About Light Parameters

Type

Three types are available in the Type list and have specific parameters.

For more information about light types, see About Creating a Light.

Spot

When you select the Spot type, the light is emitted in a specified direction with a certain spreading

The following parameters are available for nonphysical lights:

Parameter

Description

Angle

Enables you to define the half-angle of the cone, that is the angle between the axis of revolution and the cone edge (for spot light sources only). The angle is a value between 0 and 90 degrees. This means that a value of 90 degrees would generate a light source equivalent to a punctual light source.

Color

Sets a color for the light spread.

Intensity

Defines the light intensity using the slider or by entering a value directly in the box.

The intensity of a light source is the maximal lightness value of three colors (ambient, diffuse, and specular).

The light color is computed by multiplying the Red, Green, and Blue values you defined in the previous step by the intensity value. Therefore, this parameter allows you to adjust the luminosity of the light source, while preserving its chrominance ("color") component.

You can enter values comprised between 0 and 1: the higher the value, the more saturated (that is, the whiter) the light. More precisely, as soon as the intensity value exceeds 1, the color starts saturating.

For examples comparing 2 different sets of RGB values with 3 different intensities, see Color Model.

Falloff

None: no lighting end, which means that the light energy is constant and infinite.
Linear: the light energy decreases linearly with a 1/r ratio ("r" = distance to the light origin) and stops at the attenuation end. For instance, if the energy received at a distance of 10 mm from the light origin is equal to 1/10th of the light energy at the light origin.

End

Defines in millimeters the maximum distance for light attenuation (that is, the distance to the center from which the light source does not illuminate).

Start Ratio

Defines as a ratio the minimum distance for light attenuation. For example:

0 corresponds to an attenuation starting from the center.
0.5 corresponds to an attenuation starting from the middle.
1 corresponds to an attenuation starting from the end, that is a null attenuation.

Angle Ratio

Defines as a fraction of the light angle the angle to the light axis from which the light starts to attenuate. For example:

0 corresponds to an attenuation starting from the axis.
0.5 corresponds to an attenuation starting from the half-angle.
1 corresponds to an attenuation starting from the end, that is a null attenuation.

Tip: You can also position your pointer over one of the side lines, then drag the segment to modify the angle ratio.

Exponent

Simulates the angular attenuation of a spot light intensity from the inner cone to the external cone. By this it means, the exponent parameter controls the slope of the intensity angular attenuation between the external cone defined by the spot angle and the inner cone defined by the angle ratio according to the following formula: cos(angle)^Exponent. Its value range is [0-128].

The following parameters are available for physical lights:


Parameter	Description
Angle	Enables you to define the half-angle of the cone, that is the angle between the axis of revolution and the cone edge (for spot light sources only). The angle is a value between 0 and 90 degrees. This means that a value of 90 degrees would generate a light source equivalent to a punctual light source.
Color Model	Lets you choose between: Temperature RGB See RGB Color.
Multiplier	Specifies a multiplier coefficient.
Color	Specifies a color for the light spread.
Temperature	Describes the light as a temperature. Drag the slider, or enter the appropriate value in the box. You can specify the color temperature of the light in Kelvin (K) to compute the light color.
Intensity Unit	Specifies the unit for light intensity. You can choose between Candela (cd unit) or Lumen (lm unit).
Luminous Intensity	Specifies the intensity of the light in candela or in lumen.
Angular Attenuation	For physical lights.

Point

When you click the Point type, the light is emitted with the same intensity in every direction.

The following parameters are available for nonphysical lights:


Parameter	Description
Color	Sets a color for the light spread.
Intensity	Defines the light intensity using the slider or by entering a value directly in the box. The intensity of a light source is the maximal lightness value of three colors (ambient, diffuse, and specular). The light color is computed by multiplying the Red, Green, and Blue values you defined in the previous step by the intensity value. Therefore, this parameter allows you to adjust the luminosity of the light source, while preserving its chrominance ("color") component. You can enter values comprised between 0 and 1: the higher the value, the more saturated (that is, the whiter) the light. More precisely, as soon as the intensity value exceeds 1, the color starts saturating. For examples comparing 2 different sets of RGB values with 3 different intensities, see Color Model.
Falloff	None: no lighting end, which means that the light energy is constant and infinite. Linear: the light energy decreases linearly with a 1/r ratio ("r" = distance to the light origin) and stops at the attenuation end. For instance, if the energy received at a distance of 10 mm from the light origin is equal to 1/10th of the light energy at the light origin.
End	Defines in millimeters the maximum distance for light attenuation (that is, the distance to the center from which the light source does not illuminate).
Start Ratio	Defines as a ratio the minimum distance for light attenuation. For example: 0 corresponds to an attenuation starting from the center. 0.5 corresponds to an attenuation starting from the middle. 1 corresponds to an attenuation starting from the end, that is a null attenuation.

The following parameters are available for physical lights:


Parameter	Description
Color Model	Lets you choose between: Temperature RGB See RGB Color.
Multiplier	Sets a multiplier coefficient.
Color	Sets a color for the light spread.
Temperature	Describes the light as a temperature. Drag the slider, or enter the appropriate value in the box. You can select the color temperature of the light in Kelvin (K) to compute the light color.
Intensity Unit	Sets the unit for light intensity. You can choose between Candela (cd unit) orLumen (lm unit).
Luminous Intensity	Sets the intensity of the light in candela or in lumen.

Photometric

When you select the Photometric type, the light intensity is measured according to the light direction and it is stored by the manufacturer in an IES file or a EULUMDAT file.

The following parameters are available for photometric lights:


Parameter	Description
Color	Sets a color for the light spread.
Light Profile	Defines the light profile. Click to search for an IES file. A preview of the light profile is displayed. It corresponds to a light representation that matches with the characteristics contained in the IES file attached.
Luminous Intensity	Sets the intensity of the light in candela or in lumen.
Intensity Unit	Sets the unit for light intensity. You can choose between Candela (cd unit) or Lumen (lm unit).

Color Model

Two colors models are available: RGB and Temperature. If you select the Sun type, this parameter is not available.

RGB Color

Light is associated with an RGB color value that does not involve wavelengths or spectral rendering.

The light color is computed by multiplying the Red, Green, and Blue values by the intensity value. Therefore, this parameter adjusts the luminosity of the light and preserves its chrominance component. The default color is white.

(1) Add to Favorites
(2) Color comparator
(3) Main component selector
(4) Color selector
(5) Main component slider
(6) Color component values

Use the slider to modify the color intensity, or click to choose another color in the Color dialog box. Three tabs are available:

Definition lets you enter a value comprised between 0 and 255 for any of these boxes.
The overall color of a light source is composed of three independent colors: diffuse, ambient, and specular. The values of these colors are given in RGB mode or in HLS mode by three real positive values:
- HSL (Hue, Saturation, Luminance) model is an intuitive, easy to use tool for describing or modifying a color.
  Hue is the "color" of the color. It is the name by which the color is designated and is used to define the required color.
  Saturation is the intensity of the color. The higher the number, the more intense the color.
  It is used to tune the purity of the color.
  Luminance is the brightness of the color, that is, the degree to which the pure color is diluted by white or black. The larger the number, the lighter the color. It is used to adjust intensity.
- RGB (Red, Green, Blue) model is a more physical model. It is based on the tri-stimulus theory of the human perception system. This model is usually used to define, with a high precision, the three primary components of the color.
Favorites displays all the favorite colors selected by you by clicking Add To Favorites in the Definition and Catalog tabs.
Catalog displays standard colors defined. You can add a color to your favorites by clicking Add To Favorites.

When the intensity value exceeds 1, the color saturates.

In the first example, all color components (R, G, and B) are multiplied by intensity:



R,G,B = 10,100,10 Intensity = 0.2 Result = 5,50,5	R,G,B = 10,100,10 Intensity = 0.5 Result = 10,100,10	R,G,B = 10,100,10 Intensity = 1 Result = 30,255,30

In this second example, when the intensity is higher than 1, R and B are the only components to be multiplied because G is already saturated:



R,G,B = 40,255,40 Intensity = 0.2 Result = 20,128,20	R,G,B = 40,255,40 Intensity = 0.5 Result = 40,255,40	R,G,B = 40,255,40 Intensity = 1 Result = 120,255,120

To define more precisely the intensity, click to open the Intensity dialog box.

This dialog box lets you define three coefficients using the slider or by entering a value directly in the corresponding box:

Ambient: defines the intensity of light emitted in any direction by the object, even if not lit by any light source
Diffuse: defines the intensity of light diffused by the object when lit by a light source
Specular: defines the intensity and color of light reflected in one particular direction. This coefficient affects the highlight on shiny surfaces.

These three coefficients only impact the material aspect. They are identical to those used to define material lighting properties (and are used in combination with them) but have no influence on the material definition.