Unusual DGI value Changes

asked 2019-05-02 21:16:59 -0500

updated 2019-05-03 19:06:57 -0500

Dear Community,

I am trying to obtain glare index of a defined reference point in a single space, the problem comes when I am changing the illuminance set point for lighting control. As you see below, first and second columns show DGI values for a single day when illuminance set point are set as 500 and 1000 lux respectively, which causes changes in DGI value that does not have anything to do with lighting control in theory.

P.S. Third column shows the difference linked with a graph.

Any thoughts?

Regards

C:\fakepath\DGI.png

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"In the last relationship, the background luminance is approximated as the larger of the background luminance from daylight and the average background luminance that would be produced by the electric lighting at full power if the illuminance on the room surfaces were equal to the setpoint illuminance. In a more detailed calculation, where the luminance of each room surface is separately determined, B(i ) would be better approximated as an area-weighted luminance of the surfaces surrounding a window, taking into account the luminance contribution from the electric lights." PAGE 127, Engineering

Amir Tabadkani ( 2019-05-06 19:19:10 -0500 )edit

Based on above calculation method for glare index, what does it mean ? I am not quite sure if I understood it completely

Amir Tabadkani ( 2019-05-06 19:19:50 -0500 )edit

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answered 2019-05-09 14:04:53 -0500

Samuel de Vries
346 ●2 ●6

DGI assesses glare on the basis of contrast within the distribution of luminance within the field of view of the occupant, where high luminance parts of the view (sun or bright sky) contribute positively and background luminance (the general brightness of the overall image) contributed negatively to the DGI value. In other words: the higher the ambient light from artificial lighting/daylight the less bothered an occupant might be about a glare source such as the sun. I cannot follow the numbers you show but what I expect is that with more artificial lighting DGI values should be lower.

This presentation by Jan Wienold might help: https://www.radiance-online.org/commu...

Note also the limitations of DGI as an indicator. Personally I would also be careful in using Energyplus for performance predictions of glare.

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Dear Samuel,

Thanks for your reply. Based on DGI formulation, it should assess glare due to both artificial and natural light, and it takes artificial lighting contribution portion in Background Luminance (Lb), as mentioned in original glare equation by Hopkinson and here also, but then I have two further questions:

Amir Tabadkani ( 2019-05-09 18:52:30 -0500 )edit

a. How E+ calculates the luminance of electrical lightings? Does E+ takes the lighting level that we define in Watts and lighting schedule as the source to calculate?, however, again it is the power and is not related to luminance, Or the only other related field is the Daylighting:Control setting which dim the lights based on reference point illuminance threshold, but again how it takes the luminance of artifical lighting?

Amir Tabadkani ( 2019-05-09 19:00:01 -0500 )edit

b. As mentioned again in Eng. document, the maximum contribution portion of Lb in glare index, is at most 1, however, as you see in the picture I attached it is more than 1 in most hours.

I agree with you DGI itself is not reliable enough for glare calculation especially most papers used other software, but at the moment if I want to define a multi-objective control algorithm, my only option is EMS currently.For sure, I will use Radiance and Honeybee for a better calculation of glare and visual comfort, but I need to first figure out what are the possibilities and limitations we have with E+

Amir Tabadkani ( 2019-05-09 19:02:21 -0500 )edit

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