> I have known the value of solar irradiance in 817nm band is 1.08279
> W/(m2*nm).How can I define it?

817nm is on the verge of not being visible at all to humans, you can 
look up the XYZ colour value of it from here:

http://www.cvrl.org/database/data/cienewxyz/lin2012xyz10e_5_7sf.htm

XYZ=(3.815429E-06,1.523114E-06,0.000000E+00)

Compare the size of those numbers to the ones around 500 nm, they are tiny.

If you want a realistic image output in sRGB then you can easily convert 
the XYZ values to RGB.

Wikipedia has the matrix equation here:

http://en.wikipedia.org/wiki/SRGB#The_forward_transformation_.28CIE_xyY_or_CIE_XYZ_to_sRGB.29

That will give you linear RGB which POV will accept with the "rgb" 
keyword, there is no need to do the second step for gamma correction 
(although you could if you wanted and then pass the values to POV with 
the "srgb" keyword, the results should be identical).

> "light_source {<sun_x,sun_y,sun_z> color rgb
> 1.08279 } "I set statement as above in pov,but I doublt it is wrong,because
> 1.08279 is a solar irradiance,color rgb is color ，but I donnot know how
> to convert them or how to define solar irradiance.beg a hot to help me !thank
> you!

The "scale" of lighting in the scene is up to you, a bit like the 
exposure of a camera. I suspect with a 817nm light source you'll need to 
multiply up the brightness by a large amount to get anything visible though.

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