UR-Pro filters info?

[bghazzal]

SUMMARY

 

Returning to the filtering / transmission info we now have on the UR-Pro Cyan filter, we can try to summarize the data as is:
 
- In terms of spectrum, the UR-Pro Cyan does indeed function like fluorofilter, with a similar profile.
The filter blocks the UV spectrum, and strongly restricts cyan to green transmission, while maintaining a medium transmission of dark/purple blues, and strong transmission of orange/red wavelengths, thus having a marked warming effect and smoothing spectrum alterations of ambient light in blue water (which helps the camera white balance).
Practically, the working result is that UR-Pro Cyan preserves some deep blues for the water column, while nudging the cyan/green cast to a warmer yellow-orange tones, which gives a good base for colour grading to aesthetically pleasing results.
In this, it is not the most colour-accurate (the Keldan Spectrum filter is clearly more accurate), but is nonethless great and very practical for underwater video use (which explains its popularity).

- The UR-Pro Cyan induces a general light loss in the roughly -1.6 to -2 stop / Ev range (being a colour filter, this is also relative to wavelength transmission, as discussed here)

- Physically, the UR-Pro Cyan was made of acrylic or glass (not gel), with an orange to reddish tint, and was the steepness of the curve seems to indicate that it was an optical grade filter.

If we take the available transmission data from the filter patent and order it on the wavelength/colour spectrum, we end up the following:

UR-PRO CYAN FILTER WAVELENGTH TO TRANSMISSION, MAIN DATA POINTS

Below 370 nm:  0%

UV ULTRAVIOLET: BELOW 400 nm
Below 400 nm: 25%
VIOLET: 400-450 nm
Below 410 nm: 27%
Below 450 nm: 12%
BLUE: 450-500 nm
Below 470 nm: 8%
Below 500 nm: 4%
CYAN: 500-550 nm
Below 520 nm: 7%
Below 550 nm: 18%
GREEN: 550-580nm
Below 570 nm: 50%
YELLOW: 580 - 600 nm
Below 600 nm: 87%
ORANGE: 600 - 650 nm
Below 700 nm: 90%
RED: 650 - 700 nm

 

-  We are also given a rough visual representation of the UR-Pro Cyan filter's transmission curve, which can be rendered as is on the wavelength spectrum:

UR-PRO CYAN FILTER SPECTRUM TRANSMISSION CURVE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

- Combining these data sets, we can add the following projections for wavelength transmission cut-off (known data point are in bold type, projections are in italic type)

 

UR-PRO WAVELENGTH TO TRANSMISSION, KNOWN DATA POINTS AND PROJECTIONS

 

 

 

ALTERNATIVES

 

In terms of possible alternatives, there seems to be a marked difference between optical grade photographic filters, which have might tighter, precise cut-off points, and design specificities like fully cutting off the UV spectrum, and lighting filter gels, which have a more sloped curve, with less marked cut-off points.

 

- A relatively cheap and somewhat more accessible option would be to combine existing lighting gels such as fluorescent filters (FL) / orange colour temperature adjustment filters (CTO), or orange to pale red colour conversion filters (CC), with a UV blocking filter to steepen the curve.
This seems promising, as illustrated above.
 

However, one thing to look out for when combining lighting gels would be light exposure / stop loss.

A loss -3 stops / Ev would be a reasonable limit. For smaller sensor cams like action cams, it would be best to stay in the -1.5 to -2 Ev / stop range, as anything stronger would introduce to much noise when shooting in ambient light.

Some options to look into would be Lee and Rosco filters, such as the (discontinued...) Lee Pale Red 166 combined with the Lee 266 UV, or the Rosco Cinegel  3310 Fluorofilter, or other CTO / CC filters of their respective ranges (Rosco 166 Pale Red, Roscolux and Cinegel CTOs, etc.)

- Another option would be to look into existing optical grade filters for photography (color correction, fluorescent filters, such as the Tiffen 47B filter for example) or those designed for other optical applications (microscopy, astronomy...). These would require no additional UV filters, but these will usually be in glass form, so less flexible, and more expensive.

 

- Lastly, there would be the option of trying to tweak existing photo filter gels (Magic Filter or Keldan) to bring them closer to the UR-Pro Cyan, combining them with a minus green lighting gel for instance.

From my tests on grading flat action-cam footage, Magic Filter's handling of magenta (inverted, when to other filters) is problematic at depth for my video grading use (more details here and here) but a filter like the Keldan Spectrum SF-1.5, combined with a gel giving it a bit more "bite" (a minus green gel?) seems like a promising base for experimentation.

 

cheers!

 

ben

 

Edited July 11 by bghazzal

[DreiFish]

Good investigative work and good summary, Ben. I got the CTO and minus green filters in various strengths -- just finding the time and energy to test them is proving challenging 🙂