bob115 Posted April 9, 2014 Share Posted April 9, 2014 Has anyone used this before? I had a couple questions. Are the par calculations done with the distance through water? Or through air? I'm guessing though water because it has commercial units listed and they're all reef lights. Or is there a better one out there? Link to comment
jedimasterben Posted April 9, 2014 Share Posted April 9, 2014 Don't even look at any of the PAR/watt calculations. Use it for approximating spectra only. All PAR measurements you see for all commercial lights are through air, not water. Link to comment
bob115 Posted April 9, 2014 Author Share Posted April 9, 2014 So you're telling me I need to do the math myself? This'll be fun Link to comment
glennr1978 Posted April 9, 2014 Share Posted April 9, 2014 Don't even look at any of the PAR/watt calculations. Use it for approximating spectra only. All PAR measurements you see for all commercial lights are through air, not water. Hey Ben, do you know what the variance is for LED PAR readings? I'm using an apogee meter and have been told that the readings for LED's are lower than the actual value, and that with MH the reading will be higher than the actual value. I hope that makes sense... I'm switching some corals over from my LED tank to my MH tank and it would be nice to know if I'm putting them under the right amount of light initially. This thread just reminded me to ask you about that... Link to comment
jedimasterben Posted April 9, 2014 Share Posted April 9, 2014 If you can figure out the math needed for blue LEDs, clue me in on that one lol. There's already a fairly accurate one for white LEDs: http://www.plantedtank.net/forums/showthread.php?p=1803323#post1803323 Link to comment
jedimasterben Posted April 9, 2014 Share Posted April 9, 2014 Hey Ben, do you know what the variance is for LED PAR readings? I'm using an apogee meter and have been told that the readings for LED's are lower than the actual value, and that with MH the reading will be higher than the actual value. I hope that makes sense... I'm switching some corals over from my LED tank to my MH tank and it would be nice to know if I'm putting them under the right amount of light initially. This thread just reminded me to ask you about that... Depends on the wavelength. This is the defined response curve provided by Apogee: The black line is the 'perfect' response, aka needs no correction, the blue is the actual response. So, for instance, a 450nm royal blue LED will read at around 87% of its actual reading, so if the meter said you get 150 PAR, you are actually getting 170ish. For white LEDs, I just use a 5% error correction. Link to comment
glennr1978 Posted April 9, 2014 Share Posted April 9, 2014 Depends on the wavelength. This is the defined response curve provided by Apogee: The black line is the 'perfect' response, aka needs no correction, the blue is the actual response. So, for instance, a 450nm royal blue LED will read at around 87% of its actual reading, so if the meter said you get 150 PAR, you are actually getting 170ish. For white LEDs, I just use a 5% error correction. Thanks Ben . Link to comment
bob115 Posted April 9, 2014 Author Share Posted April 9, 2014 So does anyone know the 'standard' way companies rate the luminous flux of LEDs? Like how far away from the die, and so forth. Link to comment
jedimasterben Posted April 9, 2014 Share Posted April 9, 2014 Flux is a measure of the output, that doesn't change with distance. Link to comment
bob115 Posted April 10, 2014 Author Share Posted April 10, 2014 Flux will change. Luminosity won't. Flux is the amount of light energy passing though a given area in a given time. General unit is W/m^2 Luminosity is the amount of light energy emitted per given time. General unit is W Flux will change because the farther you are from the source the more the light is going to spread out, and therefore the less will go through said surface. Flux can be related to luminosity by: flux = luminosity /4(pi)r^2 At least I'm pretty sure that's how it works. The reason I need to know is because most LEDs are rated by their luminous flux and not their radiant flux. Luminous flux isn't helpful for what we want to know about our LEDs. Radiant flux is related to luminous flux by the photopic luminosity function. Which I'll use to give me the actual amount of light energy passing through a surface. Link to comment
bob115 Posted April 10, 2014 Author Share Posted April 10, 2014 Well I've contacted Cree and asked how they rate their LEDs, so we'll see if I can get an answer. Link to comment
evilc66 Posted April 10, 2014 Share Posted April 10, 2014 All LEDs from the manufacturer are tested in an integrating sphere. It's a standard form of measurement (absolute irradiance) and does not take into account the distance from the source to the sensor. It's irrelevant in this test. Link to comment
bob115 Posted April 11, 2014 Author Share Posted April 11, 2014 All LEDs from the manufacturer are tested in an integrating sphere. It's a standard form of measurement (absolute irradiance) and does not take into account the distance from the source to the sensor. It's irrelevant in this test. Ok so the luminous flux rating that they give is measured in such a way that its also the total output. Cool. Well that makes things easy. Link to comment
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