Determining the colours on a bedika cloth is a wisdom that has been passed on from generation to generation. Preferably a bedika cloth is checked by the light of the sun ('white light'). I've have been to some Rabbanim who rely on an electrical white light source as well as direct sunlight, to check a bedika cloth.

Would it therefore be possible (under the correct photography conditions) to standardise photography settings such that one can take a 'white-light-equivalent' photograph to be sent to a Rav via digital media such as email, SMS, Whatsapp, etc.

To probe further... if the conditions are agreed upon, can a computer program be used to determine whether a bedika cloth is tahor or tamei?

  • Update: In OU Torah Tidbits Rabbi Gershon Weitzman (Machon Puah) addresses this question (only 'part 1' is currently published). He says that non-red colours (e.g. green, yellow) may be distinguished digitally, but that shades of red needs human inspection. He does, however, say that the situation is more complicated and the remainder of the answer will follow next week. – bondonk Feb 13 '16 at 22:00

"White light" is a very tricky term. Rabbi Dovid Miller advises a combination of fluorescent and incandescent lights to get the right artificial color balance that will approximate sunlight.

Digital color-matching is known to be non-trivial; involving getting something photographed under the right light, captured properly, and then displayed properly on the other end. Graphics professionals spend a whole lot of money on computer monitors that a are more color-true than what the ordinary consumer needs.

So I really don't see the current technology making this doable in any cost-effective way.

More broadly, though, some colors will be "yes", some colors will be "no", and some colors will be "judgment call." From what I understand, most yoatzot halacha, for instance, can check for obvious yes or nos, but will then refer to a posek for anything else. So a computer could at best serve as a triage tool for human experts (many of whom prefer to see as many as possible, so they can train the next generation).

In short, it's not really practical today.

  • Whether its practical is one thing, and 'possible'is another. For some who find it hard to get to a Rav it may be much more practical to send an image, if it is indeed 'possible' – bondonk Sep 7 '15 at 18:17

Here is a great article on why the color spectrum represented by monitors or printers is simply not an accurate reflection of the true color spectrum, and that "primary-colors" based system, whether the RBG of monitors or the CMYB of printers, cannot actually replicate real life. To just give one illustration of the problem:

Illustration of the limited capture of the color spectrum in primary color schemes.

So even the Pantone colors are still not capturing anything reasonably close to the full color range. The basic issue described in the article is that although the eye (in most people) has three color receptors for red, green and blue, there is no specific wavelength on the light spectrum that can activate one and only one of these receptors, rather it interacts with all of them to varying degrees.

Two paints that appear to be the same color can, when combined, produce a new, different color, because each material produces different wavelengths that each activate the color receptors to about the same degree.

So until we can digitally capture the full light spectrum and replicate it with high fidelity, this is simply not a feasible technology for examining bedika cloths.

  • Many eyes dont fully capture the full colour range. Still, some colours are more obvious than others. In the first instance, a computer may still be able to say that a 96% "absolute red" colour of 5x5cm is tamei, leaving the "possibly tamei" bedika cloths for real people to scrutinise – bondonk Dec 28 '15 at 19:32

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