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angled light cosine correction
Old 09-16-2005, 07:50 AM   #1 (permalink)
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Hey All,

I dont know if this really affects what we do however, I thought I would ask. As a beam of light is tilted so that it strikes a surface at an angle, the area illuminated by the beam increases. Since the number of quanta of light in the beam remains constant, the illumination on the surface decreases. This decrease is proportional to the cosine of the angle, which the incident beam makes with a perpendicular to the surface.

So knowing this ....when we place a hair light above and angled down...or a main light high and angled down, Do any of you all worry about this when taking light meter readings or is my scientific mind just in hpyerdrive?

Thanks,
Michael
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Re: angled light cosine correction
Old 09-16-2005, 09:43 AM   #2 (permalink)
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You're beyond Hyperdrive... In fact, I think you've "Gone Plaid" [img]/ubbthreads/images/graemlins/tongue.gif[/img]
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Re: angled light cosine correction
Old 09-16-2005, 09:48 AM   #3 (permalink)
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Your math problem is giving me tired head. The meter will read that correctly, so you can put your fears to rest. Here is something the meter will not read and which will have an effect on the situation. A principle of lighting is that the angle of incidence equals the angle of reflection. A light from behind skims off the subject and into the lens. It will always appear brighter than a light from the front of the same intensity. In fact, a hair light a full stop down from the main will still appear brighter from camera angle. The meter (assuming incident) does not read this correctly as it is always pointed at the light. A spot reading from camera position would read this light correctly. A very sensitive and exact meter is the human eye. It is a great comparitor. You can easily judge the relationship of the lights visually IF your model lights are proportional to the flash. The meter reads quantity of light, which the eye is not so good at. I meter the main and set everything else visually. Special cases would be the only
exceptions. In my image, all lights are the same intensity.

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Re: angled light cosine correction
Old 09-16-2005, 10:24 AM   #4 (permalink)
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While your theory is correct, it may not make much difference in the long run.

If you spot meter from a distance, then you would get the appropriate reading.

If you used an incident meter from the head position, it likewise should take it into effect.

My meter has a "dome" which can be retracted.

You can get slightly different readings depending upon whether you have the dome up (which is catching all light, including bounce back from around the room, versus retracted which is more precise, but in the end, the difference is fairly negligle.

Mark
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Re: angled light cosine correction
Old 09-16-2005, 04:04 PM   #5 (permalink)
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I have thought about just this problem in the past many times. The difficulty that I have is that since the model's head, or any other part, is probably not flat, the angle and distance that has to travel is subject to a correction based on the overall curvature presented within the plane of the now oval shaped light. This means that the number of quanta within the beam will have different strengths and the problem cannot be solved with a linear equation. That is why I use a light meter, bracket, and take lots of shots of the same thing! Stimulating though, haven't heard "quanta" for a while.

More meds please
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Re: angled light cosine correction
Old 09-16-2005, 04:31 PM   #6 (permalink)
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[ QUOTE ]
Do any of you all worry about this when taking light meter readings or is my scientific mind just in hpyerdrive?

[/ QUOTE ]

i wasn't gonna say something to that effect, but since you did.... uhhh... yes.
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Re: angled light cosine correction
Old 09-17-2005, 12:50 AM   #7 (permalink)
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Kinda figured that Jimmy....lol. Someone one asked me "do u see the light" and my answer was always a consistant and resounding no. Until...I was able to put the science of light behind what we did. Once I was able to do that...i started seeing the light better.
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Re: angled light cosine correction
Old 09-17-2005, 12:54 AM   #8 (permalink)
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Tnansk for the post...I didnt even take into consideration any geometric or linear shapes the light might actually strike..I was thinking about this from a flat surface perspective..lol. Now I need meds...thanks alot...next time dont help....lmao (u know im kidding)..thats for the food for thought.
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Re: angled light cosine correction
Old 09-17-2005, 12:57 AM   #9 (permalink)
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so your saying in effect with the dome up it meaures incident and reflective?
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Re: angled light cosine correction
Old 09-17-2005, 12:58 AM   #10 (permalink)
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lllloooollll.....im speechless.
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