SRE - Concepts

Spectral Radiant Emittance (SRE)  represents the intensity of light when a color other than white (100%) is selected. When a color is assigned to a light source, it behaves like a color filter, decreasing the output of the luminaire as indicated by the SRE figure. Note: The exception to this decrease in light output is when the color is selected from the Source Color tab in the Color/SRE Selector dialog.

The colors displayed in the Spectral Radiant Emittance dialog are automatically gamma corrected based on the following equation:

where C′ is the gamma-corrected displayed color value and C is the specified RGB value.

Luminous Area vs. Housing Color

The Color Selection dialogs show actual colors or gray-scale shades that correspond to a particular Reflectance or Spectral Radiant Emittance (SRE). All surface colors in AGi32 are based on their Reflectance or Spectral Radiant Emittance. Reflectance is a property of a non-luminous surface, such as the luminaire housing or ceiling or wall surface. Spectral Radiant Emittance is the light transmittance value of a transparent or translucent surface, such as the lensed portion of a recessed fluorescent luminaire, downlight, or floodlight, based on the combination of its red, green and blue spectral transmission characteristics.

Luminous Area

When luminous surfaces are specified, they are assigned an SRE value and a corresponding color that indicates the color of the emitted light.

The luminous area of a luminaire symbol may be assigned a luminous color. The default luminous color of a symbol is white (SRE = 1). When assigning a luminous color other than white:

Computer graphics cannot easily simulate light Spectral Power Distribution (SPD) data, whether from a lamp, sunlight, or color filters. However, it is possible to convert SPD data to an RGB equivalent. While this is not a perfect conversion, it is usually adequate for convincing visualizations. See the Modeling Lamp Color (CCT) and/or Luminous Area Color as a Gel or Filter heading below for additional information.


Modeling Lamp Color (CCT)

When the Source Color method is used to model lamp Correlated Color Temperature (CCT), the emitted lumens are NOT decreased.

There are three important caveats associated with this process:

  1. Lamp spectral and surface spectroradiometric data would be required to accurately model the effects of color reflectance. In other words, a Spectral Power Distribution (SPD) curve for the light source and a similar curve describing the spectral reflectance characteristics of each of the surfaces in the space would be required for an accurate depiction of the effect of lamp color.
  2. There is currently no way to accurately reproduce the visual effects of lamp color using RGB monitors or any other 3-color  medium.

AGi32's Automatic adjustment (Display Properties, Color Temperature tab) will adjust the color appearance of the rendered image based on lamp color selected in the Color/SRE dialog. This is done to account for the chromatic adaptation that occurs in real life. The adjustment is based on a mathematical formula that predicts the change in appearance of most colors due to chromatic adaptation as color temperature is changed. Alternatively,as this is a subjective estimate only, the user may set the environment CCT to any value that will give the desired visual results. See Display Properties for more information on making adjustments.

Important: The accuracy of the Automatic adjustment is highest for sources with a high CRI (70 or higher). The lower the CRI, the more difficult it becomes to predict the chromatic adaptation color shifts as the color temperature is changed.

Compatibility with earlier versions: If an AGI file version 2.2 or later, with Source color applied, is opened in an earlier version of AGi32, the Source Color will be applied as a Color Filter, with the associated reduction in light output. To make the version 2.2 file compatible, set the Source Color (CCT) to None (Color/SRE dialog).


Luminous Area Color as a Gel or Filter

When the Color Filters, Color Mixer or Favorite Colors methods are used to apply a color to a luminous surface, the emitted lumens are decreased. This is acceptable when using color to model the effect of a colored gel over the light source, since the gel has a transmittance value associated with it.

There are some important caveats associated with this process:

  1. Luminous color depreciates lumen output, as would be expected when using a filter or gel.
  2. Light source (lamp plus gel) spectral data and surface spectroradiometric data would be required to accurately model the effects of color reflectance. In other words, a Spectral Power Distribution (SPD) curve for the light source-gel combination and a similar curve describing the spectral reflectance characteristics of each of the surfaces in the space would be required for an accurate depiction of the effect of gel or filter color.
  3. There is currently no way to accurately reproduce the visual effects of gel or filter color using RGB monitors or any other 3-color  media.

Rosco and Lee Color Filters

You may select a Rosco or Lee color filter by name or number. RGB values for Rosco color filters were derived from proprietary filter spectral power distributions provided by Rosco Laboratories, Inc. RGB values for Lee filters were derived from the filter CIE Tristimulus values (X, Y, Z) obtained from www.leefilters.com.

The RGB values are based on industry standard ITU BT.709 phosphor chromaticities for HDTV and computer monitor displays with a white point of 6500K.


Equation for Surface Reflectance and SRE

The relationship used by AGi32 between Color and Reflectance (including SRE) is based upon color weighting factors specified in ITU-R Recommendation BT-709, "Basic Parameter Values for the Studio and for International Programme Exchange (1990)," [formerly CCIR Rec. 709], ITU, 1211 Geneva 20, Switzerland.

The weighting factors are representative of human visual sensitivity to the color phosphors used in contemporary computer monitors.

Reflectance = (0.2125R + 0.7154G + 0.0721B) / 255


SRE Values for LED Dominant Wavelengths

The RGB values listed in the tables below will allow you to simulate the perceived LED color associated with each dominant wavelength color and standard LED colors.

Note: Applying these values will act as a color filter on the luminaire and reduce the light emitted. Therefore, it is important to remember to add the reciprocal of the SRE value as a multiplier in the LLF calculator (as a User Defined Factor) in order to maintain the light output from the luminaire.

LED Color and RGB decimal Values

AGi32 Scaled RGB Values

Royal Blue (0.05, 0.00, 0.95)

( 13, 0, 255)

Blue (0.00, 0.11, 0.89)

(0, 31, 255)

Cyan (0.00, 0.63, 0.37)

(0, 255, 149)

Green (0.00, 0.77, 0.23)

(0, 255, 76)

Amber (0.70, 0.30, 0.00)

(255, 109, 0)

Red-Orange (0.97, 0.00, 0.03)

(255, 0, 8)

Red (0.92, 0.00, 0.08)

(255, 0, 22)