Redscale raster display

As mentioned elsewhere, the planetarium programme Stellarium has a “night mode” to prevent dazzle when observing the real night sky. This is basically red. Remarkably though, other than Nixie tubes and LEDs, displays tend to be reluctant to invade this end of the visual spectrum unless they also use other colours.

My suggestion is that a display be made which only shows colours at the red end of the spectrum, in a fairly novel but also quite low-tech way, which is only visible to the dark-adapted eye. Uses could be as follows: darkrooms (not terribly useful nowadays), watching videos in the dark when one wishes not to disturb sleeping partners, children or simply wants to fall asleep more easily, observing the night sky without being dazzled, spying on people without being detected, alarm clocks, night time radio and media player displays and watching nocturnal animals without disturbing them. It appears at first sight to have two other advantages: low energy use and low “technology” use.

The human eye can become very sensitive to light if in complete darkness. A commonly-quoted example is that vision can allegedly detect a candle flame from twenty kilometres. Provided an object emits enough light, it can be visible at a size well under one minute of arc in the visual field, for example stars, all of whose apparent diameters are below a second of arc by many orders of magnitude.

They are also good examples of dim light sources. A sixth magnitude star has a luminosity of ten nanolumens per square metre, whereas Venus, the brightest object in the night sky other than the Moon, is around a seventh of a millilumen per square metre. These are really, really dim lights, but all easily visible to the human eye under the right conditions. Most of them cannot be clearly perceived to be of a particular colour due to the poor function of cone cells under such conditions, and the resolution of the eye is also low due to the way rod cells are connected up to the optic nerve.

The display consists of a rectangular grid of pixels. Each pixel is a very small helical strand of wire like a filament bulb, less than a hundred microns long, offering a standardised degree of resistance, behind a convex lens to reduce the gaps which would otherwise exist, in an inert or evacuated atmosphere, each one being discrete and replaceable. Different light levels vary continuously from off up to the magnitude of Venus. The light is generated by heating the pixel with the electric current until it is red hot, but only of a sufficient temperature to give off enough photons to appear as bright as somewhere between a faint star and Venus. Damage is avoided by the helical shape, just as with a light filament. Colour temperature would vary but at such low light conditions this would probably not be visible so much as colour as something akin to a non-linear variation in luminosity, which could be taken into consideration.

The whole display is fairly low resolution by the standards of a computer monitor because visual acuity would in any case be lower in darkness. Changes in light level on the display would need to take into account the relatively long persistence of the pixels and the possibility of expansion and contraction causing the pixels to fail. Hence the frame rate of videos would have to be slower than normal. Text displays, rather than scrolling, should change in a page-flipping style, or with an input buffer at the bottom of the display.