[Filter Test SEU Gen2 #2] White Balance, Raw Histogram & Andrea's "White Signature"

[Andrea B.]

Last Updated [2018.07.19 10.22 EDT. Added phrase about in-camera wb.]

 

[Filter Test SEU Gen2 #1] Introduction to the SEU Gen2

[Filter Test SEU Gen2 #2] White Balance, Raw Histogram & Andrea's "White Signature"

[Filter Test SEU Gen2 #3] Filter Speed & A Windy Bull's-eye

[Filter Test SEU Gen2 #4] Dealing with the Usual Dichroic Effects

[Filter Test SEU Gen2 #5] Landscape Interlude

[Filter Test SEU Gen2 #6] Monochrome Museum Comparison

[Filter Test SEU Gen2 #7] Measured Filter Transmission

[Filter Test SEU Gen2 #8] Dichroic Reflection Detour

[Filter Test SEU Gen2 #9A] Longpass Stack Wandering Discussion. See #9B for results.

[Filter Test SEU Gen2 #9B] Longpass Stack Results

[Filter Test SEU Gen2 #10] What good is a filter test without a Rudbeckia?

[Filter Test SEU Gen2 #11] A Fascinating New Rudbeckian View

[Filter Test SEU Gen2 #12] Summary

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SEU Gen 2 White Balance, Raw Histogram & Andrea's "White Signature"

 

The first thing to do for any new UV-pass filter (and at the beginning of any shooting session) is to photograph a good reflective standard which is stable under UV, visible and IR light. This enables the creation of false color white balance presets in the converter of choice or in those cameras capable of measuring white balance through a UV-pass filter. My Nikon camera full spectrum conversions suffer from the well-known inability to do this in-camera under a UV-pass filter, so shooting reflective standards or targets is a must. However, this is a good practice to follow regardless of your camera brand. I have found that some cameras are not always accurate in their white balance measurement under certain filtration. By 'accurate', I mean that white balance measured in-camera over a white reflective target does not always match the white balance obtained from the camera's own converter when the white balance tool is applied to the target area.

 

I also like to look at the Raw Digger non-white-balanced raw color of a UV-pass filter over Spectralon. Each UV-pass filter has a different 'white signature'. The notion of a filter having this raw color 'white signature' over a white standard has actually proved useful a couple of times when I've been lax about getting my files named and have had to untangle which filter made which photo.

 

 

WHITE BALANCE TEST: For each filter, photograph a good reflective white standard.

 

Question: What does the converter white balanced photo look like for each UV-pass filter? Were there any problems in using a typical converter to obtain the white balance?

 

Answer: The SEU Gen2 files white balance nicely as do the other two filters' files. We will see later that the SEU Gen2 white balance gives some false color tints not seen in these CC Passport photos which do not really show all of any particular filter's array of false colors.

 

"WHITE SIGNATURE" TEST: For each filter, photograph a good reflective white standard, extract its raw color in the photo before white balance and find the color's location on the color wheel.

 

Question: No question really. I'm simply curious about the raw color cast for each filter over UV-reflective white. I've been informally calling this the "white signature".

 

Subject: Edmund 5" Spectralon Square & Color Checker Passport (2014)

The Spectralon was recently sanded.

 

Location & Sunlight:

• Latitude: North 44° 16' 57.2"
  
• Longitude: West 68° 18' 52.2"
  
• Alitude: 18' = 5.5 meters
  
• GPS Date: 2018/07/13 15:16:15
  
• Solarmeter UVA+B Model 5.0: 5.2-5.4 mW/cm², pointed at the sun.
  My goodness! That's one of the highest readings I've ever gotten. This sea level thing is real!
  

Gear: D610 + UV-Nikkor 105/4.5 + UV-Pass Filter + Sun

 

UV-Pass Filters: In my initial tests with the SEU Gen2 I chose two other commercially manufactured UV-pass filters for comparison, the classic BaaderU and the recent KolariU.

• BaaderU :: 350fwhm60 :: approx 80% at peak :: OD > 3.5??
  
• KolariU :: 365fwhm40 :: approx 50% at peak :: OD > 3.5
  
• SEU Gen2 :: 392fwhm50 :: approx 74% at peak :: OD > 4.5
  

Exposure: f/8 at various speeds @ ISO-100, Nikon ADL=off

We have to be careful not to blow out the white standard shots (whether PTFE or Spectralon) even if our camera might have enough headroom to provide excellent highlight recovery because some white balance tools will not work over a 3-channel blowout. I used the monochrome histogram as a rough guide to see when my exposure length was approaching danger.

 

Conversion: Capture NX2. The white balance marquee tool was drawn over most of the Spectralon rectangle. For uniform comparisons, the white luminance was set to 95% and the black/white points adjusted if needed. Those settings may have changed in the sRGB JPG created for posting here.

 

Raw Color: Raw Digger. The export was made with the No Profile setting to show the raw color before white balance. The color sample was taken in Adobe Photoshop Elements 11 where the labeling and the full saturation patch were added. The color wheel label may vary by a few degrees. If more precision is needed (for what I ask with a smile?), then use the Raw Digger sampling technique and convert the average RGB counts to an RGB triplet.

 

Comment_1: This is just for fun. It gives us a very broad feel for how the UV-pass filters are transmitting, but I don't attach deep significance to this. Broadband filters with peaks at 350 or 365 nm have an orange-red raw color over white Spectralon. Filters with higher peaks at 380 or 390 nm or with some violet/blue passage have more blue or magenta in the raw color over white Spectralon. (For the record, I'm getting more of a yellow-orange from my narrowband 340 nm filter.)

 

Comment_2: Inclusion of the Color Checker Passport in a UV photo is certainly not needed, but it seems to have become traditional. Also, I needed to make a visible photo for D610 color profiling, so why disturb the setup when switching to the UV-pass filtration?

 

 

Summary Chart

Results at a glance. Reminder: the color wheel values probably vary by a degree or 2 or 3.

 

 

SEU Gen2 White Balance

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

 

 

KolariU White Balance

f/8 for 1/2" @ ISO-100

 

 

BaaderU White Balance

f/8 for 1/3" @ ISO-100

 

 

SEU Gen2 Raw Color

Just call me Magenta. B)

But it could be her sister Violet in disguise because of the way our cameras record around 400nm.

 

 

KolariU Raw Color

An almost red white signature.

 

 

BaaderU Raw Color

I can see this as red-orange, yes?

 

 

SEU Gen2 Raw Histogram

This histogram was made from a Raw Digger sample placed over a large central portion of white Spectralon. Like the smaller sample above, the raw false colour is almost a pure magenta. The exposure on the day I made this with the D610 and UV-Nikkor was f/11 for 1/2" @ ISO-100. I got a color wheel location of 304° and a fully saturated RGB triplet of (255,0,242). The two greens are averaged for this histogram.

[Andrea B.]

Comments now open for this topic.

[Andrea B.]

Last Updated [2018.07.18::18.31.EDT. Added question answer & comment.]

[Andy Perrin]

What is with these grooovy fonts, Andrea? Are you having fun?

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I did wonder how these compare to SEU Gen 1, though. Seems like it would make sense to compare this filter to its immediate predecessor?

[Andrea B.]

Are you having fun?

 

You bet!!!!

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I can certainly add a color cast reference for the StraightEdgeU, first generation. I waffled back and forth on doing that and landed on the side of "no" about including it in any test because the 1st SEU is no longer sold. I wanted to use available filters in all the tests. But I'm curious about the color cast too.

[Andrea B.]

Last Updated [2018.07.19 10.22 EDT. Added phrase about in-camera wb.]