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Light Reflectance Values for Ultrabounce, Muslin rags...


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Hi,

I'm currently finishing off a soft-light large reflector fall-off calculator.

The calculator, in premise, works by modelling the reflecting soft surface as being composed of many point sources, say m*x, whose total light output is the same as total luminous flux of the original light sources (minus losses). If the total luminous flux of the sheet is L, then the luminous flux of a single point source is L / m*n

Of course above is heavily simplified, the goal was to take into account the majority of the variables one can compute while not making user input absurd. The only factor not taken into account is lamberts cosine law in terms of an assumption the point sources emit light isotropically in a solid angle of 2(pi) steradians (only in front of the surface). The calculator calculates light-output along the normal of the reflector - with that if the calculated point is 'off' normal one would need to take into account the appropriate light loss.

In terms of reflector material, that's where I'm stuck as I don't have a large inventory of industry standard rags (doing this while on holiday in a very remote region). A simplified way of calculating light loss from each reflector material (ultra bounce, muslin, grey card) is to just compute from it's LRV (Light reflectance value - such as a standard grey card being 18%). I'm currently calculating from muslin and assuming a LRV of 73.5% as that's the reflectance value of cotton fibre. However, if anyone had at there disposal industry standard Muslin rags (with fireproofing etc), Ultrabounce and so on and would be willing to do a few tests for me, that'd be incredibly useful!

There are 'proper' ways of doing it, but, from my tests with non-standard material if one has a spot meter (preferably one that gives an FC output), lights and a grey card it can be easily found.

My set-up works by setting up a grey card and lighting appropriately so that the projected light is, within reason, even. I then try to have my card read at 50fc, approx T4, 400ASA, 180 degrees. I then set up the tested rag directly adjacent or directly behind the grey card and take another reading. Typically (when the grey card is set-up in front of the material) I'll take a reading, remove the card and then take a reading in the exact same place on the material.

https://en.wikipedia.org/wiki/Light_reflectance_value

If anyone has any spare time and fancies contributing some information to this project it would be fantastic and greatly appreciated!

G

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I can help with this. I'll try to plot out some time soon.

On the subject of a calculator. I've been looking for a solution as well. I don't understand all the math terms, but I can say from my tests that the angle of the surface relative to camera determines the light falloff. Or cosine.

Specularity comes from simply how uniformly smooth the surface is. But if that surface is then wrinkled, like on Styrofoam insulation, we consider it "specular" but it is just a wrinkled smooth surface. Provided the texture is uniform, then it will meter like a hard light.

We should talk more on the subject indeed.

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Gabriel,  

You had a minor "brain fade."  Footcandles are Light Incident to subject.  Spot meters read Footlamberts then translate to f-stops. (Brightness or reflected light measurement).

The relationship of 'lamberts to 'candles was explained in the 1960's ASC manual, (when B&W was king), but because I had a minus 15 for the eight days I took Algebra,  I never quite understood it.  As I think on it now, it was probably because I didn't have a meter that read footlamberts directly.

Spectra and Sekonic do have meters that read fl directly.  (758 Cine and above).

You have an interesting project going.  One that raises more questions.  I see walking an arc (like a sniper's range card) measuring and notating Light Incident -- falloff-- at various distances from your reflector.  

 

 

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5 hours ago, Stephen Sanchez said:

I can help with this. I'll try to plot out some time soon.

On the subject of a calculator. I've been looking for a solution as well. I don't understand all the math terms, but I can say from my tests that the angle of the surface relative to camera determines the light falloff. Or cosine.

Specularity comes from simply how uniformly smooth the surface is. But if that surface is then wrinkled, like on Styrofoam insulation, we consider it "specular" but it is just a wrinkled smooth surface. Provided the texture is uniform, then it will meter like a hard light.

We should talk more on the subject indeed.

Yes! The angle does however, I find in most lighting scenarios DoP's try... (or potentially should) try and keep the normal of a bounce perpendicular to the subject. If they don't generally the light is sharper and the projected light is uneven (unless of course this is a desired effect... which does somewhat elude me) note the infamous Roger Deakin's cove does make a lot more sense!

Funnily enough though, lamberts cosine law is pretty simple to calculate. In fact in terms of what we do a simple rule is if you're subject is more than 60 degrees from your reflectors normal (a hypothetical direct line being projected perpendicularly from the reflector) you loose a stop and at that point it does increase exponentially. 

In terms of specularity, I believe you're correct! I remember reading a post you wrote a while back. I'm not too ofay with the exact point a surface becomes 'near' lambertian to which this calculator is <85% accurate, but I've noted most non-specular materials such as Ultra bounce are near enough however, materials such as silver stipple is another question.

The issue is well is as soon as you need to start inputting the normal of your incoming source in relation to the normal of your reflector the inputs become more complex. It's fine for people that know, but the goal of this preliminary one is for it to be a (while slightly inaccurate in terms of a quarter of a stop which I believe should be negligible) a fool proof calculator with the only information required is available from a fixtures photometric chart. 

Would be great to talk more! As always talking makes one think which of course helps.

My email is gabjol@me.com

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1 minute ago, Gabriel Devereux said:

Yes! The angle does however, I find in most lighting scenarios DoP's try... (or potentially should) try and keep the normal of a bounce perpendicular to the subject. If they don't generally the light is sharper and the projected light is uneven (unless of course this is a desired effect... which does somewhat elude me) note the infamous Roger Deakin's cove does make a lot more sense!

Funnily enough though, lamberts cosine law is pretty simple to calculate. In fact in terms of what we do a simple rule is if you're subject is more than 60 degrees from your reflectors normal (a hypothetical direct line being projected perpendicularly from the reflector) you loose a stop and at that point it does increase exponentially. 

In terms of specularity, I believe you're correct! I remember reading a post you wrote a while back. I'm not too ofay with the exact point a surface becomes 'near' lambertian to which this calculator is <85% accurate, but I've noted most non-specular materials such as Ultra bounce are near enough however, materials such as silver stipple is another question.

The issue is well is as soon as you need to start inputting the normal of your incoming source in relation to the normal of your reflector the inputs become more complex. It's fine for people that know, but the goal of this preliminary one is for it to be a (while slightly inaccurate in terms of a quarter of a stop which I believe should be negligible) a fool proof calculator with the only information required is available from a fixtures photometric chart. 

Would be great to talk more! As always talking makes one think which of course helps.

My email is gabjol@me.com

I will note, one thing I'm playing with at the moment is massive reflectors like 40' by's.

As of course if you're subject is 10' away from a 40' (which is a bit nuts) lamberts cosine law would be required to take into account the outer edges and such. Should be easy enough to compute. 

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1 hour ago, Eric Eader said:

Gabriel,  

You had a minor "brain fade."  Footcandles are Light Incident to subject.  Spot meters read Footlamberts then translate to f-stops. (Brightness or reflected light measurement).

The relationship of 'lamberts to 'candles was explained in the 1960's ASC manual, (when B&W was king), but because I had a minus 15 for the eight days I took Algebra,  I never quite understood it.  As I think on it now, it was probably because I didn't have a meter that read footlamberts directly.

Spectra and Sekonic do have meters that read fl directly.  (758 Cine and above).

You have an interesting project going.  One that raises more questions.  I see walking an arc (like a sniper's range card) measuring and notating Light Incident -- falloff-- at various distances from your reflector.  

 

 

Thank you!

I've been using a photon counter and using some fun math taking into account certain standards to arrive at photographic values. 

However, foot lamberts is as perfect! If not more in this scenario.

WIth Ev being a constant in this scenario. The variable is R (which as a reference grey card as shown bellow would be a reflectivity of 0.18, I believe muslin should read about a 0.735 which is the figure I'm currently playing with but, unsure!)

L_\mathrm v = E_\mathrm v \times R,

L_\mathrm v is the luminance, in foot-lamberts,
E_\mathrm v is the illuminance, in foot-candles, and
R is the reflectivity, expressed as a fractional number (for example, a grey card with 18% reflectivity would have R = 0.18).
 
I've never used a spot meter with foot lamberts (or foot candles). In a sense, according to some law of quantum dynamics (which I've forgot) a photon is destroyed and created on reflection - in a sense you're measuring after destruction and the newly formed photon count is of course theoretically less than and the ratio in a sense between the two is the reflectivity. If one doesn't have a spot meter theoretically you could achieve the same with a light meter (that you wouldn't mind clamping to a controlled point).
 
Thank you again!
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7 hours ago, Gabriel Devereux said:

In a sense, according to some law of quantum dynamics (which I've forgot) a photon is destroyed and created on reflection - in a sense you're measuring after destruction and the newly formed photon count is of course theoretically less than and the ratio in a sense between the two is the reflectivity. If one doesn't have a spot meter theoretically you could achieve the same with a light meter (that you wouldn't mind clamping to a controlled point).

 

I've guessed that it takes energy to retransmit the photon after impact.

I'd recommend getting a spot meter. I own an older L788. It gives 3° and 1°. Which has been perfect for isolating small elements from the whole. It gives CDm2 reflectance measurement. Which is on the same SI measurement scale as lux. (Hence why I use lux instead of FC.)

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22 hours ago, Stephen Sanchez said:

I've guessed that it takes energy to retransmit the photon after impact.

I'd recommend getting a spot meter. I own an older L788. It gives 3° and 1°. Which has been perfect for isolating small elements from the whole. It gives CDm2 reflectance measurement. Which is on the same SI measurement scale as lux. (Hence why I use lux instead of FC.)

It's interesting, I'm trying to find the exact law I poorly quoted above. 

The calculator currently gives an output in LUX. It's written and from my tests, working (in terms of near accurate approximations). Shoot me an email when you're free (have put it above) and I'll send it through as would be great for you to have a play!

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