How does stray light influence high absorbance measurements?

“Real verses “Measured” Absorbance - The plot above shows how stray light not only sets the upper absorbance level, but also is responsible for deviations from Beer’s Law. The black dotted line is the theoretical Beer’s Law relationship without any stray light affects; whereas, the solid red line has incorporated an instrumental stray light component of 0.001 %T. Note that the linear Beer’s Law relationship is maintained up to an absorbance of about 4. After a short region of increasing decline, the measured absorbances plateau as they approach the stray light values for the instrument. Stray light photometric artifacts always yield lower than actual absorbance values.

The percent error from Beer’s Law linearity due to the consequences of stray light is graphed above. The value for the stray light in this plot is the same as the previous page, 0.001 %T. Note that the error does not become significant (i.e. 1%) until around 4 absorbance, One percent is typically the linearity cutoff, since above 1% the error increases dramatically with small increases in absorbance.