> I want to simulate the reflectance image or radiance image. My step is:
> 1,construct geometric model
> 2,render geometric model
> Then I have defined  solar irradiance,

The sun's light is a combination of all wavelengths, in your example in 
another thread it was only considering a single wavelength (817nm). You 
need to get the irradiance for all visible wavelengths and then sum up 
the resulting XYZ values before converting to sRGB. This will give you a 
much larger srgb value with a colour close to the middle of the srgb 
space. Not surprisingly, someone has done this before, lookup the colour 
coordinates of various white points like D50, D65 etc.

> the results in a HDR image,all is OK. But the result is not what I wanted ,the
> value is smaller than what I need. I am so confusing.

AFAIK the HDR file format has no units for the colour values, and POV 
certainly doesn't, so the size of the values is arbitrary. If I were you 
I'd do the following two steps:
1. Modify your light source colour to match the sun (start with <1,1,1> 
for a quick test)
2. Modify the scaling factor of the light source (eg FACTOR*<1,1,1>) 
until the output values match what you want.

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