# What is the Last Plate of the Color Blindness Test For? **URL:** \ **Category:** Factual Questions\ **Created:** [July 10, 2003, 12:45am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362 "2003-07-10T00:45:26Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![homercles](https://avatars.discourse-cdn.com/v4/letter/h/ed8c4c/32.png) [@homercles](https://boards.straightdope.com/u/homercles)\ **Post date:** [July 11, 2003, 10:07pm UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/21 "2003-07-11T22:07:58Z")
> [@](#): > > \*Originally posted by moriah \* > \*\*Also, blinking very fast will also reveal the 2 to the color sighted. > > Peace. \*\* Ha! Excellent observation :D. But do you know why? Im a color vision scientist and will be happy to tell, but i would like to hear your explaination first 😉 ---
**Author:** ![NTChrist](https://avatars.discourse-cdn.com/v4/letter/n/e274bd/32.png) [@NTChrist](https://boards.straightdope.com/u/NTChrist)\ **Post date:** [July 12, 2003, 12:31am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/22 "2003-07-12T00:31:29Z")
> [@](#): > > \*Originally posted by Nanoda \* > \*\*I think I mentioned it in another thread, but I always enjoy remembering the time in grade 12, Bio 30, when some guys in the back of the class (you know the type), colluded to cheat on a colour blindness test. The front of the class never felt so good… \*\* Hey! You can’t do that! No starting a story without finishing it! P.S. which HS in Edmonton did you attend? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [July 12, 2003, 12:46am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/23 "2003-07-12T00:46:11Z")
> [@](#): > > \*Originally posted by homercles \* > \*\*Ha! Excellent observation :D. But do you know why? > > Im a color vision scientist and will be happy to tell, but i would like to hear your explaination first 😉 \*\* Well, looking at the [resonse curves](http://www.color-tec.com/color/p7.htm) for the three different types of color-sensing cone cells, it is imediately apparent that the blue-sensing cones are significantly more sensitive than the red- or green-sensing cones. I can conclude, therefore, that rapid blinking allows insufficient light to fully activate the red and green cells, while still permitting a normal blue response. Is this correct? ---
**Author:** ![homercles](https://avatars.discourse-cdn.com/v4/letter/h/ed8c4c/32.png) [@homercles](https://boards.straightdope.com/u/homercles)\ **Post date:** [July 12, 2003, 1:07am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/24 "2003-07-12T01:07:44Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\*Well, looking at the [resonse curves](http://www.color-tec.com/color/p7.htm) for the three different types of color-sensing cone cells, it is imediately apparent that the blue-sensing cones are significantly more sensitive than the red- or green-sensing cones. I can conclude, therefore, that rapid blinking allows insufficient light to fully activate the red and green cells, while still permitting a normal blue response. Is this correct? \*\* OH! So close, but nope sorry. The real case is actually quite the opposite of that HORRENDOUS site. All things being equal, much more short wavelength (“blue” but please never use that terminology again) energy is “absorbed” by the cornea, lens, and macular pigment, relative to the longer wavelengths. So, the total absorbition measured at the cones is substantially reduced at shorter wavelengths. Thus, that figure (showing B G R responses) is profoundly inaccurate. If you want more accurate links just ask. HINT: what does blinking your eyes really fast versus really slow change? ---
**Author:** ![netscape\_6](https://avatars.discourse-cdn.com/v4/letter/n/8c91f0/32.png) [@netscape\_6](https://boards.straightdope.com/u/netscape_6)\ **Post date:** [July 12, 2003, 2:09am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/25 "2003-07-12T02:09:36Z")
> [@](#): > > \*Originally posted by homercles \* > \*\* > HINT: what does blinking your eyes really fast versus really slow change? \*\* what wave lengths have a chance to get in? ---
**Author:** ![homercles](https://avatars.discourse-cdn.com/v4/letter/h/ed8c4c/32.png) [@homercles](https://boards.straightdope.com/u/homercles)\ **Post date:** [July 12, 2003, 2:25am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/26 "2003-07-12T02:25:18Z")
> [@](#): > > \*Originally posted by netscape 6 \* > \*\*what wave lengths have a chance to get in? \*\* Uh, nope. Think about what a computer monitor does at different refresh rates. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [July 12, 2003, 2:40am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/27 "2003-07-12T02:40:48Z")
I think I see where you’re going with this. You’re referring to the effect of afterimages? When you look at an object, then close your eyes, you see a brief negative-image afterimage. Blinking quickly superimposes the afterimage on the real image in the brain, altering the perceived color balance. Better? Incidentally, I found [this](http://webvision.med.utah.edu/) excellent color-vision reference in my searching for more information. ---
**Author:** ![williambaskerville](https://avatars.discourse-cdn.com/v4/letter/w/f14d63/32.png) [@williambaskerville](https://boards.straightdope.com/u/williambaskerville)\ **Post date:** [July 12, 2003, 10:54am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/28 "2003-07-12T10:54:41Z")
I don’t see anything in that last image, no 5, no 2, just dots. I’ve had color blindness tests and been passed as normal…but I AM blind in one eye. Is there a depth-perception aspect to that one ? ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [July 12, 2003, 10:57am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/29 "2003-07-12T10:57:42Z")
Forget the damn two - how do I see the _five?_ Yes, I know colorblindness is foever. \>sigh\< ---
**Author:** ![voltaire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voltaire/32/313_2.png) [@voltaire](https://boards.straightdope.com/u/voltaire)\ **Post date:** [July 12, 2003, 12:08pm UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/30 "2003-07-12T12:08:12Z")
Put me down for seeing an 8 in that last one…So what does _that_ mean? ---
**Author:** ![bustmeanoreo](https://avatars.discourse-cdn.com/v4/letter/b/c67d28/32.png) [@bustmeanoreo](https://boards.straightdope.com/u/bustmeanoreo)\ **Post date:** [July 12, 2003, 2:01pm UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/31 "2003-07-12T14:01:48Z")
i’m seeing an 8 in the last one also, but if i close one eye, i see the two…what the heck does that mean…i’m colour blind in one eye??? ---
**Author:** ![homercles](https://avatars.discourse-cdn.com/v4/letter/h/ed8c4c/32.png) [@homercles](https://boards.straightdope.com/u/homercles)\ **Post date:** [July 12, 2003, 4:57pm UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/32 "2003-07-12T16:57:17Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > **I think I see where you’re going with this. You’re referring to the effect of afterimages? When you look at an object, then close your eyes, you see a brief negative-image afterimage. Blinking quickly superimposes the afterimage on the real image in the brain, altering the perceived color balance. Better?** Uh, im not so sure i understand that explaination. But the real reason why you can see the 2 when you blink really fast is that the LM opponent system has a lower sensitivity to temporal frequencies (temporal frequencies being the rate at which a light can be switched on and off). By blinking really fast youre basically “shutting off” the LM opponent system and just relying on the pure luminance information. Make sense? > [@](#): > > Incidentally, I found [this](http://webvision.med.utah.edu/) excellent color-vision reference in my searching for more information. Yes, that site is MUCH better. ---
**Author:** ![moriah](https://avatars.discourse-cdn.com/v4/letter/m/b2d939/32.png) [@moriah](https://boards.straightdope.com/u/moriah)\ **Post date:** [July 12, 2003, 6:45pm UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/33 "2003-07-12T18:45:47Z")
In the last figure there is a 5 made out of green and blue dots in a field of red, orange, and yellow dots. For the color blind, the 2 is made out of lighter and brighter dots, and relies on contrast, not color to be seen. Where the 2 and 5 overlap, light blue and green dots are used. Where there is part of the 5, but the 2 does not overlap, dark green is used. Where there is 2, but no 5, light yellow is used. Those who see 8, are overlapping the two images. Those who see 3 are partially overlapping the two images. Take out the row of light yellow in your mind’s eye to see the 5. ## Peace. There, I’m done blinking rapidly deciphering the construction of that test. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [July 12, 2003, 7:02pm UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/34 "2003-07-12T19:02:23Z")
[Here](http://www.gitchat.org/Color5-2.jpg) is that plate rendered as a greyscale image only, confirming **homercles** ’ explanation. The 2 is clearly visible, though it helps to sit back from the monitor. ---
**Author:** ![homercles](https://avatars.discourse-cdn.com/v4/letter/h/ed8c4c/32.png) [@homercles](https://boards.straightdope.com/u/homercles)\ **Post date:** [July 12, 2003, 10:27pm UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/35 "2003-07-12T22:27:40Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\*[Here](http://www.gitchat.org/Color5-2.jpg) is that plate rendered as a greyscale image only, confirming **homercles** ’ explanation. The 2 is clearly visible, though it helps to sit back from the monitor. \*\* Ah yes, validation is a beautiful thing 😃 ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [July 13, 2003, 10:15am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/36 "2003-07-13T10:15:22Z")
I can see the 2 _much_ clearer on the color plate, ya’ll don’t know what you’re missing. And on the monochrome plate I can now see a 3. Anyhow… if an individual is merely green _deficient_, meaning their eyes do respond to green, just not very well, then by manipulating the color balance on a monitor would you be able to adjust the picture (by adding green) until the person was able to see the image as a normally sighted person would? At which point I’d probably say “ewwwww - _hideous_” because I’m used to seeing the world like that, but that’s a different matter. ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [July 13, 2003, 2:12pm UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/37 "2003-07-13T14:12:38Z")
A good way to see the two (in the normal color plate _or_ the greyscale version) directly, i.e. without doing anything special with your eyes, is to view it on an LCD display, then tilt the screen (or your head) away from the normal viewing angle; at a certain angle, the dots making up the two will seem to vanish, leaving it quite nicely outlined. But maybe that’s cheating. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [July 13, 2003, 9:55pm UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/38 "2003-07-13T21:55:22Z")
Gee, I dunno, **Chorpler** , did it blast past you that I don’t need to do anything special to see the 2 - that’s _all I see?_ What the heck was that other number so-called normal people are supposed to see again?.. I think it would be really educational for someone to come up with a gizmo that not only allows normals to see my world, but for me to see the “normal” world. Now that would be cool! (But, like I said, I’d probably go back to seeing my way, as that’s what I’ve gotten very used over about 4 decades.) ---
**Author:** ![Super\_Gnat](https://avatars.discourse-cdn.com/v4/letter/s/e274bd/32.png) [@Super\_Gnat](https://boards.straightdope.com/u/Super_Gnat)\ **Post date:** [July 14, 2003, 12:30am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/39 "2003-07-14T00:30:29Z")
First I saw the five, but then I saw the two (and went “Whoa!”), but now I can’t see the five anymore. ☹ ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [July 14, 2003, 3:44am UTC](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362/40 "2003-07-14T03:44:11Z")
> [@](#): > > \*Originally posted by The Green Feather \* > \*\*The green traffic lights are starting to look more blue. Or is that just me? \*\* Your local street department may be replacing the incandescent lamps in the traffic signals with LED ‘lamps’. They last much longer, and use way less electricity, so result in significant cost savings for the city. But the colors in the LED ‘lamps’ are not always the same shade as the incandescent lamps & lenses were. So you could indeed be seeing traffic lights of a slightly different color. [Previous page](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362.md?page=1) [Next page](https://boards.straightdope.com/t/what-is-the-last-plate-of-the-color-blindness-test-for/187362.md?page=3)