The Black Body emission node uses Temperature (expressed via the Kelvin scale) and Power to control the color and intensity of the light, respectively. All objects radiate energy warmer than 0 Kelvin, but this energy is not visible to human vision. The average body temperature for a human, expressed in Kelvin, is 310.15 K — too low for our eyes to register as a source of illumination (as this value is actually in the infrared spectrum), though we do perceive the warmth we generate. The lowest end of the visible spectrum is right around 700 K (embers). A match flame is 1700 K and a standard "Edison" incandescent light bulb is 2400 K. In other words, when temperature increases beyond a certain point, visible light is emitted, starting with warmer reddish values all the way through cooler blue values. 


The term balck body refers to an ideal object that absorbs all light and heat falling on it without reflecting either of these back into the environment. However, most surfaces emit thermal radiation across a predictable spectrum when heated. This spectrum is based on the surface temperature. Lighting models use this concept due to the nature of heated objects shifting color from red to yellow to blue then white as they get hotter.


Figure 1 shows a temperature emission value of around 3900k being emiited from a Black Body node connected to a Universal material on the TV object.  


Black Body Emission

Figure 1: A Black Body emission node connected to the Emission pin on a Diffuse material


Black Body Emission Parameters

Texture - Sets the light source's efficiency to either a value or texture. Keep in mind that real-world lights are not 100% efficient at delivering power at their specified wattage - a 100-watt light bulb does not deliver 100 watts of light. This parameter enters the real-world values.

Power - The light source's wattage. Each light should be set to their real-world wattage - for example, set a desk lamp to 25 watts, a ceiling lamp to 100 watts, and an LED light to 0.25 watts.

Surface Brightness - Causes emitters to keep a constant surface brightness, independent of the emitter's surface area.

Keep Instance Power - Enabling this option with Surface Brightness disabled and uniform scaling applied to the object causes Power to remain constant.

Double Sided - Allows emitters to emit light from the front and back sides.

Temperature - The temperature (in Kelvin) of the Black Body emission's light.

Normalize - Ensures all normal vectors have the same length for the Black Body emission - this keeps the emitted light's luminance constant if the temperature varies. this is enabled by default.

Distribution - Controls the light pattern. This can be set to a Greyscale or RGB image for loading an image texture or IES file. the image texture's Projection nodes adjust the light's orientation and direction.

Sampling Rate - Choose what light sources receive more samples.

Light Pass ID - The Light Pass ID captures the respective emitter's contribution.

Visible On Diffuse - Enables light source visibility on diffuse surfaces. Black Body or Texture emission light sources can cast illumination or shadows on diffuse objects. Disabling this option disables emission - it's invisible in diffuse reflections, but is still visible on specular reflections. It's also excluded from the Direct light calculation. This option is enabled by default.

Visible On Specular - Enables the light source's visibility on specular surfaces, and hides emitters on specular reflections/refractions. This is enabled by default.

Visible on Scattering Volumes - If enabled, the illumination is visible on scattering volumes.

Transparent Emission - Light sources cast illuminations, even if the light source is on transparent material. If unchecked, a transparent surface will "glow" but not cast illumination into the scene.

Cast Shadows - Enables surfaces within the illumination of the light source to cast shadows. If disabled, the light source will illuminate surfaces but the surfaces will not cast a shadow from the light source.