|
Dear Dr. Green,
Thank you for the e-mail. I am curious if you know the wavelength of the fluorescent light that is emitted? Color perception, or lack of it, in the blue end of the spectrum varies widely between people. I have tried an experiment where I was looking at the solar spectrum. I then allowed the spectrometer to tune to shorter wavelengths until I could not see the blue spectra anymore. What I notice is that at some point the blue fades to a whitish color. I am still seeing light but I can't make out the color of it anymore. I have not ruled out scattered light, but what I saw matches your description of the fluoresing rocks. What I think/guess is happening is as the spectral transmission/response of the cornea+rod cells falls off the blue color perception fades to white and the white light is being perceived by the rods which can't tell color, but are more sensitive to blue light than the cone cells are.
Another reason for not seeing well in the deep blue is as one ages their cornea/lens will yellow slightly. This happens mostly in older people 50+??. If this is the case of not seeing the light then one shouldn't see the white light either since their cornea will not transmit this light to the rod cells. Have you ever heard of the gandma with the over tinted blue dyed hair.
By the way I happen to have extreme blue vision in that I can see down to about 380 nm (a 1:1000 statistics on the bell curve), where 90+% of the population will see maybe to about 390nm. I have red/green protan color vision (red weak-anomalous trichromat) and do not know if seeing better in the blue is the result of me being protan color blind.
In any case knowing the wavelength of the emitting light, is it a strong spectral line or broad spectral emission? This would tell me much about how the eye would respond to the light. People's vision in the range of ~420-390 nm will have large variations, from not seeing any light to having no difficulting see the light.
I hope this helps.
Dennis
Fascinating subject. Yes, I have a few thoughts, but I can’t claim any of them have merit – they’re little more than informed guesses, or thought experiments. Much of the following you folks already know, but here goes anyway:
- First, the shorter the wavelength, the more the light is refracted by our eyes. That means that the “focal spot” on the retina, at the back of our eye, is a compromise: light of longer wavelengths will have a focal point behind the retina, and light of short wavelength (blue, violet) in front of it. With monochromatic red or blue light that can lead to fuzzy images. Decades ago I noticed that I couldn’t focus on the blue landing lights used at airports – no matter how hard I tried, those lights always looked fuzzy to me. Now I don’t know if having a fuzzy image has anything to do with color perception, but at least we can safely say that blue and violet light entering the eye will impinge on a larger area of the retina than will yellowish-green light, and will therefore appear less bright, and thus our cones will have more difficulty perceiving its color.
- To repeat Dr. Gallagher’s suggestion: Below a certain threshold intensity our eyes lose the ability to perceive color. That’s why so many dimly fluorescent minerals appear gray. The actual color of their fluorescence may become apparent if a time-exposure photograph is taken, but we can’t tell using just our eyes. Similarly, an obviously colored phosphorescence can quickly dim to gray, but it’s not the actual color of the phosphorescent light that has changed, just our perception of it. Perhaps people who see scheelite as fluorescing white have a higher cutoff for color vision than “normal” people. I doubt this is the explanation, though – scheelite is a brightly fluorescing mineral, and if some people’s cones don’t work well in viewing it, then they wouldn’t work well for nearly all other fluorescent minerals either. Result: they’d see all, or nearly all, fluorescent minerals as white.
- If we view a small colored object against a larger background, our eyes tend to see the reciprocal color of the background. That’s why Franklin barite in small grains, against a field of red-fluorescent calcite, appears to fluoresce pale blue rather than pale yellow. Like Howie (see original message) I wonder if the same holds for the totally black background against which we tend to view fluorescent minerals – would our eyes then tend to see white? And would some people then see white instead of the blue of pure scheelite?
- From experiment we know that the shift from photopic (day-adapted) to scotopic (night-adapted) vision takes longer for some people than others. In any event it usually takes a half hour or more for one’s eyes to become fully dark-adapted. In daylight our eyes are most sensitive to yellowish-green light, and our sensitivity falls off to either side, to the red and the blue. At night our entire sensitivity curve shifts toward the blue, so we become more blue-sensitive, and when fully adapted to night vision have difficulty seeing red at all. Perhaps those people who can’t see the blue of scheelite are “locked” into photopic vision longer than normal; their vision is not yet blue-shifted. Do any of you know if it’s common for people with normal photopic vision to have poor scotopic vision?
For what it’s worth, I see the fluorescence of pure scheelite as decidedly blue (bluer than the daylight sky on most sunlit days) and would never think of calling it “white”.
The only thing I know for sure is that it’s common for people to perceive the fluorescence of scheelite as white instead of blue. I’ll have to pay closer attention to see if the same holds for other colors, particularly the strongly unsaturated ones. How many people perceive Franklin barite as white instead of cream, or can’t see the green in the greenish-white phosphorescence of some gypsum and aragonite? I’m lucky in that my eyes seem sensitive to many variations of “off-white”, but it’s also true that I’ve trained myself to recognize these differences because they’re important. Ah well, lots to think about, much to observe – another reason this hobby is so fascinating.
Cheers- Earl
|
|
A single color specimen is easy to photograph, but take one of the more desireable multi-color specimens, with some colors unsaturated and dim and you’ve got a challenge. Do you over expose the brighter colors to pick up the dimmer ones?
 This exquisite specimen of Margarosonite is perhaps one of the harder minerals to capture properly. The bright green willemite overwhelms the pale sky blue and red FL of the margarosanite and subtle areas of pink margarosanite.
Below are 9 different exposures of this piece (along with some other specimens) to show the range of colors the camera captures for the same piece. Observe how colors change depending on the length of exposure. The smaller images to the right were taken without the larger specimens present to show how othe FL minerals can even affect how less saturated colors are affected by simple relections.
The image to the right is how the piece looks to the human eye (at least - my eye).
|
|
 A Piece of Willemite/Calcite from Miller Canyon, AZ. Unlike Franklin calcite, areas of this calcite are paler and exhibit varying tones. In order to show the pinkish coloration at the right side, the picture was slightly over exposed (red turned to yellow, green too light). If the photo was taken with only attention paid to the peak brightness of the calcite, the right side would barely show up. But your eyes are capable of adjusting to these varying intensities - so we comprimise. (A better job could’ve been done on this photo, but it’s a good example of the problems encountered in digital photography of FL minerals.
|
