Questions tagged [light-emitting-diodes]

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19 views

LED flashlight Lens / Reflector Choice for Collimation (Large throw)

I am trying to build a LED flashlight with a large throw (beam of light should reach very far, be as parallel as possible). I am using a led with approx 84watt (10x10 COB, 100Watt rated) which should ...
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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 ...
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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 ...
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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 ...
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2answers
133 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 ...
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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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1answer
371 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
58 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 ...
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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 ...
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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 ...
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0answers
76 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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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 ...
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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 ...
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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
347 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
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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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2answers
109 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 ...
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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 ...
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1answer
263 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
58 views

Would 2 people wearing identical night vision goggles be able to see each other from afar? [closed]

From what I understand, this is how night vision goggles (NVGs) function: The environment is illuminated by forward facing IR LEDs. A portion of the light bouncing off of the scene passes through an ...
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0answers
18 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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23 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 ...
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3answers
155 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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1answer
46 views

Can RGB screen color exist in real physical world?

For example the bright royal blue color; R0 G10 B220, HEX: #000ae1. I know this color cannot be printed nor painted. But can this blue color exist in the real world other than on a digital screen? For ...
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1answer
627 views

Does interchangeable wavelength LED exist in NIR?

Does exist a led material that allows to interchange the emitted wavelength in near IR spectrum?
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1answer
171 views

If 2 beams (of very low intensity) were to cross each other will the resultant point at which they meet be of higher intensity?

For example, if I take 2 laser pointers and arrange them in such a way that their beams cross each other (forming a kind of "x") will the resultant colour at the point at which they intersect be of a ...
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2answers
201 views

Why does the p-region have a higher energy level than the n-region in a diode?

Looking at the image below, I'm curious as to why the p-type region is at a higher energy level than the n-type region when no external voltage is applied. How is that? The E-field, if I'm not ...
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1answer
71 views

Is the spatial coherence of light a requirement for creating a well-collimated light beam?

I was wondering if, for creating a well collimated light beam, the spatial coherence play a role. In particular, I understand that a laser beam can be well-collimated with respect to light coming from ...
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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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31 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
191 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
49 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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2answers
119 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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2answers
17k 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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1answer
39 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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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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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 ...
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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?
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1answer
258 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
47 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
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 ...
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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 ...
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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
2k 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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2answers
789 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 ...