very attractive. This lent it to custom applications like military avionics, where the nature of the input signal was not important and the developer was free to use any signaling style they wished. In the avionics role the penetron had other advantages as well. Its use of phosphors in layers instead of stripes meant that it had higher resolution, three times that of the RCA system. This was very useful for radar display and IFF systems, where the images were often overlaid with textual cues that required high resolution to be easily readable. Additionally, since all of the signal reached the screen in a penetron, as opposed to 15% of it in a shadow mask tube, for any given amount of power the penetron was much brighter. This was a major advantage in the avionics role where power budgets were often quite limited, yet the displays were often hit with direct sunlight and needed to be very bright. The lack of the shadow mask also meant the penetron was much more robust mechanically, and didn't suffer from color shifting under g-loads. Penetrons were used from the late 1960s to the mid-1980s, mostly for radar or IFF systems where two-color displays (green/red/yellow) were commonly used. Improvements in conventional shadow masks removed most of its advantages during this period. Better focusing allowed the size of the holes in the shadow mask to increase in proportion to the opaque area, which improved display brightness. Brightness was further improved with the introduction of newer phosphors. Problems with doming were addressed through the use of invar shadow masks that were mechanically robust and attached to the tube using a strong metal frame. Other uses Penetron displays were also offered as an option on some graphics terminals, where the high speed color-switching was not required and the penetron's limited gamut was not a concern. IDI offered such displays as an $8,000 option on their IDIgraph and IDIIOM series terminals. Tektronix, a major manufacturer of oscilloscopes, offered a limited gamut of color in some of its CRT oscilloscopes, using Penetron-type technology. Description In most versions of the penetron the tube has an inner layer of red and outer layer of green, separated by a thin dielectric layer. A complete image is produced by scanning twice, once with the gun set to a lower power that is stopped in the red layer, and then again at a higher