"I'd like to fill a room with ambient daylight by bouncing LED light off the ceiling."

EDIT: Poor wording, but by "fill", "ambient" and "daylight", I'm not considering the intensity, but just how to focus and direct whatever light output I have, to achieve a "general brightness from above" similar to being under a bright sky, avoiding direct glare. For instance, should I try to cover most of the ceiling with a wide beam, or direct a narrower beam towards its center? And ultimately, how to choose lens/reflector components for COB LEDs.

"Initially I would test with 15x10W LEDs aimed diagonally upwards from the top of a shelf."

EDIT: I'm not expecting to simulate sunlight with 150W, it's just seems like a reasonable place to start in terms of cost and managing the cooling requirements. There is an article about DIY SAD treatment by bouncing 150W directional light off the ceiling, using a ceramic metal-halide bulb. In 2017 it seems possible to achieve 100 lm/W and CRI: 95 with LEDs at a similar cost. The author finally suggests using 25,000-50,000 lm, which would be 2-4 times more than 150W.

"If the LEDs' emission angle is 120°, how important are lenses and/or reflectors for this kind of setup, and what do I need to consider when choosing them?"

EDIT: I understand that this question might not be appropriate here, what I'm looking for is the big picture of how to find and select a lens for COB LEDs. I don't think the Physics SE would be suitable for that, either. I apologize and respect any decision by the mods. :)

Thank you for the detailed and insightful answers!

  • \$\begingroup\$ I'm voting to close this question as off-topic because it is really about optics, not electronics. \$\endgroup\$
    – Dave Tweed
    Mar 16, 2017 at 21:37

2 Answers 2


I'd like to "fill" a room with ambient "daylight"

Well, ambient daylight can be over 1 kW per square metre so 15x 10 watt LEDs (at 30% light conversion efficiency) is about 50 watts. However, The World Meteorological Organization regards 120 watts per square meter as the minumim to count as "sunshine" so this helps a bit but how far you go down the scale towards 120 watts per sq metre is your call.

Also, in a room that could be filled with normal daylight to the standard you hope to achieve maybe it would need (estimate) 6 square metres of windows. That would be 6 x 120 watts or 720 watts and remember this is the bottom end of the Meteorological Organization's limit.

So maybe a kW through 6 sq metres would be OK. Take a look at this graph: -

enter image description here

A lot of the incident light power is made up from infra red so, if you do some more research you can estimate the equivalent visible light power. Maybe it's about 50% of the 1 kW so possibly 500 watts of light diffused into the room would give the impression of being lit from real sunshine.

Of course the LEDs are only about 30% efficient so you will need an input power of about 1500 watts. You are quite possibly a long way short of the overall power needed to give the impression of daylight.

  • \$\begingroup\$ About 42% is in the visible 400-700nm spectrum. en.wikipedia.org/wiki/Sunlight \$\endgroup\$ Mar 16, 2017 at 13:34
  • \$\begingroup\$ @Misunderstood cool, I wasn't far off with my guesstimate! \$\endgroup\$
    – Andy aka
    Mar 16, 2017 at 13:35
  • \$\begingroup\$ This LED fixture at 500+ watts can put out about 1/6 of sunlight's irradiance on a 4'x4' area. On pg 15 it shows a plot of PPFD (Photosynthetic Photon Flux Density). I believe sunlight is about 2000 PPFD. PPFD is only visible spectrum. This fixture would cost about $1,000 to build. $400 LEDs, $200 heatsinks. cree.com/~/media/Files/Cree/LED-Components-and-Modules/XLamp/… \$\endgroup\$ Mar 16, 2017 at 13:57
  • \$\begingroup\$ Thank you, this is good info to have. Rather than going straight for monster solutions (500W COBs and whatnot) for this hobby project I think it's better for me to add some 10K lm at a time and add more modules later if it turns out well. Updated the question with some clarifications. \$\endgroup\$
    – Toerndev
    Mar 18, 2017 at 16:18

Real sunlight comes from a very distance, almost point-like source. It casts a rather sharp shadow.

Diffuse daylight (like when it's foggy) comes from all skyward directions.

The diffuse light will provide a more uniform brightness (less shadows), but the lack of highlights and shadows can reduce visibility in some cases (for example, a bumpy surface can begin to look totally flat).

What do you want to achieve? An imitation of sunlight or totally uniform brightness? Your choice of the word "ambient" seems to indicate uniform, shadowless brightness, but you won't achieve this by simply pointing a row of LEDs at the ceiling from a top shelf. The ceiling close to the LED strip will be very bright. As the intensity drops off with the distance squared, it will get pretty dim elsewhere at the ceiling. At an angle of 45 degrees, brightness will already be down to 50% (compared to right above the LED). So if the LEDs are 50cm below the ceiling, you will have the 50% brightness at a horizontal distance of 50cm.

Obviously, the effect can be played with by placing the LEDs at a lower height. When the LEDs are placed on the floor (250cm below the ceiling), the 50% line will be at a horizontal distance of 250cm. Also obvious, a 120 degree emission angle will in such a case mean that the light will most likely just blind innocent bystanders.

To provide an sort of approximation to a uniform lighting of a flat surface, you can use an asymmetrical reflector (bouncing the LED light first at the reflector, and from there to the ceiling). Here's an example of a conventional fixture which does just that: http://www.adblighting.com/files/DS7023-E%20AC%201000%20Cyclorama%20data%20sheet.pdf

Note how the tubular "bulb" is close to one edge (easy to see) and how the reflector is not a simple parabolic one, but one with a steep, short slope on one side of the light source and a rather flat, long slope on the other side of "bulb" (that's pretty hard to see in the brochure).

The orientation of the lamp as shown in the brochure is meant to illuminate a wall from a point just below the ceiling. To illuminate a floor, it be mounted facing straight down. Straight up for a ceiling. Many inexperienced users will erroneously tilt the lamp to face more diagonally into the direction in which the light should go, causing uneven lighting. Still, such fixtures are an easy solution in all cases, as they cannot be mounted at an arbitrary distance for an arbitrary to be illuminated.

Unlike the ordinary floodlights, which can cheaply be bought about everywhere, these things are also expensive.

In cases where a light designed needs a (typically temporary) non-standard asymmetrically shaped reflector, aluminium foil can be used. A rather thick and sturdy aluminium foil is available for profession lighting purposes, at a price where even professional light designed will wince. You might start experimenting with several layers of ordinary household aluminium foil. Heck, even professionals will do that when working on a less than unlimited budget!

  • \$\begingroup\$ Thanks for sharing this. I wonder how one might mount LEDs so that they face the inside of the reflector, and/or combine with aluminum foil. It might be hard to get heat sinks and any fans out of the way this way. I've tried to clarify the question to answer your questions. \$\endgroup\$
    – Toerndev
    Mar 18, 2017 at 16:06

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