Image sensor response to light temperature color

[sertel]

Hi all,
I have a special application where a fluorescent light is illuminating the scene and I need to keep the illumination level as low as possible. I can, however, change the light color temperature and I wonder if the digital camera image sensor is like human eye that is more sensitive to some color than others.
If that is correct, what color temperature would be the best? (This is not a white balance question)
Thank you in advance!

[Manfred M]

The Bayer array used on most digital cameras is somewhat like the human eye in that it is more sensitive to green than other colours. 50% of the photoreceptors are keyed to green, 25% to red and 25% to blue.

[sertel]

Thank you GrumpyDiver, I much appreciate your help!!!

[xpatUSA]

Hi all,
I have a special application where a fluorescent light is illuminating the scene and I need to keep the illumination level as low as possible. I can, however, change the light color temperature and I wonder if the digital camera image sensor is like human eye that is more sensitive to some color than others.
If that is correct, what color temperature would be the best? (This is not a white balance question)
Thank you in advance!

Manfred is correct of course, but . . .

I would have thought that the objective of lowering the illumination level is better served by putting a neutral-color diffuser between the lamp and the subect, for example a sheet of tracing paper or a piece of thin white cloth. While realizing that the question is not about white balance per se , nevertheless changing the lamp color temperature would force you to change the white balance if you don't want a color cast in the output image.

[sertel]

Hi Ted,
let me clarify: I am using low (lumens) level light because I have reflections problems and at the same time I do not have room enough for a bigger illumination device. So I am trying to get the most with the less...
I appreciate your contribution,
Sergio in Houston,TX

[Dave Humphries]

Hi "sertel",

Before I start, could I request a first name please?
It helps because we're all so friendly here and we like to know what your name is and where you are located
(there are fields available for this if you click Edit Profile)

UPDATE: How did you do that? Read my mind?

I'll start again;

Hi Sergio,

Now to the question you asked:
The colour temperature (CT) of a fluorescent light is determined by the phosphors inside the tube, so changing the CT means changing the tube. The normal range of white tube CTs offered isn't that huge, unless the single colour special 'effects' variety of tubes are being considered.

I believe, but haven't tried, that changing to an alternate 'white' CT isn't going to make enough difference to be worth the effort or expense.

I further believe that changing to a single colour, effects tube (e.g. green) will just result in single colour images (i.e. impossible to balance for a normal colour image), so unless monochrome conversion is intended, this probably also isn't what is desired.

Welcome to the CiC forums from ....

[sertel]

Hi Dave,
thank you for your warm welcome to the forum. I am really impressed by the quality and friendly style of the forum that indeed already helped me a lot.
As Manfred pointed, green would be the best choice and based on that, I ordered the closest (color) fluorescent lamp: 6,500 °K. I am waiting for it to make a test and I will publish in the forum.
By the way, 1000Bulbs.com have a very nice variety of lamps and excellent price ($1.67 for a F13T5 6500K tube).
Thank you for your advise! Sincerely,
Sergio

[xpatUSA]

Hi Ted,
let me clarify: I am using low (lumens) level light because I have reflections problems and at the same time I do not have room enough for a bigger illumination device. So I am trying to get the most with the less...

Yes Sergio , I know about reflections because I take pics of wristwatches, for example:



Here's the rig that took the shot, pardon the mess!



Actually, that is the same watch, I believe.

The lamps are LED which will perhaps irritate purists but the point here is the avoidance of reflected highlights that burn out large parts of the image. The answer is the diffusion of the illumination, not the changing of it's color temperature. The 6500K lamp you have coming will be a disappointment - it will look very bright and have almost a bluish cast, like those horrible headlamps that kids like on cars. If you use diffusers in the same way as above, moving the lamp away from the subject reduces the illumination on the subject by a square law and also increases the diffusion (with floodlights anyway).

A wide angle flood might help with your problem, or a regular desk lamp with the direct view to the bulb blocked out. Also check out those home-made diffusers hanging on the back wall. I used to put those over 13W CFL's in those same lamps.

Notice also the barely visible tracing paper hanging in front of the 30W Philips TL950 overhead tube, makes lots of difference!

Good luck, do post your results . .

[Bobobird]

Interesting thread and info.

I do not do indoor stuff due to lack of space but always good to know about such things as they may apply in other situations.

@Ted - I am intrigued by the camera position. How is it being held in that position?

[jcuknz]

Hi Ted,
let me clarify: I am using low (lumens) level light because I have reflections problems and at the same time I do not have room enough for a bigger illumination device. So I am trying to get the most with the less...
I appreciate your contribution,
Sergio in Houston,TX

Without knowing what you are photographing it is hard but I would suggest that if you have room to photograph the subject you have room for a light tent ... one/two/three thicknessed of muslin cloth with a hole for the camera lens ... you then organise the lights outside the tent. The idea being that the whole subject reflects instead of just parts of it. This may result in rather soft results which you then harden up in editing. If you want shadows to reveal the shape of the object you lay opaque paper over the tent to cast a revealing shadow. This was demonstrated in a NZ magazine a few years ago by a photographer who used the backing paper from rolls of 120 film.

This is what expatUSA has done with the watch but carefully organised the white diffusion screens in the positions that the watch reflects them to get that lovely even glow.
There must be an upstand from the tripod to mount either the camera or perhap better the lens to, I have seen such collars advertised for lenses without tripod mounting threads and know SRB-GRITURN.com offer them.

[Mike Buckley]

I have reflections problems and at the same time I do not have room enough for a bigger illumination device.

This is what expatUSA has done with the watch but carefully organised the white diffusion screens in the positions that the watch reflects them to get that lovely even glow.

I highly recommend the book, "Light: Magic and Science," now in its second edition.

[Manfred M]

I would tend to agree with the vairous writers. I've occassionally been called on to do some reprographics work of artwork behind glass. It is certainly not at the miniature scale of Ted's work, but the setup is not all that different looking. The artwork is hung on walls and I use a pair softboxes at about 45° from the subject that are set up just out of the frame. I shoot at night to ensure that there are no external light sources hitting the art.



As your can see, no reflection of the glass. This picture was actually taken hand-held as I was not after "perfection", but just generally demonstrating the technique. I normally use a tripod in these settings.

[jcuknz]

While we are talkinmg about light tents below is my version of the bucket tent I read about complete with plastic magnifying glass as close-up lens. The motor was shiney but not like a watch. It did need Post processing and what with soft daylight through the window and the bucket it was rather soft. One placed the object on cloth over bricks [whatever] to bring it up into the focus range and with the x10 zoom of my FZ50 ... hole cut to match lens.

if one wanted highlights I guess one could drill another hole or two/three big enough for a small LED torch to shine through but I didn't progress that state of the art

[FrankMi]

It is very helpful to see just HOW folks get the images they post. Thank you Ted, Manfred, and jcuknz!

[darekk]

I haven't red this thread carefuly, maybe someone has answered this question or this post is not an answer and has nothing to do with this thread ...
In case of RAW format you can change color temperature using software. Canon's Digital Photo Professional can do it (see attachment), Nikon or Sony for example probably have their own software doing that, maybe Adobe Lightroom can.
In case of Digital Photo Professional you can even apply custom color recipe saved earlier in the .vrd file.



Maybe you could use color conversion filter:

6500K (your fluorescent lamp):
M = 1000000 / 6500 = 153.85 M (or MK-1) = 15.4 dM (decaMireds)

5700K (+/- natural light)
M = 1000000 / 5700 = 175.44 M (or MK-1) = 17.5 dM (decaMireds)

So you have to use 17.544 - 15.385 = +2.159 dM (or +21.59 MK-1) conversion filter (pink)

http://en.wikipedia.org/wiki/Mired

Here is sample color correction filter (warming) with strength of +18 M (or MK-1 or +1.8 dM):
http://www.schneideroptics.com/Ecomm...D=624&IID=3123
probably this is KR 1.8 filter.

Theoretically it is supposed to shift color temperature from 6500K to 5819K:
153.85 + 18 = 171.85
1000000 / 171.85 = 5819
(this number is too accurate, such calculations are very approximate ...)

More color conversion filters:
http://www.bhphotovideo.com/c/search...rch=yes&sts=ta

The rest you can fix digitally.

Actually probably the best way is to use custom color balance if your camera is supported with such option. Take picture of the Kodak Gray Card lightened by that fluorescent lamp and make it white color standard.
Or color conversion filter + custom color balance ! To avoid digital modification of very deformed light, perhaps not containing many colors when saved in camera's processor, because probably they are "rounded" to one of limited number of integers (maybe "weak" colors, "infusions" to "stronger" ones can be lost ?) ... Improve it preliminary using filter and only tune what is converted to digital information.

[xpatUSA]

@Ted - I am intrigued by the camera position. How is it being held in that position?

It's a Giottos ball-head mount, so the camera is adjustable to any angle.