Neutral Density Filters & NightVision
Submitted: Monday, 20th June 2005 by Dennis
In the mid to late 1990’s, my initial, casual interest in astronomy began to develop into a deeper passion, coinciding with the increasing availability of affordable computerised telescopes and the general popularisation of digital astronomy. Now, the digital age of astronomy is well and truly a fundamental part of the rich and diverse landscape of amateur astronomy. However, there is a downside – the bright glare of a computer display is unacceptably obtrusive at astrocamps. Until recently, I had been experimenting with several unsatisfactory solutions to optimally dim my computer display, to prevent me from being ignominiously expelled from the observing paddock by irate eye-ballers. These traditional astronomers jealously guard their hard won night vision by wearing sun glasses when they adjourn to the kitchen for hot drinks – that’s how dedicated they are. The first time I saw them, gathered in the gloom sipping coffee, I thought I had stumbled into a meeting of the local Mafia!
The graphics card on my geriatric Digital notebook computer does not support the red, night-vision mode of Starry Night Pro, which changes the standard Windows colour scheme to one which utilises subdued reds and other dull colours to dim the display to an acceptable level. However, I eventually found a free plug-in on the internet which when used in conjunction with a sheet of red cellophane, attenuated the glow of my display sufficiently to satisfy the twin needs of readability and unobtrusiveness. But, as my collection of astronomy software grew, to include The Sky, SkyTools and CCDSoft, the night vision plug-in and red cellophane combination did not always work well as I tabbed between each program. The red cellophane would make certain icons, drop down menus and text difficult to read depending on how each software author had used colours in their particular implementation of night vision mode.
Neutral Density Solution
Then, I discovered neutral density filters! These reduce the intensity of light across all wavelengths equally, by a fixed amount, without suppressing or enhancing particular wavelengths of light at the expense of others. Let me explain.
When we view white text on a red background in normal daylight conditions, the contrast between the white text and red background is so great that we can easily read the text. However, when we view the same text through a red filter, it becomes very difficult to read. Why? The red filter allows the red wavelengths of the red background to transmit so efficiently that the contrast between the white text and red background is reduced to such an extent that the text cannot easily be made out. Furthermore, contrast between different shades of blue is significantly reduced, so that light blue text on a darker blue background also becomes difficult to read. To help illustrate this, I took a photograph of some books on my bookshelf in normal daylight, and then through a red gel filter as used in theatre lighting. The results speak for themselves, please refer to Figures 1 and 2 below. Notice how the (white) titles on some of the (red) spines have “disappeared” in Figure 2.
This is where the neutral density filter can help save the day, or should that be night? By dimming all wavelengths equally, the relationship between the colours as used by the computer program remains unchanged, so that you can view the display as the software author intended when you invoke their particular implementation of night vision mode. The result? The eye-ball Mafia are happy to share the night skies with the digital kids on the block and we also get to read our computer displays with relative ease.
I purchased the x 2 neutral density gel filter from the Production Shop in Brisbane, which sells theatre lighting supplies. Their telephone number is 07 3216 1340. The gel filters come in 1 metre wide rolls, and the smallest off-cut is ˝ metre long, costing around $15.00. These are theatre grade gel sheets and are therefore optically correct and tough, very resistant to tearing compared to coloured cellophane, which tears if you simply show it a pair of scissors! A x2 ND filter reduces the intensity of transmitted light by a factor of 2, whereas a x3 will reduce the intensity of a computer display to 1/3.
To hold the gel filter flat, I cut a correct sized piece for my Dell notebook computer display, and then glued the cut sheet into a plastic frame that used to house a mesh style anti-glare filter found lying around in my junk box. See the image on the right for details. This helps keep the filter flat and prevents any stray, unfiltered light from spilling out around the sides of the ND gel sheet.
A final bonus was that the ND filter also slightly improved the general contrast over the whole notebook display, making it easier to read in the darkness of night.
Clear skies and astro-optimised filters!