illuminate output from DIVA Daylight Grasshopper component and RGB in .dat file

Hello, I"m new to DIVA.

My issue is that illuminance output from the Grasshopper component and Illuminance (lux) calculated by me based on the RGB data set stored in .dat file are slightly different.

I've already found the following articles and tried both but my self-calculated illuminance is always a bit different from those output from the Grasshopper component.

(R*0.265074126 + G*0.670114631 + B*0.064811243) * 179 = illuminance.

(R*0.265 + G*0.67 + B*0.065)*179

What is odd is that the percentage of this small difference does not take exactly same value for each sensor node. It ranges from  -7% to +8%. (I mean if this is the matter of coefficients, the ratio between self-calculated value and those output from Grasshopper component must stay same over the sensor nodes.)

For example, when RGB in .dat file is

1.222799e+001 1.222799e+001 1.222799e+001

the illuminance by Grasshopper DIVA Daylight component is 2156.33245.

(Simulation type is Illuminance)

But my calculation is 2188.81021

Does anybody suggest me the precise formula used in the Grasshopper DIVA Daylight component?

Btw, as one can see in the attached image, the gradation images generated from both illuminance data sets visually seem same. So this difference might be ignore-able.

Thank you.

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