Today I looked at this fixture and hood more closely. The original Perfecto hood and fixture has glass between the light and the water, mounted on a lip in the hood. The lip is about .38 inch wide all around the opening for the light to go through. The opening is 3 inches by 18.25 inches, so the lip reduces that opening by about 30% in area. That is acting as an aperture to reduce the light entering the tank. So, I trimmed the lip out with a utility knife.
I did some measuring of the light from the fixtue, using a PAR meter, and charted the results:
The red dashed line is what I should get if I replace the mylar reflector material with plain aluminum foil. And, that would, in total, represent a major improvement in the amount of light this fixture provides at the substrate level.
What is the actual PAR reading after the Mylar was replaced with aluminum foil?? Just curious.
Reading from the begining and seeing your setup, reflectors would seem to not be effecient enough to really improve your lighting.
By making a reflector the full lenght of the hood will be better. This will give you full reflection of all available light in the hood. Just placing a reflector behind the light and no other place you are not really reflecting all the light that the bulb is displacing.
Also having a rounded reflector also impeds the reflection of available light. The light will bounce back into the light instead of reflecting into the water column. What needs to be done is to have angles that will better reflect light into the water.
I really think the bulB is also too close to the reflector cutting down the reflected light. What is happenig is when the reflector is too close to the light and you have such a large light the relfected light is being blocked by the bulb.
My final suggestion is to, if you can, move the light further way from the reflector and also make a sheet metal forming fixture with beveled angles to form angles on the reflector. This will give you the ability of reflecting the light away from the bulb and into the water.
The forming tool will be used by placing the sheet metal on the fixture and holding it down with a clamp. Then go at it with a hard rubber mallet forming the sheet metal around the wood forming tool and making sure you have good sharp corners at the angles.
Enough I think I can write a book here on this subject.
Also one last thing, you did the light testing out of the water. What are the results when you test with the probe submersed, if you can submerse it. This is the real test that should be done. Light is reflected out of the water at the surface of the water unless it is at a certain angle,where the light penetrates the water, which I don't remember right now.