One more thing, I have a nice quality luxmeter. Do I need it in order to set up the light ?
Best idea is to use PAR-meter, instead. PAR corresponds to Photosynthetic Available Radiation, which measures the amount of photons in micromols corresponding to the wavelength range between 400 to 700 nm (the visible light). A luxmeter will integrate the total number of photons within its spectral response, which relies in a priori knowledge of the spectral information of the source, i.e. most of luxmeter have different options to track light, depending on the light source you are monitoring, so no very good for aquariums in which the spectral information of a given light can be very specific, and then many errors can come from this kind of measurements. Additionally, the information in lux is very difficult to translate to photon density, as it is integrating all the intensities per squared centimeter for specific wavelengths and for a given angular fraction and normalized to a given power. In plain words: luxmeter gives you information about the intensity of light respect to a source of reference, meanwhile a PAR-meter will give you the counting of photons per unit of surface and time within a given spectral band.
Regarding your other questions, with a PAR-meter, you can track the amount of light at substrate level, which is commonly fixed as adequate when reaches 40 PAR. Some people not growing carpeting plants aim less. This usually can be used to adjust parameters like the height, or dimming the lights with some tricks if is too much, and so on. Without that, the only way I see to control that is by essay/error: Playing with time, intensity and height of the lamp until you get it right. I think most people do this, as most do not have a PAR-meter.
Regarding the way to handle the light within the cycle, I think there is no much difference. A concept that usually is lost in translation is that what matters is the total amount of photosynthetic photons the plant get within a cycle, which can be obtained of various ways:
1. Strong lights, shorter periods of time.
2. Weaker lights, longer periods of time.
3. Distance of the lamp to the surface.
4. Height of the tank.
5. Spectrum of the bulbs.
Making the evolution of the light to evolve as in nature during the day it could seem better, but I really doubt makes any difference. However, doing so, it can become much complicated to tune the light, as you add also another variable to the system, which is a time dependent photon flow.
However, as said, using PAR can be misleading, because it does not matter your PAR if you do not control the time. In other words: you can have a different PAR but get success by just adjusting the times the lights are on. If less than 40, longer hours, if more than 40, less hours. I think PAR 40 is aimed for tanks with lights on 10h per day, and that can be of reference.
Hope this help.
Cheers,
Manuel
