Spectrograph Study of a Rosco 74 Night Blue Filter

[colinbm]

Spectrograph Study of a Rosco 74 Night Blue Filter.

I am trying to work out where the 'gold' comes from in the vegetation with the Rosco 74 Night Blue Filter.

This thread is what I am referring to.....Rosco 74 Night Blue

 

 

Lets look at a spectrograph of the full sunlight today.

 

Next is a spectrograph of the Rosco 74 Night Blue filter in full sunlight.

 

Out of interest, I took a spectrograph of reflected sunlight off a couple of trees.

 

Then, I took a spectrograph of reflected sunlight off a couple of trees, with the Rosco 74 Night Blue filter.

 

Continuing the test, I moved in on one of the trees & from close-up at 100mm I took a spectrum of a leaf in reflected sunlight.

 

And to conclude this test is a spectrum of the leaf at 100mm in reflected sun with the Rosco 74 Night Blue filter in place.

 

Back to my question, "Where does the 'Gold' come from ?"

[colinbm]

Just to add to the fun of it all, here is the spectrograph of the sunlight that was transmitted through the leaf.

 

[Andy Perrin]

Well that does it, I'm throwing away my R72 and just sticking a leaf over the lens next time.

[colinbm]

Well that does it, I'm throwing away my R72 and just sticking a leaf over the lens next time.

 

Yes, the tree doesn't seem to have much need for IR.

[Nisei]

Thanks Colin

I've posted some examples in my gallery.

I'm also wondering where the gold comes from.

And I'm even more surprised why it shifts to red (like with other blue filters) when stacked with a GRB3 filter.

Using this stacked with Lee 115 I only got better contrast in my pictures, not a dramatic change in tint.

[dabateman]

Well that does it, I'm throwing away my R72 and just sticking a leaf over the lens next time.

 

I wonder what a landscape image would look like using your triwave and a leaf filter?

Might be fun. Or really blurry.

 

[dabateman]

You have to put this into context of an average full spectrum converted camera sensor response.

[colinbm]

Dave is your graph is an un-converted camera ?

 

I have thought of the overlaying of a converted cameras response & what that may take away, but I have no clue on how to approach that.

 

Yellow comes from the additive colours of red & green, so is the 'gold' coming from the additive colour of IR & Blue/Green ?

[Stefano]

Yellow comes from the additive colours of red & green, so is the 'gold' coming from the additive colour of IR & Blue/Green ?

It could be. To be sure you (or Nisei) could try to stack it with a longpass filter, like a red filter, and see if foliage turns white.

[Stefano]

I wonder what a landscape image would look like using your triwave and a leaf filter?

Might be fun. Or really blurry.

You could actually make a chlorophyll filter, extracting chlorophyll with alcohol and putting a "liquid cell" of chlorophyll and alcohol in front of the camera. Probably not much different than Hoya R72 for a standard camera, I don't know what to expect in SWIR.

[Nisei]

It could be. To be sure you (or Nisei) could try to stack it with a longpass filter, like a red filter, and see if foliage turns white.

Adding a red filter makes the image almost black and white.

[Stefano]

Adding a red filter makes the image almost black and white.

Did you keep the same WB? Anyway, that shows that the blue portion contributes to the colors. Foliage reflects more green than blue, so it appears yellow, and the sky diffuses more blue than green, so it appears blue. The filter passes some green, so it can pick up the green reflection of foliage a bit.

[Stefano]

Did you keep the same WB? Anyway, that shows that the blue portion contributes to the colors. Foliage reflects more green than blue, so it appears yellow, and the sky diffuses more blue than green, so it appears blue. The filter passes some green, so it can pick up the green reflection of foliage a bit.

I guess the WB for this image isn't too different from a standard, "sunny" WB. Infrared is seen as reddish by a camera, plus the green from the foliage gives you yellow, so I think Colin is right.

 

I didn't want to quore myself, but it's ok anyway.

[Nisei]

Did you keep the same WB?

I didn't.

Tried that now and when using the WB of the 74 filter stacked with a red filter the whole image turns into a monochrome yellow tint. So the foliage stays about the same as without a red filter and anything white also shifts to this same yellow tint.

 

I guess the WB for this image isn't too different from a standard, "sunny" WB.

Tried the "sunny/daylight" preset with #74 but this gives a very heavy pink/magenta tint.

[Stefano]

I didn't.

Tried that now and when using the WB of the 74 filter with a stacked red filter the whole image turns into a monochrome yellow tint. So the foliage stays about the same as without a red filter and anything white also shifts to this same yellow tint.

So the yellow is given by IR, and foliage doesn't reflect enough blue/cyan to change the color.

[Nisei]

So the yellow is given by IR, and foliage doesn't reflect enough blue/cyan to change the color.

So why is it then that nearly all other blue filters are producing red foliage?

[colinbm]

I need to learn this CIE 1931 & 1976 business....

Here is the report from the spectrum of the leaf at 100mm in reflected sun with the Rosco 74 Night Blue filter.

Is this showing the white balance ?

 

 

[Stefano]

So why is it then that nearly all other blue filters are producing red foliage?

I don't know. How does BG3 render foliage? It should be quite similar to Rosco 74.

[Nisei]

I don't know. How does BG3 render foliage? It should be quite similar to Rosco 74.

Steve tried that here.

[colinbm]

Looking at the report again, from the spectrum of the leaf at 100mm in reflected sun with the Rosco 74 Night Blue filter.

It has the 'Dominant Wavelength at 574.98nm, is this the 'Gold'.

 

[ulf]

Dave is your graph is an un-converted camera ?

No it is the sensor sensitivity for some sensor like in a full spectrum converted camera.

 

For UV we are left with the rather low sensitivity below 400nm.

That is the reason to the need of several seconds of exposure time with properly filtered for UV, while a VIS image need something like a 1/1000 as long exposure time.