CHAPTER XIII 



ACTION SPECTRA AND ABSORPTION 



SPECTRA 



Harold F. Blum, jVational Cancer Institute and Princeton University 



A . Introduction 417 



B . Absorption Spectra 420 



1 . Quantitative Measurement of Absorption Spectra 422 



Absorption Laws and Units 422 



Measuremtrnts 425 



2 . Absorption Spectra of Substances in Living Systems 427 



3 . Effect of Light Scattering 428 



Attenuation CoofficicMit 430 



4. Fluorescence 430 



C . Action Spectra 431 



1 . Sources and Measurement of Radiation 432 



2. Photochemical and Photobiological Aspects 435 



Reciprocity Law 435 



Quantum Efficiency and Quantum Effectiveness 437 



3 . Estimating Absorption 438 



4 . General Remarks 445 



References 448 



Addendum 449 



A. INTRODUCTION 



Photochemical reactions taking place in hving systems underUe a 

 number of phenomena, ^vhich may be classed together as photobiologi- 

 cal -processes ; some are of the greatest importance to the living organ- 

 ism, others relatively trivial. Action spectra and absorption spectra, 

 to the consideration of which this chapter is devoted, are useful tools 

 in the study of the photochemical reactions concerned in these proces- 

 ses. To understand their use only the elements of photochemistry 

 need be understood, although the photochemical reactions themselves 

 may be quite complicated. The first step in any photochemical 

 process, in vitro or in vivo, known as the primary reaction, is repre- 

 sented by: 



417 



