The smaller the wavelength of the photon, the more energy it carries. So the work that a single photon can do is related to it's wavelength and is a known, and measureable, amount. Light intensity, on the other hand, refers to the number of photons measured at a given time and has nothing to do with wavelength. You can shine shine a brighter light at an object are more photons available but the photons themselves still carry the same amount of energy. We are interested in the energy that can be used in a chemical reactions, specifically chemical reactions inside living cells.
Energy enters chemical reactions in one of two ways.
This energy can come from collisions with other molecules or, in a photochemical reaction, the energy is supplied by photons. Heat of reaction Energy released by a chemical reaction that is then available to activate further reactions through collisions. The energy in a mole of photons is measured in units called einsteins . This one to one relationship is a key to understanding how light and matter can interact in photobiology 1 and is known an Einstein's Law of Photochemistry. The energy one einstein of light carries can be easily determined if the wavelength is known.