PSC 131 Color Qs

What wavelengths of light are in our visible spectrum?

400-700nm

Color deficiencies can arise from a number of different cone anomalies. What is a monochromat? Dichromat?

A monochromat is someone with one one cone type. A dichromat is someone with only two cone types. Most people with typical color vision are trichromats - people with three different cone types.

How would a monchromat or dichromat differ from a trichromat on a color matching task?

A monochromat would only need one light source to match every color given to him/ her on the color matching task.

How does the color matching experiment support the trichromatic theory?

Color matching experiments in which there is a given color and subjects have to match that color using different sources of light. The experiments showed that every color could be matched perfectly by the human eye if there were three sources of light that could be adjusted for short, medium, and long wavelengths of light.

What are metamers?

Colors that look identical even though they have different combinations of wavelengths of light.

What is the problem of univariance?

If we only had one photoreceptor, many different wavelengths of light would look the same.

If you only had one cone type, how would the colors that you normally perceive be affected (this is the topic of the lab)?

More reflected light would be perceived as brighter and less light as darker, but only objects that reflected wavelengths of light that could be picked up by the one cone type would be seen as having any color or brightness. Thus, everything would like grayscale of wavelengths picked up by that cone.

Now let's look at the world under the low pressure sodium light again, but make some predictions. • How will red look? • How will blue look? • How will yellow look? Why?

Red and yellow will both look dark black

If an object absorbs short and medium wavelength lights in the blues and greens, what color will the object appear to be?

Reddish, because only the long wavelengths of light will be reflected off the object and into the observer's eyes.

What property of light gives rise to our perception of different colors?

Reflectance of different wavelengths of light. Reflectance describes the light wavelengths that bounce off the objects that we perceive. Other wavelengths, if present, are absorbed by the object and therefore do not contribute to our perception of their color.

What is a tetrachromat?

Someone with 4 cone types.

What determines what colors we perceive?

The colors we see are determined by the combination of light that enters into our eye - either directly from a light source, or reflected off objects.

What is the trichromatic theory of color vision?

The idea that we need three different type of receptors to recreate all the colors we see.

How does the demo simulate color blindness? And how is it different?

The low pressure sodium lamp only emits one wavelength of light so this is the only light that can reflect off of any object. It simulates color blindness (a monochromat) because a monochromat only has one cone type and so can only absorb light within the range of that cone sensitivity. Both produce similar outcomes in that only very limited wavelengths of light are being absorbed by photoreceptors and contribute to perception of color. They are different because in the demo, there is only one light source. In colorbllind individuals, there are many wavelengths of light in the environment, but they can only absorb a very limited set.

What are opponent colors? How do afterimages indicate their presence? And what do they suggest?

The opponent-process theory states that the cone photoreceptors are linked together to form three opposing colour pairs: blue/yellow, red/green, and black/white. When one member of the colour pair is "fatigued" by extended inspection, inhibition of its corresponding pair member is reduced. This increases the relative activity level of the unfatigued pair member and results in its colour being perceived.

What color will you see if you mix blue and yellow light? What about blue and yellow paint? Why?

• Mixing blue and yellow light will produce something close to white because yellow includes both medium and long wavelengths of light and blue includes the short wavelengths of light. Mixing blue and yellow paints gives rise to the perception of green, which is composed of medium wavelengths of light. This is because both blue and yellow pigments reflect a range of medium wavelengths of light. • White light, which is the combination of all the different wavelengths.