[opendtv] Re: News: Will LEDs Light Your Productions?

Hi Jeroen, I realised what you were doing, which is why you got a smiley.

Yes, the same is true for cameras. For a camera to have perfect colour
analysis, the spectral sensitivities of the RGB sensors are the
colour-matching functions of the system (display) primaries. You get those
by taking the CIE 1931 x(bar) y(bar) colour-matching functions and passing
them through the XYZ-RGB matrix derived for the system by standard maths (I
know that you know this, because it's all in the book chapters I sent you).
There is no room for altering these curves, they are defined by the maths
and the CIE tabulations that describe human vision. The impossibility of
having these curves results from the need for negative lobes, which can only
be simulated or approximated by using a 3x3 matrix. The matrix will not get
it "right", it can't it isn't possible, it can only get to an acceptable
compromise.

However, suppose you have a "perfect" camera with the correct curves. Then,
if you light your scene with spiky sources and display the camera signals on
a proper monitor, you'll see the same (incorrect) colours that you would see
in the scene. It's all very simple, the spectrum that reaches the sensor is
the convolution of the illuminant, the object colour reflectivity, and the
transmissivity of any optics in the camera. That spectrum is shared through
filters (either dichroics and absorptive shaping filters for 3ccds, or
absorptive dyes for singles) onto the sensors, which integrate the spectral
content. You can easily do all of that in Excel to see what the signals
become (that's how I cross-check my own maths).

Although LEDs initially are spectral emitters (single spikes) there's
nothing to stop a manufacturer putting low-energy phosphors in to broaden
the spectral content. It's exactly the same as for discharge lamps.

For more on illuminant sources, try looking here.
http://www.squ1.com/index.php?http://www.squ1.com/lighting/lamps.html
The spectral distributions are pretty accurate, the ones that correspond
with measurements I've made are very close. Although the spikes are not
high, they're broad; that's because the measurements were made at 5 or 10nm
intervals, so each measurement is an integration of the light in a 5 or 10nm
range, thus the spikes are actually much higher and narrower, but that
doesn't really matter much.

----- Original Message ----- 
From: <jeroen.stessen@xxxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Monday, October 25, 2004 3:50 PM
Subject: [opendtv] Re: News: Will LEDs Light Your Productions?
> > trying to teach it to you for some years now, but I don't seem to be
> having
> > much success :-)
>
> I know that...  I was just teasing you to try to get you to be
> a little bit more accurate in what you say or how you say it.
> I was afraid that you meant to say that spiky spectra are okay.
>
> Interesting what you say about monochromatic primaries, that they
> would reveal differences in colour perception between observers.
> I suppose that the same could be said of cameras then, if you fed
> them with monochromatic light sources ? They would become super-
> critical ? Fortunately this is not a realistic use case.
>
> About these LEDs: theoretically an LED could be sold as a "white
> lamp" if it produced only a line of yellow and a line of blue on
> opposite sides of a white point. If you shine it directly into
> your eye then you will certainly see white light. But it is still
> useless as an illuminator for surface colours. Who will guarantee
> me that these LEDs that this thread is about have anywhere near a
> flat spectrum ? I would like to see some evidence for that...
>
> Best regards,
> -- Jeroen
>
> +-------------------------------+----------------------------------------+
> | From:     Jeroen H. Stessen   | E-mail:  Jeroen.Stessen@xxxxxxxxxxx    |
> | Building: SFJ-5.22 Eindhoven  | Philips Digital Systems Laboratories   |
> | Phone:    ++31.40.2732739     | Visiting & mail address: Glaslaan 2    |
> | Mobile:   ++31.6.44680021     | NL 5616 LW Eindhoven, the Netherlands  |
> | Pager:    ++31.6.65133818     | Website: http://www.pdsl.philips.com/  |
> +-------------------------------+----------------------------------------+
>
>
>
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