Cesky: |
Not many people know that there are 2 purple colors:
1) "False purple" - it is not really the purple color we see in a rainbow, for example. It is formed from 2 spectral components - red and blue. Not that these components somehow mix to form purple. But in such a way that it irritates the eye in a similar way to real purple. The fake purple is displayed by the monitor or printed by the printer because they have no way of displaying the real purple.
2) "Real purple" - is the purple we see in the rainbow. It has the shortest wavelength and highest frequency of the visible colors. It lies at the end of the spectrum, near the ultraviolet. The monitor cannot display the ultraviolet components, so it has to simulate the purple colour by using a "false purple", by lighting up the red and blue colours.
For this reason, we can observe that some cameras will take a picture of some purples as we see them, but will take a picture of another purple as blue. Yet to the eye, the colours look exactly the same. This is because of whether it is a "true" purple, lying close to the UV colors, or a "false" purple as displayed by the monitor or printer. Some cameras may have limited sensitivity in the UV colour range - either by imperfections in the red sensor, which has no secondary sensitivity in the higher frequencies, or by a UV filter that limits the UV components.
With color, because of digital expression, people have the misconception that color is made up of RGB components. Normal daylight is actually a broad spectrum of frequencies, you could say continuous (spectral lines are narrow and dense). What we think of as daylight white light looks something like this (sunlight also contains UV and IR components and other frequencies far from the visible part):
When light bounces off the surface of an object, the light is modulated - different parts of the spectrum are reflected with different intensities. This does not mean that only the yellow component is reflected and the others absorbed. Again, the whole spectrum (including UV and IR) is reflected, just with different intensities. For example, reflected light from an object that we consider yellow:
The human eye is not capable of perceiving the entire spectrum of light. When such broad-spectrum light hits the cones, it irritates the cones with different intensities depending on which band each cone is most sensitive to. There are 3 types of cones, the most sensitive in the band around the red colour, the green colour and the blue colour, in the visible part of the spectrum i.e. about 380 to 750 nm. By comparing the irritation intensities of the cones with each other, the brain then evaluates the colour. Note the small bump of the red component on the left of the graph - this bump is responsible for our perception of "true purple" as purple and not blue.
Cones are oscillators oscillated by light at a certain frequency. The red cone has the lowest frequency (=highest wavelength) and oscillates also at the second harmonic of its fundamental frequency, i.e. at the opposite end near the UV region. Normally there would be a second peak of sensitivity (somewhere at 300 nm), but due to the limitation of the sensitivity range, only a small "bump" remains at the end of the visible region. But still, because of it, we are able to perceive the "true purple color" - like the irritation of the red and blue cones, but without the green cone.
When we want to capture a colour image with a camera, we use sensors that are sensitive in the same areas as the human eye - i.e. RGB. This produces 3 signals with different intensities. The monitor then displays the dots in RGB colours and these irritate the cones of the eye in the same way as the original light. But the light from the monitor is NOT the same light as normal daylight. The color from the monitor just simulates a similar irritation to the cones as the original light irritates them. The original light is a broad continuous spectrum, while the light from the monitor is 3 spectral components of RGB. So we are not actually capturing the original colors, just the same irritation as the eye has.
Animals have a different color perception and therefore would see the colors from the monitor completely different than in reality. Even humans have slightly different cone sensitivities and their oscillation frequencies, which is why color evaluation from a monitor can vary from person to person. They may argue that the color on the monitor is different from the original color from real life because each person has a slightly different color tuning. If one person tunes the shade of color to the original, the other person would tune it a little differently. In doing so, they will argue that everyone is right, and indeed everyone will have it right. Each will see the same shade of color differently than the other. RGB is just a sort of empirically determined standard and may differ from real people.
And thus the purple color and the purple color are not the same. Sensors are not able to accurately copy the spectral waveform of the eye's sensitivity (partly because it is not measurable, it is only determined empirically), nor is it technically possible to do so accurately. Even if the sensor has a corresponding maximum at the correct wavelength, it may have a different sensitivity to the second harmonic (e.g. it extends more into the UV region) and this will change the interpretation of violet colours. Correct violet tends to be a big problem with cameras. The excitation frequency may be already in the UV region, which will still partially irritate the red cone (at the 2nd harmonic), but the photocell may not pick it up anymore, because its range is limited by the red filter to the lower bands of the spectrum.
Thus - there may be purple colors that the camera takes as well as the eye, but there may be purple colors that the same camera takes as blue, even though the eye sees purple. This is because colors in the real world are not three RGB components (as perceived by the eye), but a broad spectrum of frequencies.
Miroslav Nemecek
