# How to set emission strength to reproduce real LED light

I have made a Samsung 3 chip led module and its say 12VDC. How a 12vdc is measured in blender. What setting i can provide to get the accuracy of 12vdc light output.

Information regarding led

• 12VDC = 12 Volts Direct Current, it is a unit of electric potential, not light intensity – Duarte Farrajota Ramos Mar 7 '18 at 16:51
• According to the linked spec, it produces 110lm of light so the question becomes “how to adjust the emission to equate to a light output of 110lm”. – Rich Sedman Mar 7 '18 at 17:15

The Voltage the light is operating on doesn't tell much as the power also depends on the current flowing through and the light emitted on the efficiency of the light.

Luckily we get enough information from the datasheet of the light. Its light output is 110 lumens. That is illuminance, not irradiance that Cycles is based on, so we need to convert:

What kind of units does the Cycles emission strength use?

To convert, we need to know the spectral composition of light it produces, because the conversion is dependent on the wavelength. Looking up the LM561B 5630 LED datasheet, we can get the chart for the 6500K LEDs your modul houses:

The luminosity function looks like this:

The peak is 555nm green where 1W produces 683 lumens.

Now you have everything for nice mathematical integration exercise, if you want to be precise.

But honestly, IMHO, I doubt Cycles is even slightly this scientifically accurate. In manual the emission strength is supposed to be Irradiance, but Cycles does not deal with non-visible wavelengths, it is not even a spectral renderer, and it's white-point is sRGB white which is 6500K, which does not correspond to real-life black-body white.

Even for the luminosity function there are multiple CIE standards.

Your best option is not to be accurate, plug the LEDs power rating of 1.32 Watts per module, and slightly lower that because some of that goes to heat. Lastly tweak it so it looks good.

If you need to be visually accurate, you need the light's IES profile to use with render engines:

How reproduce a real led lamp with IES photometric data?

And if you need to be scientifically accurate, you shouldn't be using Cycles.

• "Even for the luminosity function there are multiple CIE standards." Luminance is well defined for any RGB colourspace. In the case of REC.709 lights, average luminance is 0.2126R + 0.7152G + 0.0722B. – troy_s Mar 8 '18 at 2:01
• @troy_s Yeah that is luminance of RGB light shining in that color space. Trying to accurately match and reproduce real world light is a different task. – Jaroslav Jerryno Novotny Mar 8 '18 at 9:14
• “The CIE photopic luminosity function y(λ) or V(λ) is a standard function established by the Commission Internationale de l'Éclairage (CIE) and may be used to convert radiant energy into luminous (i.e., visible) energy. It also forms the central color matching function in the CIE 1931 color space.” Luminance is "real world light". In this case, encoded to three lights belonging to a particular colour space. – troy_s Mar 9 '18 at 2:01