Geometry term a TV tube's front-profile shape?

August 2, 2006 11:40 AM

Is there a geometric name for the "bulging rectangle" shape, most commonly found as the front-profile outline of classic (CRT-type) TV tubes?

You know: that rectangular "television" shape with curved left & right sides, and slightly less-curved top & bottom sides.

This shape is now often found as an icon on computer monitor/projector menus to symbolize "pincushion control", among other things. Is there a term in math (geometry) to describe this particular shape?

posted by skyboy to science & nature (13 comments total)

My daughter calls this a squoval...a square oval, but I don't know if it has a real name.
posted by alltomorrowsparties at 12:04 PM on August 2, 2006

Enzyteangle? Sorry, I don't know.
posted by JigSawMan at 12:13 PM on August 2, 2006

reuleaux polygon ?
posted by sergeant sandwich at 12:22 PM on August 2, 2006

They are probably sphere-surface sections, or in the case of Trinitron tubes, cylinder-surface sections.
posted by b1tr0t at 12:29 PM on August 2, 2006

I think the current shape of tubes is just an evolution from early tubes, which were, in fact, round-faced. Of course, they wanted to emulate the aspect of a movie screen so, over the decades, they worked within available technology to create a more-and-more rectangular-shaped face.
I suspect that the vestigial curvature you see has something to do with building in the necessary strength to the tube, in order to hold the vacuum. Sort of the way an arch can better support force.
All that said, I'm not sure there is an actual word that defines it...other than "tv tube", I suppose.

That shape is an icon because it is a near-universally-understood image. Sort of the same way the old-fashioned cradle-style rotary phone is still used as an icon for "phone" It's ingrained.

But, then again, IANAEngineer, and am speaking completely out of my ass on this one.
posted by Thorzdad at 1:00 PM on August 2, 2006

I don't know if there's a geometric name, but the the image adjustment for monitors is sometimes called 'barrel.' (for convex) when contrasted with 'pincushion' (for concave).
posted by Eldritch at 1:16 PM on August 2, 2006

I've always heard it referred to as a pillow button.
posted by iconomy at 1:24 PM on August 2, 2006

I suspect that the vestigial curvature you see has something to do with building in the necessary strength to the tube, in order to hold the vacuum.

i think the original reason was not vacuum strength (all you need to support is 14 psi, which is not so much. glass is pretty strong) but rather that the electron gun in the back of the TV is effectively a point source, so a spherical shape is necessary for consistent transit time from gun to screen. for a perfectly flat screen you'd have to adjust the timing when the beam was near the edges of the screen or you'd end up with a distorted image.

i don't know for sure, but i suspect modern flat-screen CRTs do exactly this somehow.
posted by sergeant sandwich at 1:27 PM on August 2, 2006

It was actually a beam focus issue, Sarge. Early sets couldn't modulate the beam focus fast enough to track a non-spherical shadow mask. Trinitrons were the first sets to do it much better, and current flat-tube sets (most of which likely use electrostatic rather than electromagnetic focus) do it best of all.

I don't think that the outline shape of the tube, though, has a formal name: current sets don't actually *have* their tube face in that shape.
posted by baylink at 2:20 PM on August 2, 2006

If you seek a commonly used word, it seems like this is a type of lozenge. Originally in heraldry, lozenge meant a non-square rhombus (or diamond), but more recently it's been used with rectangles having convex (angular or curving) sides, as in this Google image search.

Or you could make up your own word. How about zworykin?
posted by rob511 at 4:38 PM on August 2, 2006

You can model some convincing TV shapes using a superellipse.
posted by StickyCarpet at 5:15 PM on August 2, 2006

Here's a Java graphing toy.
posted by StickyCarpet at 5:26 PM on August 2, 2006

yeah, squoval i call it too
posted by londongeezer at 7:42 AM on August 3, 2006