# Thermal Absorptance vs Thermal Hemispheric Emissivity in EnergyPlus Objects: Implications in writing code for codebase

Hello!
Wanted to confirm / get some clarification in terms of nomenclature when writing code for LBT, topically honeybee_energy:

In:

``````WindowProperty:FrameAndDivider,
TestFrameAndDivider, ! Frame/Divider Name
0.05, ! Frame Width
0.04, ! Frame Outside Projection
0.03, ! Frame Inside Projection
5.0, ! Frame Conductance
1.3, ! Ratio of Frame-Edge Glass Conductance to Center-Of-Glass Conductance
0.8, ! Frame Solar Absorptance
0.8, ! Frame Visible Absorptance
0.9, ! Frame Thermal Emissivity    !!!!!!!!!!!!!!!!!!
DividedLite, ! Divider Type
0.03, ! Divider Width
2, ! Number of Horizontal Dividers
2, ! Number of Vertical Dividers
0.03, ! Divider Outside Projection
0.03, ! Divider Inside Projection
5.0, ! Divider Conductance
1.3, ! Ratio of Divider-Edge Glass Conductance to Center-Of-Glass Conductance
0.8, ! Divider Solar Absorptance
0.8, ! Divider Visible Absorptance
0.9, ! Divider Thermal Emissivity  !!!!!!!!!!!!!!!!!!
0.7, ! Outside Reveal Solar Absorptance
0.25, ! Inside Sill Depth (m)
0.6, ! Inside Sill Solar Absorptance
0.2, ! Inside Reveal Depth (m)
0.5; ! Inside Reveal Solar Absorptance
``````

Note the nomenclature: ‘Thermal Emissivity’

## Now in the following, ‘Absorptance’, ‘Emissivity’; is ‘same same’:

According to Gustav Kirchhoff’s law of thermal radiation, the absorptance of a medium is identical to its emissivity: the more an object can absorb electromagnetic radiation, the more it can also emit the same kind of radiation.

The absorptance of a surface can actually depend on the direction of incoming light. Therefore, one can specify a directional absorptance and a hemispherical absorptance; the latter is an average over different directions. Also, the absorptance is generally dependent on the optical wavelength.

## …

There Is stated a Difference between Directional and Hemispherical absp/emissivity.
and in the E+ I/O reference, 'Thermal Hemispherical Emissivity is explicitly used nomenclature wise:

## 1.10.59.1.9 Field: Frame Thermal Hemispherical Emissivity

The thermal emissivity of the frame, assumed the same on the inside and outside.

## Actual question:

While writing code for WindowProperty:FrameAndDivider, would utilizing the explicit nomenclature used in the E+ object be preferred over trying to maintain synchronicity with the rest of the codebase utilizing the nomenclature “Absorptance”? Or would it be preferred to utilize nomen “Absorptance” in code instead?

Thanks!

@TrevorFedyna

Are any thermal absorptance/emissivities properties in EP defined in terms of “direction”? While there is a difference between directional and hemispherical absorptance/emissivity, I believe all thermal values will be defined strictly as hemispherical. Thermal radiation is a function of the temperature of it’s surroundings, so it’s easier to define as a hemispherical property. In contrast, the transfer of solar radiation onto a surface is partially a function of the angle between rays perpendicular to the sun position and the surface normal, so it makes sense to define solar absorptance in terms of directions.

Therefore, taking into account Kirchhoff’s law, `absorptance` already implies `hemispheric_emissivity` when referring to thermal radiation, and there isn’t a need to change the convention. .

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Awesome! Thank you! Greatly appreciate your clarification and explanation!