Questions tagged [light-emitting-diodes]

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What does the turn on voltage for LEDs depends on except for the bandgap of the material?

I am trying to find out why my LEDs show a different turn on voltage. The material emitting is the same (therefore same bandgap). The stack I have is ITO/Polyvynilcarbazole (hole injection layer)/...
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How to convert photodiode current into luminance for LEDs efficiency?

Very basic question but I am struggling. I have J-V curve and Photodiode current vs. voltage for LEDs and I would like to calculate the efficiency. How can I do that? and how to express luminance (cd/...
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Is the reverse saturation current larger for a LED or a solar cell? [closed]

I am currently preparing for an exam and found on some old exams the question whether the reverse saturation current would be larger for a LED or a solar cell. I am familiar with the expression for ...
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Lab Construction of LED

If given a semiconductor with known bandgap, how might someone construct an experimental LED within a lab? It seems common to prepare solar cells using FTO glass plates with a paste of semiconductor ...
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Is the current in reverse biased PN junction due to drift or diffusion?

Wikipedia says :https://en.m.wikipedia.org/wiki/Saturation_current The saturation current (or scale current), more accurately the reverse saturation current, is that part of the reverse current in a ...
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1answer
23 views

Black coloured region of smartphone screens

We have all used backlit screen. Indeed, you must be reading my question in one of them. Everyone of us must have encountered, for multiple times, the texts or the images with regions of hexadecimal ...
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How to design a collimated (no hot-spot) RGB light source?

I'm an electrical engineer tinkering with a tiny display that has an LED back light. It looks like a little square and it quickly cycles through red, green, blue light to produce the final display. I ...
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1answer
2k views

Where is the energy coming from to light my Christmas tree lights? [closed]

Today my wife gave me a little box of solar-powered Christmas tree lights. She bought them last year 2019 but had forgotten she had them so it was only today (a year later) that I took them out of ...
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How to calculate LED light output

I'm trying to assemble an array of LEDs to be configured as a solar simulator. I need to compare this array's output to the solar spectrum as a function of W/m^2/nm. To do this I need to go through ...
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2answers
24 views

Spectrophotometer using RGB LED

I understand that an RGB LED produces light in three constrained wavelength ranges and that any color beyond R, G, or B (orange, say) is due to the interaction of the cells in our eyes with the ...
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15 views

Relationship between Optical Power output and luminous intensity

I have an LED with the following specification which I'm using for oxygen saturation measurements. Here the optical output power is given as >1.2mW for RED LED and >0.5mW for IR LED. How to ...
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22 views

Cut-off frequency of an LED [closed]

I'm trying to solve this problem: A GaAs LED is at $ 300 K $ when the current density is 100 $A/cm^2$ . The width of the active region is $ 1 µm $. Assume that, for that current, we are in the strong ...
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28 views

Shockley diode equation - temperature influence

when looking at Shockleys ideal diode equation I wonder what "meaning" the temperature has in there? If I assume $$I(T) = I_0 \left(\exp \left( \frac{U}{k_BT/q}\right)-1 \right)$$ and vary ...
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1answer
33 views

How do LED screens emit light?

How does an LED television screen emit light? My understanding is that the screen consists of many pixels, and each pixel has red, green, and blue LEDs. An electrical current runs through every red, ...
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2answers
47 views

How do wavelengths add-up?

If I have 3 LED lights of following colors and wavengths: Red 650 nm Blue 450 nm Green 550 nm All three are placed side by side and turned ON.. then what will be the wavelength of the combined light ...
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1answer
10 views

How can I find the external quantum efficiency of an LED?

I have a setup where I can measure the optical power, in watts, the current, the voltage and the spectrum of an LED. Is it possible from this to calculate the external quantum efficiency? I realise I ...
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Model radiated power from an LED

For a range of LEDs I'd like to model their performance and predict the flux density across an area using manufacturers datasheets. Eventually I'd like to take various power measurements and calculate ...
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2answers
48 views

In a LED the electrons from the n-type combine with the p-type which creates a photon. Are the electrons 'lost' or gone when this process takes place?

In a LED the electrons in the n-type move to the p-type where they are then 'combined' with the holes in the valence shell of the p-type which produces photons. My question is that when this happens ...
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1answer
18 views

Is an LED's voltage response to current non-linear with temperature?

I have been measuring the voltage response to current of an LED. It's a point source LED with an oval emission area of 40x150um (link to datasheet https://www.daido.co.jp/en/products/point_source_led/...
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16 views

How to calculate lux value?

I tried calculating lux value for my LED by using the formula the (luminous flux / area), i obtained viewing angle and luminous flux value from LED datasheet, then with the angle i calculated the area ...
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16 views

How can I calculate the UV LED dose having few parameters such as wavelength and output power of LED?

i have UVC led with 270nm wavelength and output power of 70mW. The total length of the passage for the de-contamination is 40cm and the total time is 60 seconds. Is it possible to calculate the UV ...
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1answer
83 views

How would you make a RGB LED equally bright to a broad-spectrum white LED?

Say you had a broad spectrum white LED (all visible wavelengths) and you wanted to make a RGB LED (just red-green-blue wavelengths) that could produce white light of equal brightness to the broad-spec ...
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1answer
30 views

Infrared light beam diffusion

I have a closed box with a hole in the wall. A IR beam is directed at the front of the box. Due to a small 6mm borehole in the wall only a certain part of the beam can pass trough. The left small ...
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1answer
40 views

Characteristic functions of a non-ohmic material

We know that Ohm's laws are experimental laws. Ohm's first law for ohmic materials follows a linear approximate progression of the kind: $$\frac{\Delta V}{I}=R$$ What are the functions of type $$\...
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2answers
84 views

Would a perfectly collimated laser beam have a flat intensity distribution?

I'm trying to simulate a collimated laser beam in some FORTRAN and am wondering about the intensity distribution of a perfectly collimated beam: would it be a flat distribution (equal intensity across ...
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3answers
297 views

Why are blue light-emitting diodes important enough to warrant a Nobel prize? [closed]

I am using a miniature blue light as night bed room light since my childhood. For me blue light-emitting diodes almost look like miniature blue lights. I don't see any fundamental scientific ...
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0answers
35 views

Semiconductor structure confinement modes

Today, I would like to simulate the field intensity in an emitting structure such as a laser diode. With this field, I will be able to calculate the confinement factor of my device. I put an ...
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1answer
24 views

Reverse biased semiconductor diode: Name of the current due to which the depletion layer is getting widened?

I know, that in a reversed biased connection the reverse saturation current flowing is negligible but before the depletion layer is widened to its maximum capacity. It seems the current flowing ...
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37 views

How much R and B spectrum changes over time for LED?

I am trying to see if there is any data on the R and B spectrum change over time for a LED. For example after 5000 hrs of operation, there are few light which gets dull, which is mostly that the ...
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3answers
387 views

Is there any relation between turn on voltage of LED and the colour emitted?

It is evident from the above V-I graph that longer the wavelength of the light emitted lower is the turn on voltage. I know that turn on voltage is voltage required to have significant current that ...
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1answer
64 views

Can white light have two spectral peaks?

I was looking at a graph on the receptive ranges of the 3 types of color sensitive photoreceptors in us humans, and I had the idea that if we place only two peaks at the right places, we'll be able to ...
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2answers
193 views

Is it possible to use fresnel lens(es) to focus an LED light on a headlamp

I was hoping that someone would be willing to offer any insight or guidance they have on a project that I am working on. We are trying to use a lens mounted on a headlamp to focus a plain white light ...
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1answer
817 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 ...
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39 views

Calculate irradiation from a LED lamp

I am working with cell culture and I want to irradiate a 96-well plate with a single color led lamp (let's say the peak is at 450nm). It's just a test, so a rough estimation would do. I've been ...
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2answers
128 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”? ...
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1answer
47 views

LED flicker vs human perception

I have read this question: https://mechanics.stackexchange.com/questions/43070/why-do-the-headlights-of-supercars-looking-flashing-on-video where cdunn says: So when you slow it down, you're ...
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2answers
905 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 ...
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2answers
109 views

Viewing LEDs at great distance

Say I put a satellite in orbit with several LEDs on the outside: red, yellow, blue, white and green. Could I see them from the ground with consumer-grade binoculars or telescopes? If I can see them, ...
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1answer
54 views

Amateur student making a demo of electromagnetic induction via copper coil and magnet

I'm trying to observe electromagnetic induction by creating a copper coil solded to a diode led and waving an alnico magnet inside the copper coil's core. You can see my amateur setting below (...
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1answer
84 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
73 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
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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
345 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
101 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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169 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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31 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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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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1answer
152 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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2answers
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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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26 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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