- F =PFR/PR + PFR
- Ratio of PFR to total amount of phytochrome
- Indicates amount of phytochrome in plant tissue
- PR absorbs red light more efficiently than PFR absorbs far-red light
- PR converted to PFR quicker than PFR convert back to PR
- Sun Light: more PFR than PR. F value is ~ 0.6.