# calculating correct illuminance from irradiance

Dear all,

i am new to Radiance and i went trough oodles of manual pages, R.w.R.- book, digest etc., but i couldn�t find an
This is what i did:

1. I collected illuminance data for a *'Desisti' 1 KW* incandescent luminaire in our lighting laboratory.
2. I converted the illuminance values to luminance values and the color temperature of 3200K to RGB 1.377 0.913 0.351
-- I used the color temperatur conversion from the Radiance book p.431. I need to see the color temperature in the pictures.
3. Finally i created an '.ies file' and used 'ies2rad' to get Radiance data.
-- Now i know, that instead i could have created the' .dat ' file myself by dividing the luminance data by 179 lm/w --
4. I used 'rtrace' to measure my 'virtual' light source to verify my collected data
-- I used the same measuring distance as in the lab --
5. My lamp ( source is a 'ring' ) is at 1 0 4 pointing downwards at 0 0 -1
6. at first i used 'rtrace' for the irradiance data:

···

*
*echo "1 0 0 0 0 1" |rtrace -I -h lux_test.oct

/ 4.154708e+01 2.754719e+01 1.059043e+01 /*First question: Are these irradiance values for RGB with the unit 'w/square meter' ?*

7. now i measured the illuminance:

\$ echo '1 0 0 0 0 1' |rtrace -I -h lux_test.oct |rcalc -e '\$1=*47.4**\$1+*120**\$2+*11.6**\$3'

*5397.84338*

-- this value is really satisfying, my collected data is *5400 lux!*

*Second question: Where do the factors of *47.4/179 =* 0.26 , *120/179 =* 0.67 and *11.6/179 =* 0.065 come from?

*I only know the standard factors from video for the EBU - Phosphors. There the luma value is: Y = 0.299*R+0.587*G+0.114*B
I tried these ones, but the result worse, in fact it is *5334 lux*.

Maybe someone can help me.

Best regards,

Christian

--
Christian Fusenig
Diplomand, Medientechnik
Hamburg University Of Applied Sciences

20144 Hamburg

email: [email protected]

mobile: ++49 179 5975845

Hi Christian,

Thanks for reading through the documentation. I know it can be a frustrating exercise, trying to find what you're looking for in the many, widely dispersed sources available. It looks like you did a thorough job.

...
6. at first i used 'rtrace' for the irradiance data:
*
*echo "1 0 0 0 0 1" |rtrace -I -h lux_test.oct

/ 4.154708e+01 2.754719e+01 1.059043e+01 /*First question: Are these irradiance values for RGB with the unit 'w/square meter' ?*

Yes, sort of. If your scene consisted of purely gray surfaces with no radiation outside the visible wavelength limits, then the numbers would correspond to watts/meter^2 exactly. However, the real world has stuff going on in UV and IR wavelengths (and beyond), and these should be included in a true measure of radiant flux.

More technically, the values used in Radiance (by convention) correspond to a transformation of the CIE XYZ coordinates to an RGB color space, with a conversion factor of 179 lumens/watt to make a vague correspondence to radiometric units with the above assumptions. So long as the same conversion is applied in reverse when getting back to photometric quantities, all is well. The exact color transform used is your next question.

7. now i measured the illuminance:

\$ echo '1 0 0 0 0 1' |rtrace -I -h lux_test.oct |rcalc -e '\$1=*47.4**\$1+*120**\$2+*11.6**\$3'

*5397.84338*
-- this value is really satisfying, my collected data is *5400 lux!*

*Second question: Where do the factors of *47.4/179 =* 0.26 , *120/179 =* 0.67 and *11.6/179 =* 0.065 come from?

*I only know the standard factors from video for the EBU - Phosphors. There the luma value is: Y = 0.299*R+0.587*G+0.114*B
I tried these ones, but the result worse, in fact it is *5334 lux*.

The RGB color space used in Radiance is defined by the following constants, defined in ray/src/common/color.h:

#define CIE_x_r 0.640 /* nominal CRT primaries */
#define CIE_y_r 0.330
#define CIE_x_g 0.290
#define CIE_y_g 0.600
#define CIE_x_b 0.150
#define CIE_y_b 0.060
#define CIE_x_w 0.3333 /* use true white */
#define CIE_y_w 0.3333

These same constants are defined in ray/src/cal/cal/xyz_rgb.cal for convenient conversion between XYZ and RGB coordinates. The Y(R,G,B) function from this file reduces to:

Y(R,G,B) = 0.26510582*R + 0.67010582*G + 0.0647883598*B;

which is basically the same as what you have above. Except for the difference in the green channel chromaticities and the white point, these primaries are in fact the same as used for the standard sRGB color space. If it wouldn't have thrown everything off slightly with little benefit, I might have switched to the sRGB primaries in Radiance when this standard came out.

In fact, the color primaries in Radiance can be whatever you choose them to be. As I said, this is all by convention, and the actual calculation only converts from RGB to luminance to determine whether or not to truncate the ray tree with the -lw option. Since this is really a minor effect, it actually doesn't matter what color space you render in, and if you want to know how Radiance may be used to obtain more accurate color results, consult the following paper:

I hope this helps.
-Greg