Experiment with WB for B+W 092 IR-Pass

[Andrea B.]

In a recent topic, some suggestions were made (by me and others) about white balancing a Nikon camera when using an IR-pass filter. It had been so long since I made the experiments for such a WB that I thought I should repeat them to refresh my memory.

 

Here is the Visible scene I used for the experiment.

D610 + UV-Nikkor + Kolari IR-Blocker

f/8 for 1/250" @ ISO-100

 

 

For comparison purposes and out of curiosity, I made some WB both with and without the B+W 092 IR-pass filter. All photos are SOOC. They were made with a Neutral picture control so fairly bland looking.

 

1) WB made "against the scene" with NO filter on the lens. Lots of frosty pink/magenta.

 

 

2) WB made against the dark yellow-green juniper bush with NO filter on the lens. Still pink, no surprise. But a bit less intense than the preceding. I think grass is perhaps not as yellow-green as the juniper bush, but I do not have any grass here! 

 

 

3) WB made against the scene through the B+W 092 IR-pass filter. Clearly this is a useful WB for this IR filter.

 

 

4) WB made against the the juniper through the B+W 092 IR-pass filter. This version is pretty much like 3, although the false blue is less intense.

 

 

 

In the topic referenced above, there was some discussion about using color displays on a monitor to white balance against. So I experimented with that. 

 

5) WB against a Green Screen with no filter. Doesn't work, obviously. No surprise, as there is no IR. And WB against green pushes the balance towards magenta.

 

 

6) WB against a Yellow-Green Screen with no filter. Again, this does not work. A bit more of a blue-pink. (WB against yellow-green pushes color towards blue-magenta.)

[Andrea B.]

Next obvious thing to try. WB against a magenta screen. Should push things towards green? Let's see.

 

Well, not quite. The resultant scene under the 092 is more of an orange-yellow.

 

 

 

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It's like this ---->>>> I am convinced that if I could just find the correct color screen against which to make an unfiltered WB, then the resulting photo made with the IR-pass filter would not have warm tones.

 

 

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How about a WB made against desaturated red? If only I had realized how OBVIOUS this was at the beginning. Well, duh. Just get the red out. The result is not perfect, but not too bad.

 

 

 

Now I will try repeating that but with the WB pointer set to B6 when making the WB against the desaturated red.

Not much change from he preceding. Not bad, but there is still a color cast.

Maybe I should have tried making the WB with a B6,G6 setting?

 

 

CONCLUSION:

The best Nikon IR white balance effort -- so far -- for the 092 IR-pass filter was shown in (3) in the preceding post. That was the photo for which WB was made "against the scene" though the 092.

 

The monitor screen technique seemed promising when the WB was made against the desaturated red screen, but one iteration did not get rid of all the pinkish tones. And I need to try a B6,G6 setting.

 

Unfortunately I have run out of sunlight for this day. I will try to continue testing this monitor screen technique when the sun returns.

 

Kindly note that the best white balance of all for an IR filter will be made in the sunlight against a piece of PTFE or Spectralon with the IR-pass filter on the lens if you have a camera which can handle making a WB thru a dark filter.

A converted Nikon can make a WB against PTFE through a 720 nm longpass, but might have trouble for a cut-in higher than that. (I don't remember, but will try different IR cut-ins when the sun comes back.)

 

 

 

 

[Andrea B.]

WB made in Photo Ninja against the sky.

The frosty blue seems appropriate for the cold temps we have had recently.

[Andy Perrin]

I really don’t get the point of the whole screen-WB endeavor because we know using a piece of Spectralon or PTFE and light from the scene is optimal (you said so!), and if we want quick and dirty, clicking around in PN works fine. 

Deep IR is monochrome so WB is irrelevant, and from what you said, Nikon can probably handle 720nm. 

 

I like that frosty blue. 

 

I feel like you are trying to recover some lost memory here, Andrea, but maybe it wasn’t worth recalling and that’s why you forgot! 

 

 

 

[photoni]

my little experience (I only have Hoya R72 and Wratten 87c ~800nm) tells me that beyond 720 nm the images are BW,

what's the point of looking for a white balance? Set BW vision.

 

in many discussions he read that the most logical expose - white balancing method is with green grass... maybe because they are the majority of colored Bayer pixels?!

the real problem is the contrast, while with the RAW development software I can modify it, but not in the monitor of the camera

[nfoto]

Unless properly processed, digital IR tends to be very dull and flat despite the potential motif contrasts.

[lukaszgryglicki]

Noo, beyond 720 - there are still differences in R, G, B channels up to about 830-850 from what I remember - would have to recheck. But I was able to get color from 720, 760 filters and 850 was almost mono (if I remember correctly)

[photoni]

@lukaszgryglicki you who have an RGB and a mono, try the three filters 720 - 760 - 850...

I doubt there are differences with the mono.
in the color one the differences between the RGB channels are unpredictable, aleatory, not logical.

 

off topic: the same thing in UV, two photos of the same flower with BG39+UG1 and BG39+UG3 have completely different colors if you do two different white balance (with a teflon or anodized aluminum)
in my way of thinking you should compare the two photos with the same settings.

[lukaszgryglicki]

I meant that between 720 and 850 there are still differences in channels, so when we change weavelengths smoothly from 720 to 850 channels channes change differently, not proportinally to each other, after around 830-850 - they start to change proportionally when wavelength becomes longer, so if you plot R,G,B changes across wavelengths there will be a scalar factor that you can apply to them and then their curves will be the same, this is not the case until about 830-850nm, which makes color photos still possible from 720 nm but not from 850nm - this is what I tried to say.

 

[Andrea B.]

Andy wrote:  I really don’t get the point of the whole screen-WB endeavor.

 

CURIOSITY!

[Andrea B.]

Toni wrote:  in many discussions he read that the most logical expose - white balancing method is with green grass... maybe because they are the majority of colored Bayer pixels?!

 

Grass and most deciduous leaves very strongly reflect infrared light, so they are "white" in old film IR work. That has carried over to digital IR when shooting between, say, 680 - 750 nm.  In digital work, white grass and leaves are not mandatory, of course. We have easy ways to tweak false colors in those IR photos which have them.

 

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The reason this topic got started was that when shooting between 680 - 750 nm, it is very useful to avoid strong saturated colors which make it more difficult to judge the capture of detail and more difficult to get a proper exposure. So you can either switch to a Monochrome setting (suggested above) or you can make some kind of white balance. As is well known by all and also noted above, there is not much point to WB above 720 - 750 nm. 

[colinbm]

I have used Spirulina Powder, sprinkled on an adhesive surface, as a standard for IR Custom White Balance, to good effect.