Lux
- Thread starter Lonny
- Start date
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Lol. Now a days most just use the camera.
It's just as hard to convince photographers to use a meter too. Much less Hollywood types I work with.
Now everyone has a complex station to evaluated exposure on set. Trying to get exposure down to the most precise degree. Forgetting that there's three to five stops of latitude on the film fot the exposure. And also forgetting we made a lot of great movies without that stuff and the new cameras are easier and better. than they have ever been to use.
The parallels run deep.
mcarroll
10K Club member
My understanding is that it's in Inverse-square law thing.....50% distance = x2 intensity I think is one way to put it, but that link has some good visuals, as well as several meaningful ways the law can be expressed.
Lenses can change intensity, but not by ignoring the inverse square law.
Lenses change the intensity by changing the space covered by the light.
For example, 60º lenses (imperfectly) put the same quantity of photons into a 50% smaller space than 120º lenses, doubling the concentration, or intensity.
Switching to 30º lenses would put those photons into a space that's another 50% smaller, doubling the concentration of photons again.
But take a measurements along a consistent vector from that light source to the object being lit, no matter the lens on it, and you should still see the inverse square law at work.
I am not all that great at math or physics, so please correct me if I'm wrong.
(Reflectors on T5's or halide bulbs work similarly to lenses, though are more limited in effect.)
My understanding is that it's in Inverse-square law thing.....50% distance = x2 intensity I think is one way to put it, but that link has some good visuals, as well as several meaningful ways the law can be expressed.
Lenses can change intensity, but not by ignoring the inverse square law.
Lenses change the intensity by changing the space covered by the light.
For example, 60º lenses (imperfectly) put the same quantity of photons into a 50% smaller space than 120º lenses, doubling the concentration, or intensity.
Switching to 30º lenses would put those photons into a space that's another 50% smaller, doubling the concentration of photons again.
But take a measurements along a consistent vector from that light source to the object being lit, no matter the lens on it, and you should still see the inverse square law at work.
I am not all that great at math or physics, so please correct me if I'm wrong.
(Reflectors on T5's or halide bulbs work similarly to lenses, though are more limited in effect.)
I have tested PAR reading drop off with a Ledil Brooke-W 56deg reflector and standard plastic 60deg optic and the drop off change is quite drastic with the optic. On an 18" tall tank with the optic the PAR would be 600 at the top and 150 at the bottom. The diode with reflector at same distance would read 250 at the top and 100 at the bottom. Not to mention visually the optic cast a very visible cone of light whereas the reflecter you could not see a visual spread cone.
yup. makes sense.
creates shading,hot spots, poor coverage ,"shimmer, and likely anomalous par readings for the uninitiated.
its why I pulled most of the lenses from my leds selectively.
mcarroll
10K Club member
I guess I thought we were talking about mounting height of that fixture.....that's what dictates light at the surface, so is still a significant facet of the setup to consider. Which I think was your point, Gus....wasn't it? Light transmitted through air roughly gets inverse-squared.
In water it's significantly more complicated to estimate with a lot of accuracy, but just to have an estimate to compare with, I think I've seen some use a 50%:x4 ratio....half the distance gives 4x the intensity.
In water it's significantly more complicated to estimate with a lot of accuracy, but just to have an estimate to compare with, I think I've seen some use a 50%:x4 ratio....half the distance gives 4x the intensity.
mcarroll
10K Club member
Oh, I just noticed you were comparing an LED in a reflector...almost a different beast.
They do behave a lot different in a reflector than a T5 or halide bulb just since in an LED the vast most of the light is already being projected away from the reflector, directly at your target.
The reflector is only catching that additional 10% of waste light that's drastically misdirected and much less intense than the LED's core light. Its effect is necessarily limited because of this.
I'd guess that a halide or T5 reflector can deal with around two thirds of the primary light from your bulbs – as we know, the effect can be extremely significant.
They do behave a lot different in a reflector than a T5 or halide bulb just since in an LED the vast most of the light is already being projected away from the reflector, directly at your target.
The reflector is only catching that additional 10% of waste light that's drastically misdirected and much less intense than the LED's core light. Its effect is necessarily limited because of this.
I'd guess that a halide or T5 reflector can deal with around two thirds of the primary light from your bulbs – as we know, the effect can be extremely significant.
mcarroll
10K Club member
So apparently an LED is techically not a point-source light, but instead based on Lambert's cosine law.
Still seems pretty spherical, but technically not.
I kinda think inverse-square would still be at least roughly correct since the projections are similar, if not identical.
Anyone?
Still seems pretty spherical, but technically not.
I kinda think inverse-square would still be at least roughly correct since the projections are similar, if not identical.
Anyone?
Scott.h
Valuable Member
If you want solid par numbers with an apogee qm500 get with me in a week or so. I just ordered the exact fixture. My tank is 25 inches deep and the light will hang exactly 10" from the top of the fixture. I think the par numbers might be higher then mentioned above. Reason being.. Skip to around the 6 min mark. That's an Ati, but smaller fixture also. But at 6" you'll see 550ish par, more than any coral that I know of needs
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Please update, tag, mention, PM or what ever me. Id like to know what you get.
