I struggle to visually assess white balance, so I often include a white reference point in a test shot, which I later select in the color calibration module.
It appears to me that the such a reference point is currently available to many people in the form of used lateral flow Covid test devices*. The plastic these are made of appears to be very close to pure white, but unfortunately I do not own a colour checker against which these can be compared.
Is anybody out there willing to take a photograph of such a device alongside a calibrated color checker, then use darktable’s color picker to see how the whites compare?
Yes, I have used this trick but the tape quickly unravels and becomes dirty.
It did however lead to me acquiring a sheet of PTFE to use as a white balance reference and when I photographed this alongside a Covid test, darktable’s color picker showed them to have nearly identical RGB values.
It was this that led to me posting my original question, because I remain unsure how accurate this is, and if it is accurate, then I suspect that others could find it useful.
I have been able to borrow a SpyderCHECKR 24 and run my own comparisons.
When I use color calibration’s picker to select a small area, it sees the FlowFlex lateral flow test as 100K cooler than the white square of the SpyderCHECKR 24…
Regrettably not in the UK. They can be imported through the likes of eBay, but after paying for international shipping, import duties and value added tax, they are no longer cheap. The same applies to the Portabrace version, but these seem impossible to source over here.
My search for alternative solutions is essentially based upon this lack of availability as much as price. Using a spent FlowFlex LFT and deducting 100K from whatever the color picker says seems to be a feasible workaround.
So I now need somebody to photograph one alongside a WhiBal or Portabrace, which seemingly disagree with each other (the WhiBal is technically more accurate but apparently cooler).
For the last couple of years I have been content to use a piece of PTFE sheet and have been happy with the results it produces. I do however remain curious as to whether I can do better.
Google has today managed to find me a UK stockist of the Porta Brace White Balance card that’s not stupidly overpriced, so it will be interesting to see how it compares to my other references.
In the USA, wall paint stores have paint sample square cards around 30mm x 30mm. I’m not sure if thats something you could get near you. I know Sherwin Williams displays their RGB values for their colors
These two have the 3 RGB values as close to to equal as possible.
6995 Superwhite 232 234 234
7071 Gray Screen 200 203 203
as long as these arent properly measured during quality control you can’t be sure these are really that neutral…
And no one will spend a lot of effort to check these sample cards, since no customer expects a correct rgb value
The price of color checkers or white balance cards are results of the effort needed to provide reproducible results.
You can get cheaper it8 targets - but these usually have individual or lot based measure protocols. So don’t expect extremely precise neutral cards for just a few bucks. These are just “good enough”
Another potential pitfall with random white surfaces is the possible presence of optical brighteners. They can change the colour depending on how much UV is in the light.
Whether you need a perfect target depends on what you are doing. Product photography for third parties isn’t the same as landscapes for your own satisfaction.
Most of the printing press i work with do have great quality control for their colors. Every sheet is scanned by a vision system, one sheet every so often will be taken to a calibrated machine to double check. Of course we don’t know exactly we don’t know exactly with these paint sample ones but if you compare cards with each other the color is very consistent. I would image the paint company is interested in a consistent and quality product.
The biggest problem with all cards is aging and damage from use. Pantone swatch books should be replaced every 2 yrs and those are stored indoors.
I think these could be good enough for a hobby photographer.
Geesh, my wife gets these all the time to choose paints for the house, which is now like Liberace’s piano - more paint than wood! I can even walk to our local Sherwin-Williams store, which is saying a lot for car-bound USA…
Sometimes the simple solution is just sitting under your nose…
My Porta Brace White Balance Card has arrived, so here is another shot showing how darktable’s color calibration color picker perceives the various whites…
I can even walk to our local Sherwin-Williams store, which is saying a lot for car-bound USA…
