Area emitters

This emitter type projects light from the surface of a given object, so it is the whole geometry that is working as a light source. You can set the Color and the Intensity of your light separately. The color of the emission can be set in RBG, HSV or XYZ scales, correlated to a Kelvin value or from a HDR texture (emitting texture). The intensity (or Luminance) can be set in a number of different ways (see next section). 

Custom mode

Emission

This parameter refers to the spectrum of the light emitted, basically the emitted color. There are thee ways to specify the emission spectrum:

• Color: input the color directly, using the Color Picker wheel (clicking on the colored square allows the user to choose a color in the Maxwell Render color picker), or using a correlated color at a given Kelvin Temperature in ºK (lets you choose the color that would correspond to an emission in Kelvin degrees. Please note that choosing this option will not make any changes in intensity, just color. Low Kelvin temperatures are reddish, 6500ºK is considered white, and higher values will produce a bluish light). 
• Temperature: input the temperature in Kelvin degrees and Maxwell will set both the color and intensity corresponding to that temperature (corresponding to the light emission, color and intensity, of a black body heated at that temperature). 
• HDR Image: input a high dynamic range that will be used as emission texture. See more information about textured emission here. 

Luminance

Specifies the intensity of the light. There are several options to specify luminance, like Power & Efficacy, Lumens, Lux, Candelas and Luminance:

Luminance units

Power and Efficacy

This option allows you to specify how much electricity a light source consumes (Watts) and how efficiently it converts that electricity into visible light (Efficacy). The efficacy number specifies how many lumens are emitted per watt. For example, a common 40W incandescent light bulb will have a rather low efficacy of 12.6 lumens/ watt. This information is usually supplied by the bulb manufacturers. A more efficient energy saving bulb which is also rated at 40W will have an efficacy of perhaps 17.3 lumens/ watt, so for the same amount of energy consumed, it will emit more light.

The watts/efficacy way of specifying an emitter’s intensity is useful if you would like to mimic common light sources, where the manufacturer provides the watt rating and the efficacy rating. In the “Output” row you can see how many Lumens a particular watt/ efficacy setting will create.

Luminous power

Lumens (lm) is the SI (International System of Units) unit for luminous flux. It is a common way to specify how much light is emitted. Light manufacturers usually supply this data.

Illuminance

Lux (lum/m^2). Lux is the unit for specifying illuminance. It is defined as one lumen per square meter. This is a useful setting when you would like to increase or decrease the size of your emitter, and have it emit more or less light. If you use lumens and scale up your emitter, the same amount of lumens are emitted but from a larger area, giving the impression that the emitter is weaker. If you use Lux however, the amount of lumens increases/ decreases with the scale of your emitting surface.

Luminous Intensity

Candela (cd). Candela is considered the SI unit for luminous intensity; the power of the light emitted in a certain direction.

Luminance

Nit (cd/m^2). One “Nit” is one candela per square meter.

A note on Luminance type

It is important to remember that the amount of light emitted from an emitter is spread out across its surface.

This means that the same emitter material will look dimmer on a large emitting surface and brighter on a smaller emitting surface. You may also think something is wrong when scaling up your small emitter, since the amount of light falling on your subject looks like it has decreased:

The two emitters are using the same emitter material. On the right, the emitter emits the exact same amount of light as the smaller emitter, but since it is emitting it over a much larger area, there is less light focused on the statue, so it appears darker.

If you would like to keep your emitter the same brightness no matter the surface, then you should use the lumens/m2 units. Keep in mind though that this will actually change the amount of light emitted from the surface of the emitter when you change the emitter size.

Preset

Maxwell Render™ provides some emission presets of many standard type light sources. Please note these presets will change the color as well as the intensity of your emitters to match the light emission you are choosing.

Emitter presets