Three questions about white balance

[Max von MeiselMaus]

Manfred, as stated above, that will make the green show as grey (neutral), so that won't work if Ted wants a "true" green colour.

Ted, I don't know if you were being factitious with some of your questions regarding my suggestions or whether you really didn't know. As I am trying to help here, I am going to take it that you really didn't know and so clarify.

By "notional" daylight, I mean a colour temperature taken to represent white daylight (although, as we know and some of the discussion here has pointed out, there is no such thing as a single daylight colour. Daylight changes all the time). So, for the sake of argument, 5 500K.

And, am I sure you would need to set your WB for the ambient light? Well, yes. The colour of the ambient light would influence the apparent colour of your luminous dial, so needs to be factored out to produce an accurate representation. Try lighting it with tungsten and with daylight. The green will look very different on each occasion. Therefore, the colour of the ambient light is key.

Interesting to see those shots. What is it you are trying to achieve now with your shots of luminous dials? Are you still trying to find a way to measure luminance or are aiming for something else? If you want to show the beauty/design/effect of a luminous watch, I would sling some dim ambient light in there, dimmer than the luminous paint. If the aim is measuring the glow, no idea.

[xpatUSA]

Manfred, as stated above, that will make the green show as grey (neutral), so that won't work if Ted wants a "true" green colour.

Max, I think he was joking.

Ted, I don't know if you were being [facetious] with some of your questions regarding my suggestions or whether you really didn't know. As I am trying to help here, I am going to take it that you really didn't know and so clarify.

By "notional" daylight, I mean a colour temperature taken to represent white daylight (although, as we know and some of the discussion here has pointed out, there is no such thing as a single daylight colour. Daylight changes all the time). So, for the sake of argument, 5 500K.

I had never read the phrase "notional daylight" before and googling produced only four results, none of which were relevant to white balancing. Of course, some photographers are wont to invent new phrases, so I thought I'd better ask. Quite reasonable don't you think? Perhaps you were referring to the CIE Standard Illuminant D55 but I had no way of knowing for sure.

And, am I sure you would need to set your WB for the ambient light? Well, yes. The colour of the ambient light would influence the apparent colour of your luminous dial, so needs to be factored out to produce an accurate representation.

Please understand that the purpose of the exercise is to determine the brightness of the glow. To do that with just a camera requires the correct white balance for darkness so why would I introduce extraneous light? But, suppose I did, I must ask you how would I "factor out" out some lamp of unknown CIE x,y or u,v chromaticity at some distance away? Perhaps I could use Manfred's "tiny gray card" and do a custom white balance - but can you see the flaw in that?

Try lighting it with tungsten and with daylight. The green will look very different on each occasion. Therefore, the colour of the ambient light is key.

Yes and no. Different, yes. Very different, maybe. Un-noticeably different, quite possibly. The green would of course look "very different" if was plain green paint, per your "white balance 101".

Do you not understand that luminous paint emits light just as a lamp does, making said WB 101 inapplicable?

Interesting to see those shots. What is it you are trying to achieve now with your shots of luminous dials? Are you still trying to find a way to measure luminance or are aiming for something else?

No, nothing now, it was an interesting technical challenge at the time, that's all.

If you want to show the beauty/design/effect of a luminous watch, I would sling some dim ambient light in there, dimmer than the luminous paint. If the aim is measuring the glow, no idea.

Hopefully you can now see that ambient light is a complication, definitely not needed for my purpose.

I did once illuminate a watch with a UV lamp for effect - almost deviant art, you could say - UV sets luminous paint to glowing pretty fiercely.

Off at another tangent, I wonder what the correct WB setting is for UV "light"? Here we see an instance of the main point of this thread that, for non-luminous subjects, the correct WB for that particular shot was definitely not the best, because the unearthly reflection of the UV lamp's visible emissions was quite desirable.

Perhaps we should quit now before this thread gets blocked for too-technical OT content? I'm done anyway.

Thank you for your help.