# Simulating a 40W Light Bulb

Let's say I have a light bulb at home that I want to add as a light source.

Specs:

• 40W incandescent light bulb
• 5cm in diameter
• Color temperature of 2700 kelvin

What values should I enter into Blender to have this simulated properly?

I will accept an answer explaining this for Cycles, together with how the conversion of my numbers above into Blender was done.

Bonus points for Eevee settings as well, which are not the same IIUC.

## NOTES

Simply convert Lumens to radiant flux.

1. Set Color to white, Power to 1W [link], and Size to 25 mm
2. Enter 450 Lumens (energy emitted by a 40W incandescent lightbulb)[link]
3. Divide the Lumens by 683 to get radiant flux [link] (which is used by Blender)

You can use Blackbody node to specify the 2700K color temperature.

Sources:
Cycles, unit of light energy ATTN: Brecht
Why Watt as light value - Comment by Troy_s
CG Cookie - How physically based, photometric lighting can improve 3D renders

• Related plugin with a number of researched presets: blendermarket.com/products/extra-lights / github.com/jlampel/extra_lights May 31, 2020 at 19:59
• The "683" is incorrect. "683" is for green light (555nm) and this lamp is 2700 kelvin, or "warm white" according to some web page: en.wikipedia.org/wiki/Lumen_(unit)#Explanation Aug 21, 2020 at 17:34
• Hello, good point :). I re-checked and I still think it's correct. 683 is the maximum luminous efficacy of an RGB bulb (which is the green channel). It's used by manufacturers to measure lumens, so I used it here for realistic results. See the linked CG Cookie video :). Aug 21, 2020 at 18:17
• Video watched, and it mostly confirms your numbers (for an all white light, which is close enough to my 2700K one I guess). Still don't understand how the Wikipedia page I linked goes together with the formula in that video, but I don't have a better answer myself for now so I can't really claim you're wrong here :). Sep 9, 2020 at 19:38

If your unit setup is tuned correctly:

You should easily set up size and power:

For color temperature, you should switch to cycles. In shader editor turn on use nodes for lamp, and add Blackbody node like this:

It may convert color temperature to color.

• Hello :). I'm not sure this is correct. 40W in incandescent lightbulb is energy input, but Blender works with energy emitted from object. Thread with Brecht Brecht Van Lommel and Troy Sobotka May 30, 2020 at 10:40
• Thanks for the link @JachymMichal, updated the question to clarify that this answer is wrong. May 30, 2020 at 12:28
• Also clarified in the question that the light size is a radius, not a diameter. Just found out, that was pretty unobvious, so the light size should be "2.5cm", not "5cm". May 30, 2020 at 12:51
• Why are you using centimeters instead of meters? Jun 11, 2020 at 0:21