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There is often no sun were I live so I need some advice for building a LED matrix that can replace the sun light and possibly avoid bad wavelength. UV produces vitamin D but upon reading are-uv-leds-really-dangerous? I'm unsure about UV.

  1. Which wavelengths is preferable?
  2. How much LED power?
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Jossi
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get some 3000 kelvin (this is often called a warm yellow, or orange/yellow) colour temperature "White" LEDs, such as those from Cree, and take vitamin D supplements :). The LED matrix with about 100W of LED should be more than enough to light up a good area. Also think about putting a diffuse/cover over the LED so they are able to spread the light better. And use a reflector so you do not waste light "backwards".

KyranF
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  • yes @OlinLathrop I figured some common sense and reasoning should be applied to the situation too – KyranF Nov 21 '14 at 22:12
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You can get Cree LED's either in a enclosed in bulb type housing or as a standalone LED (or sets of LEDs). This 33w LED array has an output of 2100 lumens.

You might want to consider instead a light box designed for treatment of SAD (seasonal affective disorder). It produces an output of 10,000 lux (measured one meter from the box), and is specifically designed to simulate full daylight. SAD affects over a million people in Sweden.

Lumens and lux are not the same but are related; one lux is equal to one lumen per square meter. So you would need 10,000 lumens (five of the Cree arrays), spread out over one square meter, to equal the light box output.

Most light boxes contain fluorescent bulbs; but this one has a matrix of 108 high-power LEDs and sells for $145.

enter image description here

Here is another one that uses fluorescent bulbs and costs $140. It also outputs 10,000 lux; that is pretty much standard for all of these daylight simulators.

Given that five Cree arrays would cost $100, and you still need to provide a power supply, heat sinks (they get hot!) and a housing, I suggest getting one of these light boxes and save the trouble of building one.

tcrosley
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LED spectra are nothing like blackbody spectra (the sun is a blackbody). Blackbody spectra are smooth, with every wavelength represented to some degree. LED spectra are sharp, with their light coming either from the semiconductor junction or from some fluorescing chemicals (this is how they make white LEDs). However I doubt this makes a difference, biologically.

An LED's spectrum:

White LED Spectrum

Various blackbody spectra, with visible wavelengths called out (the sun's surface is 6000K):

Blackbody Spectrum

The Sun's light as you see it at the surface of the Earth:

Sectrum at Earth surface

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Dave
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  • That's actually a relatively untrue statement. Compared to everything but incandescents, LEDs have very smooth spectra with all wavelengths represented at least to some degree. They are much better than e.g. (compact) fluorescent lamps. It depends on the phosphors that are used, though, and some very high CCT lamps (i.e. 'sunlight' 6000K+) have an order-of-magnitude spectral power dip between blue and red. All that being said, it's still very different from blackbody radiators. – user36129 Nov 24 '14 at 10:00
  • I added some very unambiguous spectra to show the differences. – Dave Nov 25 '14 at 17:01
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Have a look at this Kickstarter, it may exactly what you are looking for, it even changes colour temperature throughout the day to match the sun

https://www.kickstarter.com/projects/sunn/sunn-lights-sync-with-the-rhythm-of-the-sun

back_ache
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  • Interesting link, but the OP has already indicated a SAD lamp at $140 is too expensive, so I don't think he'll be interested in one costing $350 to $450. – tcrosley Nov 24 '14 at 11:53
  • Good point, maybe he could use it as inspiration though to cook up his own and maybe even document and open-source his design – back_ache Nov 25 '14 at 12:33
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    The Kickstarter project gave a lot of technical specifications so I will have this answer accepted even though its indirect. I will buy different kind of LEDs that is in 2700-6500 K range. Power wise its seems that 5000-10000 lm range is good enough. With a lot of LEDs the easiest way to control intensity is to group LEDs together so that each group can be controlled by a switch. I estimate that DIY will cost max 50$ and some good use of my time. – Jossi Nov 27 '14 at 00:14