The Negative Afterimage Simulation
Introduction
This simulation is another simple one that nevertheless will allow you to perform some sophisticated investigations of the phenomenon.
Tips and Hints
1. Unfortunately, the sliders produced by JAVA (the programming language used to write these simulations) are not the best 'behaved' when using a browser to run a simulation. This is particularly true for this particular simulation. Therefore, if you use the sliders to change of the stars, you need to wait a second or two between mouse clicks. Otherwise (as you'll no doubt see, sooner or later) the sliders change appearance and behave in bizarre ways that make them impossible to use. If you wish to make large changes in the RGB values, there are two ways to minimize the chance of the sliders' misbehaving:
a. Instead of clicking repeatedly in the slider's track, 'grab' the slider itself by placing the mouse cursor on the slider and clicking the left mouse button and keeping it 'clicked' while you drag the slider to the desired value, or
b. Use the slider's textfield to make the change (remember, you'll have to click the "Go" button to cause the change(s) to take effect).
Should the sliders start misbehaving, clicking the "Reset" button should cure the problem.
2. You’ll need to allow sufficient ‘recovery time’ between viewings; a minute or two should suffice, but you’ll have to determine this for yourself. You and your lab partner(s) could take turns completing each ‘experiment’, which would allow your color vision time to recover before the next viewing session.
3. An eye-to-screen viewing distance of 30 – 50 cm is about right for most viewers, but you may adjust that to your personal tastes.
4. Make sure that the brightness and contrast settings on your computer’s monitor are set towards the upper end of their range. You can even set them to their maximum settings, if you wish. A bright, high-contrast image will facilitate your development and perception of the afterimage.
Exercises & Thought Questions:
1. Use the default settings (black stars on a white background) and stare at the cross in the center star until you get a good afterimage when you shift your gaze to the upper (all-white) panel. Note the appearance of the negative afterimage. What color are the stars? What color is the background? Can you see an afterimage of the cross? Are all of the star afterimages of equal intensity?
2. Choose a color you’d like to use for the stars and, leaving the stars’ background white for now, generate a negative afterimage as described above. Note its appearance and color of the afterimage.
3. Repeat #2, this time also changing the color of the background. Does the color of the background affect the appearance of the afterimage?
4. Repeat #2 as many times as you like, changing the color of the stars and/or background as you wish.
5. Focus on the crosshair in the center star until you get a good afterimage (15 – 30 sec should suffice), then move back from the screen while observing the afterimage. Does the size of the afterimage change? What is your explanation of this phenomenon?
6. Students could compare their individual threshold sensitivity to the different colors (what’s the smallest RGB number, individually or in combination, that they can discriminate from black, and what’s the smallest change in an RGB value they can reliably detect?) with results of other students.
7. Go to the library (or search the web) and search for the keywords “Newton’s Color Circle”, and use this information to help you interpret your results for Exercises #2 and #3.
8. This is not directly related to the negative afterimage phenomenon, but you might find it interesting to use the sliders and try to produce what are termed "non-spectral colors", i.e., colors that are not part of the spectrum) such as brown, grey, pink, or olive-green. The following links might prove useful and/or informative:
http://eies.njit.edu/~walsh/rgb.html: RGB and hexidecimal values for more than 200 colors. http://www.yorku.ca/eye/nonspect.htm: Explanation of a couple non-spectral colors. http://www.yorku.ca/eye/ciediag1.htm: Illustrates the 1931 CIE Chromaticity Diagram.