[Radiance-general] Macbethcal Reflectances

[Greg Ward]

Hi Marcus,

You must ensure that there are no lighting or gain changes between  
the scanning or capture of the Macbeth chart and the swatches,  
otherwise you will have an absolute calibration error.  If you are  
using a flatbed scanner, set the exposure to manual and fix it at a  
single value.  If you are using a camera, set it on manual mode and  
use a constant illumination with the chart in the same position as  
the swatches in your two photographs.  I often shoot outdoors to  
assure even illumination.

This is the most likely culprit.

-Greg

> From: "Marcus Jacobs" <marcdevon at hotmail.com>
> Date: October 18, 2005 12:20:13 PM PDT
>
> Dear Group,
>
> I am using macbethcal/pcomb to determine the color and reflectance  
> of various scanned paint swatches. One thing that has come to my  
> attention is that the overall reflectance seems out of range. For  
> example, a creamy white color once calibrated has an overall  
> reflectance (determined by ximage and grey(r,g,b)) equal to .93.  
> From RWR I think it was mentioned that a reflectance of greater  
> than 80% may be unrealistic. Although I have had some colors out of  
> gamut, particularly Cyan,(noticed by looking at the debug.pic from  
> macbethcal), there seems to have been no apparent issues with the  
> white, neutral 8, neutral 6.5, neutral 5, neutral 3.5, black (I am  
> assuming that these patches are used to calibrate the reflectance)  
> during the generation of the calibration file. Has anyone else  
> encountered this issue?
>
> Regards,
>
> Marcus
>