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What is PACE

The Color Equation

Color is a property of light perceived by the organisms that view it. The color of objects is influenced by how that object interacts with the incoming light illuminating it. A banana appears yellow to the human eye because the banana peel is absorbing strongly all wavelengths of visible light except for a range of the yellow wavelengths, which are being reflected back to the viewer. This allows us to visually detect the object that our brain recognizes as a "YELLOW BANANA!""

Similarly, in the ocean, water molecules strongly absorb red light, preferentially leaving behind the blue light which we see as the deep azure color of the open ocean. Explore the wide range of ocean colors in PACE's Ocean Color Image Gallery.

This, of course, contrasts with the blue sky, which is blue due to the preferential scattering of shorter wave lengths of light. Want to dig further? Check out this article from McGill University – Why is the Sky Blue? Or Better Yet, Why is the Ocean Blue?

Incoming Light Spectrum - Sunlight!

While our Sun emits a wide range of electromagnetic wavelengths from its surface into space, the sunlight that actually reaches us on earth's surface has a much reduced mix of colors – i.e., spectral composition – mostly because sections of it are absorbed by our atmosphere. Moreover, our human eyes are only able to detect the relatively narrow band of wavelengths. The spectral composition of sunlight reaching earth's surface varies in intensity at different times of day and with changing cloud cover. Clouds are complex characters when it comes to light interactions: some types act to absorb light energy and reemit it as infrared/heat, while others act to block/reflect incoming light.

What Determines the Color of an Object?

We know that the color of an object that we see is due to the light reflected off of that object. If a banana is ripe, it will reflect back yellow light. If a banana is not ripe, it will reflect more green light. If the banana is very ripe, we'll see a black banana (and probably smell it too!).

Now let's get into the numbers. We know that colors are a result of wavelengths along the spectrum of visible light. Scientists use an instrument called a spectrometer to measure the magnitudes of wavelengths of light that are being reflected off of an object – read more here about PACE's spectrometer, the Ocean Color Instrument. We can see that a green banana peel reflects more light around a wavelength of 550 nm whereas a ripe banana reflects more light in the wavelengths of 675 nm. Graphs of the wavelengths of light reflected off of objects are called their spectral signature. Just like a human fingerprint, spectral signatures are unique for each object. In this way, if scientists know the spectral signature of optical constituents in the ocean (e.g., phytoplankton, silt) and atmosphere (e.g., smoke), they have a good chance of recognizing the "stuff" that interacted with the light signal captured by PACE. See how well you can match spectral signatures in the PACE Card Matching Game.

 How Does Human Vision Work?

The Path of the Light: Light reflected off objects enters our eyes through the tough protective covering of our cornea and passes through the opening of our pupil. The amount of light entering our eyes is controlled by a pigmented circular muscle called the iris. Under low light conditions the iris will open wide, dilating the pupil to let in more light. Conversely, in bright light conditions the iris constricts the pupil. Once inside the eye, the light is focused by a flexible lens onto the back inside wall of the eye, the retina. Light-detecting cells in the retina are called photoreceptors – rods give us "night vision", sometimes called black and white vision, and cones allow us multispectral, color vision.

The Colors We Can See: Eyes are only able to detect a short range of the electromagnetic spectrum.

For more details about color vision, check out the Color Vision section of the Ocean Optics Textbook .

 Advanced Topic: Looking at PACE Data in RGB

Humans are hardwired to see in a trichromatic spectrum, or red, green, and blue vision (RGB). So, it is not a surprising that computer monitors, printers, and digital cameras are often designed to show color in terms of RGB. Here’s how it works. The light sources within computer monitors and TV screens are made up of groups of three pixel sets. Each set of three pixels has a red light, green light, and blue light. Different colors are then produced by controlling how much light is emitted from each of these pixels, much like mixing colors of paint. Check out the image to the right to see how altering RGB combinations of each of these colors is used to create a full color wheel.

Looking for more? Check out the Ocean Optics Textbook  for more on how computers display color in RGB.

How will PACE turn energy into images? Find out the basics on page 12 of this resource: NASA Tour the Electromagnetic Spectrum.

<< Section I: Light Properties    Section III: Light & the Atmosphere >>