Besides the amount of radiation reaching a plant in the PAR region of the spectrum, it is also important to consider the quality of such radiation. Radiation reaching a plant contains entropy as well as energy, and combining those two concepts the exergy can be determined. This sort of analysis is known as exergy analysis or second law analysis, and the exergy represents a measure of the useful work, i.e., the useful part of radiation which can be transformed into other forms of energy.
The spectral distribution of the exergy of radiation is defined as:
One of the advantages of working with the exergy is that it depends not only on the temperature of the emitter (the Sun), , but also on the temperature of the receiving body (the plant), , i.e., it includes the fact that the plant is emitting radiation. Naming  and , the exergy emissive power of radiation in a region is determined as:
Where  is a special function called the polylogarithm. 
By definition, the exergy obtained by the receiving body is always lower than the energy radiated by the emitting blackbody, as a consequence of the entropy content in radiation.
Thus, as a consequence of the entropy content, not all the radiation reaching the Earth's surface is "useful" to produce work. Therefore, the efficiency of a process involving radiation should be measured against its exergy, not its energy.
Using the expression above, the optimal efficiency or second law efficiency for the conversion of radiation to work in the PAR region (from  400 nm to  700 nm), for a blackbody at  = 5800 K and an organism at  = 300 K is determined as:
about 8.3% lower than the value considered until now, as a direct consequence of the fact that the organisms which are using solar radiation are also emitting radiation as a consequence of their own temperature. Therefore, the conversion factor of the organism will be different depending on its temperature, and the exergy concept is more suitable than the energy one.