Basically is boils down to this...plants can use red light most efficiently and blue light triggers the plant to take in more CO2
. Any lamp that fails to provide light that covers both red and blue wavelengths will not grow plants as efficiently or produce the best colors or growth patterns. Many plants show different leaf sizes when grown under lamps that have a higher amount of a red or blue wavelengths. That being said, actinic do grow plants if they are combined with other full spectrum lamps. Personally I prefer adding an actinic lamp for the color rendering bonus it gives...but this is only advantageous if their are three or more lamps in your fixture. The 50/50 lamps are fine to use, they do not cause algae, but they don't supply the same intensity across all wavelengths that benefit plants the most.
Where the real problem occurs is when folks believe the stuff written online about how actinic lamps are useless and try to run these 50/50 lamps as low light tanks without CO2
...in most situation the 50/50 lamps provide too much intensity to run a tank without CO2
Actinic usually feature a single strong but often broad peak in their spectra at about 420nm, which nicely corresponds with the absorption spectrum of chlorophyll (a and b)
Here's a brief quote from Dr. Timothy A. Hovanec of Marineland:
Plant tanks require the correct lighting to be successful. The number one reason for lack of success in growing plants in an aquarium has to be the use of the wrong lamp. Plants have two types of chlorophyll, a and b. Chlorophyll a absorbs light at 405 and 640 nm. Chlorophyll b has a peak absorption at 440 and 620 nm. Plant lamps are designed to emit light at the red wavelengths to duplicate the job of the sun. But too much red color can cause aquatic plants to grow tall and thin. For best results, use a daylight (5,000 Â¡K) lamp such as an Aquasun, Ultralume 50 or Chroma 50 in combination with an actinic white or actinic day lamp. The actinic day or white lamp is a mixture of 50% actinic (blue light) and 50% daylight.