reflection of observer's head?

[lodestar]

Hello folks,Just a quick question about the "shadow of observer's head" phenomenon. While a deeply cut diamond is clearly dark in the centre, I've noticed that most diamonds (especially the well-cut ones) display a dark centre when looking directly at it (which is different from the deep-dark effect).Any comments on this? It's hard to find stuff about it on the web, and I'd be really interested to know!

 

[lodestar]

Well, regarding the head distance, I'm guessing it is about 12 inches on average. Would the dark reflection change size as you move the stone closer and then farther away?I've noticed that at times, the centre of the stone is completely white. Actually, the dark reflection is most obvious when in the car (with a dark roof overhead).Thanks for your comments!

 

[pricescope]

Hi Lodestar,Great observation. I hope Garry (Cut Nut) will share his thoughts on this issue since he is currently studying this effect.Bruce Harding had written about it 25 years ago: http://www.cutstudy.com/cut/english/faceting/ and particularly: http://www.cutstudy.com/cut/english/faceting/f1.htm#dead
You also might find interesting series of GIA publications on cut: http://www.gia.edu/giaresearch/diamond-cut-archive.cfm

 

[Garry H (Cut Nut)]

Also look at the images on www.ideal-scope.com
The darkness in an idea-scope image represents a head effect at a very close viewing distance of around 12 inches for a 6 inch diameter head size.
.
Please tell us what head distance you use to examine a diamond, as this is the major variable outside the different proportions of the diamonds themselves.
garry

 

[pricescope]

These images are simulated using DiamCalc for ideal Tolkowsky diamond in jewelry shop environment. Head distance from left to right: 16", 12" and 6"

The darkness in the center increases when head distance decreases. However for ideal cut diamond it also creates a pleasant contrast and allows you to see more fire.Diamonds with deep pavilion have very dark center though - nail-head effect. This image is simulated for a diamond with 44 degrees pavilion and 33 degrees crown angles in the jewelry store environment and 12" viewing distance.

 

[lodestar]

Wow - this is exactly what I wanted to know. Thank-you so much for addressing this rather important although seldom mentioned issue.Cheers!

 

[Garry H (Cut Nut)]

great stuff Leonid.
garry

 

[pricescope]

Thank you, teacher