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I'm trying to design a smart light with 800-1000 lm white LED strings (warm+cool) and RGB, but I'm having a hard time comparing the luminous intensity or sometimes mcd ratings.

  1. How can I select red, green and blue, or RGB combo LEDs while matching their perceived brightness?

  2. If a smart light can produce 800-1000 lm of white light, is it expected that it can produce a perceived equivalent of red, green and blue light? The big brands don't seem to list many RGB specs as far as I can tell.

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The unit for luminous flux is the Watt (photons are energy).

However human eyes have wildly different sensitivity depending on color, so a watt of green wavelength photons is much brighter than a watt of red photons, and a watt of infrared photons is invisible.

Lumen is a unit of luminous flux which takes the sensitivity of the eye into account. So two LEDs with the same lumen rating will produce the same flux of visible light no matter the color.

To produce one lumen of visible light, the actual power required (both in terms of photons and electricity to run the LED) is inverse proportional to the efficiency of the LED and the sensitivity of the eye to that color: therefore it takes a lot more electrical power to produce one lumen of red, than a lumen of green.

For the same reason, it is much easier to make a white LED efficient if it has low CRI with lots of green and not enough red and it makes people look sick. High CRI LEDs need lots of red so they are less efficient.

Light comes out of the LED in a cone, ie a solid angle, expressed in steradians. This beam angle depends on how it's built, what optics it has on it, etc. One candela is one lumen per steradian: 1 cd = 1 lm/sr

The same LED chip at the same current, producing the same lumens, can be mounted in different packages with different optics, resulting in different candela ratings, depending on beam tightness. The same amount of lumens in a tighter beam gives more candelas.

To pick a LED you can use either lumen or candelas, depending on how you're going to use the LED.

If you use the LED as a spotlight to light something like a wall, then the beam angle determines the size of the bright spot on the wall, and candela rating will determine how bright that spot is.

1 cd = 1 lm/sr

At a distance d, a solid angle of 1 sr intersects a spherical area d^2. So if the 1 cd LED is 1m away from a wall, approximating spherical to flat, if its beam angle is 1sr, it will make a bright spot about 1m^2 in area. If the LED emits 1cd, at 1m it will put 1lm/m^2 on the wall, corresponding to 1 lux.

If you look directly at the LED (at 1000lm you better not) then its visible brightness will depend purely on its candela rating. So if you want a LED for an indicator (or a bicycle rear light) you need high mcd LED. The beam angle decides whether you see the LED well (your eye is in the beam) or not.

If you put a diffuser in front of the LED, then tight beam angle and candela are useless. When using a diffuser you want your LEDs to have a wide beam angle, otherwise there will be bright spots on your diffuser, which is the opposite of what is intended.

In your smart lamp use case, presumably you will put the LEDs between a diffuser, so you should only look at lumens.

Candela rating can be used to compare two LEDs having the same beam angle, otherwise you have to convert back to lumens.

If a smart light can produce 800-1000 lm of white light, is it expected that it can produce a perceived equivalent of red, green and blue light?

1000lm white is pleasant.

If there are no other lights in the room, 1000lm pure green, red, or blue is definitely going to feel like everyone is on drugs. It would definitely be okay for a night club or an art installation, but for an everyday light, no way. RGB lights are a bit of a decorative gimmick. This is personal taste of course, but I'd say if you want to use the RGB either as decorative or to spice up the white light, 1/4th the power of your whites should be fine.

Also while we're at it, I see many WW-CW lights that do 2700K-6000K. This is useless because the correct color temperature that feels comfortable depends heavily on brightness. See Kruithof curve.

Direct sunlight is 5000K, but that color temperature only feels right at the intensity of real direct sunlight. At a more civilized intensity like what a 1000lm lightbulb produces indoors, 5000k feels way too blue and "cold".

This means a 1000lm, 2700-6000K light is way too wimpy to go over 4000K. It's better to drop the 6000K LEDs and use 5000K instead, and design the thermals to be able to light both at full power. That way you get 1000lm at 2700K (nice), 2000lm around 4000K (great) and 1000lm at 5000K (useless). The usual way is to keep the total intensity constant, which removes the very useful double brightness around 4000K feature.

At low levels, 2200K feels very cozy, like candlelight, but if you increase the power it will be way too red and unpleasant. So you can use your red LEDs to add that warmth at low levels, but they don't need to be as strong as the main white LEDs.

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  • \$\begingroup\$ That is extremely detailed and very well described. Thank you so much! If 1000lm at 5000K from 1x downlight is too wimpy, would it still feel too wimpy in a room with 4 downlights evenly spread out, or does it not scale like that? \$\endgroup\$
    – Dan
    Jul 8, 2023 at 6:04
  • \$\begingroup\$ Without knowing how bright I need each RGB colour to be, this probably means that my best bet for choosing an appropriate brightness is to make a dev board that fits behind the same diffuser I intend to use. I don’t need to have chosen the LEDs I want to use for my project, just any random LEDs with known luminous flux at some test current and voltage. That will give me a better defined requirement to work with when choosing the LEDs I will eventually use. I’m not normally a fan of the iterative approach, but I haven’t been able to think of another way. \$\endgroup\$
    – Dan
    Jul 8, 2023 at 6:06
  • \$\begingroup\$ Color temperature that feels comfy depends on lux, that's lumen/square meter of illuminated surface. So you would first decide what you want to do (from candlelight dinner to soldering tiny SMDs) then pick a suitable illumination lux level for that, multiply by the surface to illuminate and that gives how many lumen you need. For high lux it's not practical to illuminate the whole room so you'd use spotlights. Then from the lux, check Kruithof curve and pick a pleasing color temperature. \$\endgroup\$
    – bobflux
    Jul 8, 2023 at 9:21
  • \$\begingroup\$ engineeringtoolbox.com/light-level-rooms-d_708.html Say soldering SMDs needs 2000 lux, you'd put a 2000lm light over the bench and put these 2000lm over a 1m² bench area, you get 2000 lux, and 5000K would be in the middle of the "pleasing" CT range. But if you put that light in the ceiling and it has a wide beam angle you'd need many more lights to get the same lux on the bench, because it will waste a lot of light on the floor, walls, etc. What I'm saying is 5000K is only good at high lux, which is only practical with spotlights on a small area. \$\endgroup\$
    – bobflux
    Jul 8, 2023 at 9:29
  • \$\begingroup\$ If you want downlights then say in a 16m² room you put 4 1000lm downlights so 4 square meters per 1000lm, that's 250 lux. That's good for an office or living room. At 250 lux, 5000K feels too blue, 2700K feels too yellow, 3000-3200K feels "just right". So basically, diffused downlights don't make sense above 4000K. At 5000K only a high lux lighting makes sense, like a spotlight or very powerful diffused lights (throughs, panels, etc). \$\endgroup\$
    – bobflux
    Jul 8, 2023 at 9:35
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  1. Select one, and if the brightness matching isn't good enough, fix it in the software. For example, if you set all LEDs to 100% and it looks too blue, then turn down the blue until it looks right.
  2. I wouldn't expect it. If green can be as bright as white, then on white, it can't be driving the LEDs to full brightness. But if they can go brighter on white, then why not allow it and get a brighter lamp for no extra cost?

Don't assume all smart LEDs are red/green/blue. They could be red/green/blue/white, or even red/green/blue/warm white/cold white.

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  • \$\begingroup\$ Thanks but if I design an LED string of any colour that isn't bright enough to begin with due to not understanding how to spec the intensity, then it can't be fixed in software. The light I'm designing is RGB+WW+CW but I'm asking about the RGB brightness. \$\endgroup\$
    – Dan
    Jul 5, 2023 at 11:32

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