Do standard white LEDs produce a full spectrum of light?

Question

At places like Radioshack, small (and sometimes large) LEDs are available that are labeled to be "white". This is a link to a Radioshack page for such an LED: http://www.radioshack.com/5mm-white-led/2760320.html#q=white%2Bled&start=2

Does a light like this produce a full spectrum of light, such that appropriate light filters could produce any spectrum of visible light?

Quick answer (hence the comment): Nope. Most white LEDs are blue LEDs with a phosphor-coated lens. — Shamtam, Jan 18 '15 at 01:52
@Shamtam do you know what i would look for to find LEDs that do meet that criteria? Would a set of rgb LEDs diffused through an opaque cover produce real full spectrum light? — Konner Rasmussen, Jan 18 '15 at 01:59
I think there are some high-end luxeon rebel LEDs that actually produce full spectrum but those are mega expensive — Funkyguy, Jan 18 '15 at 02:01
@Funkyguy honestly im not too worried about a one time fee. the great thing about LEDs is their low power consumption... im wanting to build full spectrum LED panels for sale to people who wish to grow indoor herbs like thyme or rosemary... LEDs seem much better than halides if they can produce the correct light... — Konner Rasmussen, Jan 18 '15 at 02:07
I've seen plant lights advertised with a mix of red and blue LEDs. — pjc50, Jan 18 '15 at 02:13
You do not need a full spectrum light to grow plants. Look at an absorption spectrum for chlorophyll. Green light, for instance is not used by plants, which is why they are green. They are reflecting green light instead of absorbing it. — Eraticus, Jan 18 '15 at 02:17
possible duplicate of What type of method for producing White-Light LEDs is the most common? — Phil Frost, Jan 18 '15 at 05:04

score 7 · Answer 1 · edited Apr 13 '17 at 12:33

@Shamtam have already commented that:
(1) the light from a typical while LED doesn't match the spectrum of the sunlight
(2) white LEDs are blue LEDs with additional phosphor

This is to expand on @Shamtam's comment.

enter image description here Spectrum of a white LED showing blue light directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadband emitted by the phosphor.
(source: Wikipedia)

enter image description here Diagram of the spectrum a LED lamp (blue), a CFL (green) and an Incandescent (purple) superimposed the solar spectrum (yellow). Note that the energy used by each lamp is at least the area underneath its curve.
(source)

score 3 · Answer 2 · answered Jan 18 '15 at 05:47

Quick answer - Shantam to the contrary, yes. But let me explain.

Nick Alexeev provided a spectrum from a typical white LED. While it is not a faithful simulation of an incandescent (the big spike in blue is notable), it does contain all visible wavelengths. Your question asked "Does a light like this produce a full spectrum of light, such that appropriate light filters could produce any spectrum of visible light?", and the answer is obviously yes. If you wanted a predominately green light, for instance, it wouldn't (relatively) be very bright, but you could do it. This is not true for RGB LEDs, which would have very narrow spikes of intensity at 3 different wavelengths, with nothing in between. So, while you could easily produce a subjectively yellow light from an RGB LED source (just transmit red and green in the proper intensities), a narrow-band yellow filter would transmit nothing.

score 0 · Answer 3 · answered Jan 18 '15 at 02:14

White LEDs do not produce a full spectrum, like an incandescent light bulb does. This has to do with the process that produces the light/photons. In an LED, when an electron crosses the PN junction, the electron itself drops to a lower energy state, and the excess energy is emitted as a photon. This voltage drop is basically constant, therefore the energy of the photons emitted, and the color of light produced is basically constant. If you were to combined three LEDs, such as in an RGB LED, you still would not have a complete light spectrum, but rather three emission lines visible in a spectroscope.

This is also true of white florescent lights, which depend of three phosphor colors to produce white light.

Do standard white LEDs produce a full spectrum of light?

3 Answers3

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