Sun can hit 2000 "PAR" at water surface.. Thing is that's like high noon.. as the sun angle increases PAR decreases so it's sort of comes down to the term.. DLI or daily light integral..
Here is the formula for calculating DLI with a quantum meter.
μmol m-2s-1 x (,3600 x photoperiod) / 1,000,000 = DLI
- μmol m-2s-1 is a reading, or averaged readings, from the quantum meter
- 3,600 is the number of seconds in an hour
- Photoperiod is the period (in hours) of light exposure per 24 hours
- 1,000,000 is the number of μmols per mole
For example, 38.1 μmol m-2s-1 x 3600 seconds/hour x 12 hour photoperiod / 1000000 μmoles/mol = 1.6 mol. m-2d-1 (or 1.6 moles/day).
The more scheduled readings a user takes with their quantum meter throughout the day, the more representative the data becomes. You can substitute an averaged reading (per day) for the single reading in the above equation.
The photoperiod in the equation serves to estimate values for missed measurements, and can be removed from the equation in a scenario where readings are consistently taken throughout the day (such as item 3501 Greenhouse Growth Tracker).
Formula taken from Jim Faust? Light Management I (of III) article found in Spectrums articles section (#A050)
You can hit 40 moles of photons over a day in the open in FL in June w/ 12hrs of daylight.
A tank w/ 100PAR at the substrate over 12 hours is like 4.3...
In a reef tank you hit like 10 (using the generally accepted 200-300PARE for sps if I remember correctly)...18 w/ 500PAR over 10 hour photoperiod..
you can actually exceed "normal" in ocean on the equator daylight exposure in the upper levels of some reef tanks. I don't think a lot of reefers really understand that..or believe it.
Analyses of data revealed the following DLIs:
'Air' DLI = 30 Mol photons/Day
'Reef' DLI = 16 Mol photons/Day
See your 500 PAR if a 10 HR photoperiod actually exceeds a natural reef exposure..
Point is outside doesn't really count..
Thing is you can add as much light as your other factors of CO2 and fertilizer can handle..Problem it's easy to get unbalanced..
Not to mention each species has it's own tolerances for low and high light..
Many demanding plants apparently can be grown w/ lower light in the presence of adequate CO2.
Anyways an answer to your question..
"Sufficient PAR at the right depth
The light should produce sufficient PAR at the substrate depth of your tank.
Rough guidelines as commonly used by aquatic plant community as follows -
20-30 umols (PAR)
Low lighting - suitable for shade aquarium plants.
50 umols (PAR)
Medium Lighting. With good CO2, you can grow any commercially available plant, but may not get the most intense coloration in colored plants. Good for carpets.
90+ umols (PAR)
High lighting - Good for red/colored aquarium plants. Allows for greater density and self-shading effects. Requires good control of tank cleanliness and plant health to avoid algae issues.