characteristics and limitations of digital color photography

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Characteristics and Limitations of Digital Color Photography. Harald Brendel Munich, 21 September 2012. Light is Electromagnetic Radiation. Long wavelength (~700nm). Short wavelength (~400nm). Radiometry. Incandescent Light. Spectral Power Distribution. Incandescent Light. 3200K. - PowerPoint PPT Presentation



Characteristics and Limitations of Digital Color PhotographyHarald BrendelMunich, 21 September 2012


Light is Electromagnetic RadiationShort wavelength (~400nm)Long wavelength (~700nm)21.09.12Harald Brendel, Arnold & Richter Cine Technik2Light is electromagnetic radiation. Isaac Newton demonstrated that white light is separated into rainbow colors by a prism. Today, we know that white light is a mixture of radiation having different wavelengths ranging from blue at 400 nm to red at 700 nm. We see the short wavelengths as blue and the long wavelengths as red.



21.09.12Harald Brendel, Arnold & Richter Cine Technik3If we put an detector that measures the radiometric flux in the light from the prism, we can measure the power at a certain wavelength range. Turning the prism adjusts the center of this range.3

Incandescent Light

Spectral Power Distribution21.09.12Harald Brendel, Arnold & Richter Cine Technik4The result is called Spectral Power Distribution

Here we see the SPD of one of the oldest man made light sources.Incandescence is the emission of radiation from a hot body as a result of its temperature. The term derives from the Latin verb incandescere, to glow white.



2000KIncandescent Light

21.09.12Harald Brendel, Arnold & Richter Cine Technik5Here is the SPD of a light source that we are going to ban soon.

Compare with the SPD of the candle: more power in the visible spectrum below red because the bulbs filament is hotter than the candles wick.The filaments typical temperature is 3200K while the wick burns rather at 2000K.

Thats the origin of the term color temperature: its the temperature of the glowing body measured in Kelvin. The hotter the bluer.5


5600K21.09.12Harald Brendel, Arnold & Richter Cine Technik6SPD of direct sunlight.The sun is much hotter than the candle resulting in even more power in the blue/green region of the spectrum.6


Daylight21.09.12Harald Brendel, Arnold & Richter Cine Technik7Clouds filter some of the radiation having longer wavelength. Thats why the light on cloudy day is somewhat cooler. It results in a color temperature of 6500K.

Compare with direct sun light. 7


21.09.12Harald Brendel, Arnold & Richter Cine Technik8A photoreceptor is a device that responds to light with a signal.

If we use a known light source and a device similar to the one used to measure the SPD we can measure the spectral response curve of the receptor. In many cases we are only interested in the relative spectral response normalized to a maximum value. The figure tells us that the receptor is double as sensitive for radiation at 555 than at 510.

Figures of SPD and spectral sensitivity look similar but show two opposed principles: emission of electromagnetic radiation and response to such radiation.8Retina Brendel, Arnold & Richter Cine Technik9There are three different types of photoreceptors in the retina of the human eye, which are called cone cells or just cones.

They respond to long, middle and short wavelength regions of the spectrum and are called, L, M, and S cones.

The image on the right side shows a typical distribution of the cells. There are much less S cones than M and L cones (about 1:100).9Color Receptors

x=21.09.12Harald Brendel, Arnold & Richter Cine Technik10The signals of the cones are the primary stage of our color sensation.What happens when radiation hits the cones?

We have to multiply the SPD of the light with the spectral response curves of the receptors. Once again we are only interested in relative amounts and rescale the figure to the maximum value.

Red is the relatively tallest peak because the radiation contains a lot of power in the long wavelength range. The green peak is a bit smaller but still taller than blue.

The primary signal is equivalent to the surface under the curves. 10


BrightnessColor Sensation21.09.1211Compare again the areas under the three curves.

These are the tristimulus values. Their absolute magnitude corresponds to the brightness (and is less interesting because we adapt to a wide range of brightness). The relative strength of the three signal constitutes the perceived color attributes hue and saturation.


21.09.12Harald Brendel, Arnold & Richter Cine Technik12The upper figures show the previously explained results again: the formation of the tristimulus values for incandescent light.

For comparison lets look at the tristimulus values for daylight. Here we have a higher S component and the overall result is more balanced.12Primary Colors

21.09.12Harald Brendel, Arnold & Richter Cine Technik13Here are the SPDs and the tristimulus values of light that is seen as having red, green, and blue color, respectively. This is type of radiation is emitted from a digital projector used in a cinema.

Because of the large overlap between the M and L cones, its almost impossible to produce a pure excitation of just one of the two types of receptors. 13Additive Color Mixture

21.09.12Harald Brendel, Arnold & Richter Cine Technik14Lets see what happens when we move the three light spots together.14

Additive Color Mixture: Red and Green


21.09.12Harald Brendel, Arnold & Richter Cine Technik15We get an additive mixture of colors that produce yellow, cyan, magenta and white areas where two or three of the primary colors overlap.

Lets measure at the SPD of the yellow area. Its simply the sum of the SPD of the red and of the green light. At each wavelength we add the power of the two light sources.

What do you think will be the tristimulus values of the yellow color? You bet, its the sum of the tristimulus values of the two sources.15

Additive Color Mixture: Blue and Green

21.09.12Harald Brendel, Arnold & Richter Cine Technik16This will become really important, thats why I repeat it again for the other colors.

Cyan is the mixture of green and blue.16

Additive Color Mixture: Red and Blue

21.09.12Harald Brendel, Arnold & Richter Cine Technik17Magenta is the mixture of blue and red.17

Additive Color Mixture: Red, Green, and Blue

Harald Brendel, Arnold & Richter Cine Technik18Now, we mix three colors and we get white18RGB forms White


=21.09.12Harald Brendel, Arnold & Richter Cine Technik19Again the formation of white.

Wait, we have seen these tristimulus values before, havent we?

Yes, they are the same as those for direct sunlight. 19Metamerism

Two Different SpectraSame Tristimulus ValuesSame Color Perception(For Average Observers)Colors Are No Physical Attributes21.09.12Harald Brendel, Arnold & Richter Cine Technik20This is probably the most peculiar fact about color vision. We do not sense the wavelength of the radiation but we sense the relative amount of short, middle, and long wavelengths in the stimulus.

Two stimuli can produce the same tristimulus values, which results in the same sensation. This is called metamerism.

This works for the average observer whos cone cells match the standardized values. When two spectral are different some people may see them as having different colors even when their standard tristimulus values are the same.

Did I say two stimuli? Its actually even more confusing: an infinite number of stimuli can produce the same tristimulus values at least in the world of mathematics. If those spectra exists in the world of physics is another question. But it means that its impossible to know the SPD from the tristimulus values.

This is very different from hearing where we recognize single frequencies as notes as well as any mixtures of notes. If two frequency spectra look different we hear two different sounds.

I know this is a confusing thing about color vision but the good thing is: metamerism is the reason why can reproduce colors.


Color Reproduction

21.09.12Harald Brendel, Arnold & Richter Cine Technik21Lets look at this scene again. You see trees and lawn. The green/yellowish color is produced by the chlorophyll in the plants.

This is the SPD of light when its reflected by chlorophyll.

But even the most faithful reproduction of the scene (be it a photograph, a digital image, or a painting) will not contain any chlorophyll. But we can reproduce its color when we have a metameric pair and we can create one by mixing three primary colors in the appropriate amounts.

I show you the spectral sensitivity of the cones in comparison to the SPD of the light reflected from the foliage.

This is the SPD of the digital projector projector (and again the cones) producing the same tristimulus values.

21Additive Display

21.09.12Harald Brendel, Arnold & Richter Cine Technik22Now, you understand how to build a color display:You reduce the size of the spots where the light of the three primary colors is mixed to little dots. In each dot you adjust the amounts of primary light until the resulting SPD produces the tristimulus values of the color you want to display.22Color Gamut

Spectral (monochromatic) locusPrimary colorsMixtures of two colorsMixtures of three colors21.09.12Harald Brendel, Arnold & Richter Cine Technik23The range of possible colors such a display can produce can de displayed as a triangle in a chromaticity diagram. The curved line represents the monochromatic stimuli of the rainbow. All possible colors can be thou