs required to light a 16sq meter bed room with a 2.5m celling that doubles as a study. Assuming 75 lux is required, this converts to ...
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2answers
1k views

What are the wavelengths of the red, green, and blue lights used for making LED pixels?

LED screens in televisions and smartphones use pixels that use a combination of Red, Green, and Blue diodes that can produce any color when used in the correct proportion of intensity. My question is,...
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1answer
115 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 ...
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2answers
1k views

What does the spikes and curves in the spectral graph for compact fluorescent lights represent?

I have read from sources that the curves are generated by the phosphors in the bulb, and the spikes are caused by the mercury vapor. However, if the mercury vapor's release of uv particles combine ...
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1answer
3k views

Why does the LED light colour spectrum spike in the blue and green wavelengths but CFL lights have lots of smaller spikes across a few wavelengths?

What is the theory behind this? Incandescents produce smooth curves as they are similar to black bodies, but for these non-blackbody bulbs, what creates their unique spectral curves?
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3answers
3k views

How could I measure the colour spectrum of a light bulb and investigate how closely it matches a black body radiation curve?

Here is my research question: What is the colour/spectrum produced by each globe type? What is the temperature equivalence? How closely does a globe match a black body radiation curve? I will be ...
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1answer
145 views

Why do LEDs have only RGB (Red, Green, Blue) colors? [closed]

After some studies I've found LEDs having 3 colors RGB (Red, Green and Blue), I was very interested in this topic so this question is up. The Diagram will tell the colors in LEDs. ...
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1answer
236 views

Why can't one find UV LEDs with wavelength under 280 nm?

Recently, I read about the excellent property of the UV light to kill a lot of bacteria and then I found out that low wavelength (<280 nm) kills more bacteria than at higher wavelength, which makes ...
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1answer
339 views

What is the relationship between wavelength and PD area in optical wireless communication?

I was reading about multipath fading in optical wireless communication, and in many sources it is said that the multipath fading is negligible for two main reasons: In a system deploying direct-...
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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 ...
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2answers
411 views

What type of LED to use with a 100 micrometer pinhole for in-line digital holography?

I want to set up an in-line digital holographic microscope that is similar to the one describe in the article, A Study of Digital In-Line Holographic Microscopy for Malaria Detection, which is they ...
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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?
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2answers
122 views

LED headlight illusion

Commuting today, I noticed that the car behind me had LED headlights that appeared to "bobble" within their lenses--in other words the reflection of the car and the reflection of the headlight lamps ...
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1answer
154 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 ...